P Bergonzo

Alexandre Bongrain

Emmanuel Scorsone

A Bendali

Gaelle L Rousseau

Bazin Lissorgues

P Mailley

Y Li

T Kauffmann

F Goy

A Bongrain Ab

A Bendali Cd

G Lissorgues

B Yvert Ef

J A Sahel Cdh

S Picaud

3D shaped mechanically flexible diamond microelectrode arrays for eye implant applications: The MEDINAS projectMatrices d'électrodes en diamant pour l'interfaçage neuronal appliqué à la suppléance fonctionnelle (MEDINAS)

Keywords: Neurobiological interfaces, CVD diamond, Retina implant, Micro-Electrode Arrays, Mots clés Interfaces neurobiologiques, Diamant CVD, Implant rétinien, Micro-électrodes

published or not. The documents may come

3D shaped mechanically flexible diamond microelectrode arrays for eye implant applications: The MEDINAS project Matrices d'électrodes en diamant pour l'interfaçage neuronal appliqué à la suppléance fonctionnelle (MEDINAS)

Introduction

The increasing life expectancy is certainly a source of future medical challenges that must be addressed today. The ageing of the population is accompanied with an increasing number of neurological diseases (Alzheimer, Parkinson, dementia, retinal degeneration, deafness, etc.), while other forms of neural trauma are also largely observed (spinal cord lesions, strokes, etc.). To cope with these pathologies, innovative therapeutic solutions are still sought. Recent advances in micro and nanotechnology have opened new routes for the fabrication of Micro-Electrode Arrays (MEAs) [START_REF] Guillemaud | A Multichannel platform for recording and stimulation of large neuronal structures[END_REF], paving the way to novel treatment strategies using neural implants. Implants can be used to electrically stimulate neural networks to relay an altered neural network to restore lost functions. For example, current commercial neuronal implants aim at stimulating the thalamus with high frequency for treating the Parkinson disease [START_REF] Garcia | High-frequency stimulation in Parkinson's disease: more or less?[END_REF], or the inner ear from deaf people with auditive implants [START_REF] Rauschecker | Sending sound to the brain[END_REF]. These successful treatments for everyday life have suggested that other applications could benefit from a similar approach, including treatments of epilepsy [START_REF] Theodore | Brain stimulation for epilepsy[END_REF], dementia, headache [START_REF] Leone | Deep brain stimulation in headache[END_REF], or visual disabilities [START_REF] Humayun | Visual perception in a blind subject with a chronic microelectronic retinal prosthesis[END_REF].

Today, in the case of retinal degeneration, that affects almost 30 Million people in both EU and US, among the most frequent pathologies, photoreceptor degeneration is causing blindness in both hereditary diseases like retinitis pigmentosa and non-hereditary retinal diseases like age macular degeneration (AMD). Despite a substantial secondary degeneration of inner retinal neurons [START_REF] Humayun | Morphometric analysis of the extramacular retina from postmortem eyes with retinitis pigmentosa[END_REF], the neuronal circuit remains active and can still convey information to the higher visual centres. Retinal prostheses propose to restore vision by targeting electrical stimulation of this remaining retinal circuit. The concept has been validated in several clinical trials showing that patients were able to follow moving light targets and were able to identify known contrasted objects [START_REF] Humayun | Visual perception in a blind subject with a chronic microelectronic retinal prosthesis[END_REF], [START_REF] Veraart | Pattern recognition with the optic nerve visual prosthesis[END_REF]. The stimulating MEAs are commonly fabricated using metal electrodes directly in contact with the neurons and electrically stimulating the cells. Such an approach however exhibits several limitations, and particularly since injecting current into the neural tissue requires transforming electronic current (from electronics) into ionic current (within the neural tissue). This transformation is done at the electrode/tissue interface by two phenomena of charge transfer. First, capacitive stimulation occurs as the variations of the electrode potential attract and/or repulse ions on the tissue side. However the amount of charge that can be transferred purely by capacitive effect is small and often not sufficient to stimulate neurons. Most of charge injection is thus achieved by a second phenomenon, charges transfer: When a positive current is injected, the electrode becomes more positive, and the electrode material is then oxidized in order to translate negative ions from the tissue into a flux of electrons. Conversely, a negative current is injected by reduction of the electrode material. In order not to deteriorate the electrode, positive and negative currents injected during a sequence of stimulation should be balanced, and the stimulation current amplitude should be low enough to involve only reversible processes around the electrode material. If the electrode material hinders injection of the desired current, then other, non-reversible reactions occur, including hydrolysis of water contained in the tissue. These reactions deteriorate both the electrode (irreversible change of the material) and the neural tissue (important local changes in pH) [START_REF] Rizzo 3rd | Methods and perceptual thresholds for short-term electrical stimulation of human retina with microelectrode arrays[END_REF]. Further, metal electrodes as generally fabricated from Ir, Pt or Au, i.e. passive materials that do not provide an optimal contact to neurons. This non-optimal contact may induce reactive gliosis (Muller cells), in the vicinity of electrical implants that produce an insulating surface between the implant and the neuron. Such a process could also explain the progressive degradation of the electrodes that has often been observed [START_REF] Rizzo 3rd | Methods and perceptual thresholds for short-term electrical stimulation of human retina with microelectrode arrays[END_REF].

Diamond electrode MEAs

Within the last two years and in the frame of the MEDINAS 1 project, we demonstrated the interest of a very novel material, namely nanocrystalline diamond (NCD), towards the development of novel MEAs for stimulation and recording, and particularly within the context of retinal diseases [START_REF]The DREAMS project (www.neurons-on-diamond[END_REF], [START_REF]The MEDINAS project[END_REF]. Synthetic diamond can be deposited at low cost from methane and hydrogen in conventional microwave chemical vapour deposition systems and on various substrates from two to four inch in diameter. We demonstrated that NCD offers extreme biocompatibility and stability in physiological media [START_REF] Kopecek | Improved adhesion, growth and maturation of human bone-derived cells on nanocrystalline diamond films[END_REF]. For example, Fig. 1 shows for retina cells [START_REF]Diamond Retinal Artificial Micro-interface Structures[END_REF] a biocompatibility as high as that of glass for diamond, although it has the overwhelming advantage of being a semiconductor [START_REF] Dankerl | Diamond transistor array for extracellular recording from electrogenic cells[END_REF]. NCD can be processed using standard nanotechnology approaches for the fabrication of pixelized microelectrode arrays using the approach shown on Fig. 2. Based on the patterning of nanodiamond seeds prior to growth, it comes possible to fabricate structured MEAs where the active area is diamond. From the use of 3D shaped moulds performed in a substrate, using e.g. conventional silicon processing approaches, diamond MEAs with 3D geometries can thus be fabricated. Reporting the MEA from a sacrificial layer to a flexible substrate also enables the fabrication of flexible implants for retinal stimulation as shown on Fig. 3. Here this ultimate challenge was to make the fabrication of diamond compatible with a soft substrate material, and it was achieved using a sacrificial substrate lift-off technique, enabling the preparation of such implants on polyimide as well as on parylene. During the MEDINAS project, matrices of 64 independent pixels for the stimulation of tissues were fabricated. Further, the possibility to fabricate electrodes with 3D geometry was demonstrated, and prototypes exhibiting wells with improved performances for cell stimulation were explored (under patent app. [START_REF] Picaud | 3D neuronal prosthesis[END_REF]). Fig. 2. Schematic of the step flow process to enable patterned diamond growth on varying substrates. Here described on oxidized Si of course processable on glass, quartz, alumina, etc. The electrochemical benefits were evaluated on MEAs where symmetrical current pulses were injected while the local variation of the pH was monitored on the electrodes using the pH sensitive fluorescent dye BCECF [START_REF] Bevensee | Use of BCECF and propidium iodide to assess membrane integrity of acutely isolated CA1 neurons from rat hippocampus[END_REF] in an unbuffered media. While the charge injection limit remained in the 10-100 μC/cm 2 range with Pt microelectrodes (in agreement with e.g. [START_REF] Ls Roblee | Electrochemical guidelines for selection of protocols and electrode materials for neural stimulation[END_REF]), values as high as several hundreds of μC/cm 2 could readily be achieved using diamond microelectrodes [START_REF] Bongrain | Diamond microelectrode arrays (MEAs) for recording and stimulation of neural networks[END_REF].

Preliminary evaluation of diamond retinal implants

The first prototypes of diamond coated retinal implants were fabricated (Fig. 3) and to test their in vivo stability, we have developed a technique to introduce implant devices in the subretinal space of blind P23H rats. After 14 weeks in vivo, the eye was fixed and histological sections of the eyes were produced to visualize the retinal tissue with respect to the implant. Fig. 3 (right) illustrates the immunostaining of the section to visualize neurons (red) to be stimulated, the bipolar cells, and glial cells (green) isolating the implant from the neurons. The large red band represents the bipolar cells neurites, their cell bodies are visualized in blue with a very small red rim in between the 3D structures. These preliminary results are very encouraging because no major reactive gliosis (red) is detected in contact with the implant.

Fig. 1 .

 1 Fig. 1. Retinal cell survival on NCD diamond (left) as compared with glass (right) after one day and ten days (Insets). (Scale bars are 200 μm).

Fig. 3 .

 3 Fig. 3. In vivo implant. Left: view of patterned diamond microelectrode arrays on soft polyimide as fabricated for retinal implant fabrication. Right: section of a rat retina with an implant (yellow).

Acknowledgement

This work has benefited from the Financial support from the French National Research Agency (ANR) in the framework of the TECSAN call for projects.

Conflict of interest statement

There is no conflict of interest.


Thanh Viet Nguyen

Michel Simioni
email: michel.simioni@inrae.fr

Hung Trung Vo

Tran Van On

Thu Dau Mot

Valuing mangrove conservation attributes in Red River Delta, Vietnam: a choice experiment approach

Keywords: Mangrove preservation, Environmental services valuation, Discrete choice experiment, Xuan Thuy National Park, Vietnam JEL Classifications: Q51, Q54, Q57, O13, D12

The study aims at valuing mangrove ecosystem services in Xuan Thuy National Park, Red River Delta, Vietnam. A discrete choice experiment was employed to elicit household willingness to pay (WTP) for a community project to protect mangroves against climate change. A conditional logit model and a random parameter logit model were estimated to identify the relationships between WTP and the different attributes of the mangrove conservation project. The results suggested that local households exhibited strong preferences for mangrove coverage area and storm prevention capacity whereas biodiversity benefits were not greatly perceived by most respondents. High level of heterogeneity in household preferences was found for the high mangrove coverage, and high management level of biodiversity. Furthermore, marginal household WTPs were computed given a change in each attribute level. Hence, the findings will aid in the development of a comprehensive payment for mangrove preservation policy in Vietnam.

Introduction

Mangroves are remarkable forests that play an essential role in protecting the community from storm, reducing impact of flooding, and coastal erosion in the intertidal zone [START_REF] Prance | The Botany of Mangroves[END_REF][START_REF] Blanco | Ecosystem-Wide Impacts of Deforestation in Mangroves: The Urabá Gulf (Colombian Caribbean) Case Study[END_REF][START_REF] Menendez | The global flood protection benefits of mangroves[END_REF]. These forests have extensive root systems that not only form a great carbon reserve but also serve as an important nursery ground for marine organisms [START_REF] Cummings | Mangroves in the global climate and environmental mix[END_REF][START_REF] Polidoro | The loss of species: Mangrove extinction risk and geographic areas of global concern[END_REF]. In addition, mangroves also support local livelihoods by providing timber, foodstuff, and raw material for traditional medicine (Millennium Ecosystem Assessment, 2005;[START_REF] Veettil | Mangroves of vietnam: Historical development, current state of research and future threats[END_REF]. However, there has been a drastic reduction of coastal wetlands in the world due to natural factors and human actions that claimed coastal wetlands for agricultural and industrial development. Since the mid-1900s, 20% to 35% mangrove forests have been lost worldwide, three to four times faster than land-based forests [START_REF] Mcnally | The Potential for Mangrove Carbon Projects in Vietnam[END_REF][START_REF] Polidoro | The loss of species: Mangrove extinction risk and geographic areas of global concern[END_REF].

It has been stated that around 34 to 42% of the world mangrove vegetation is located in Southeast Asia [START_REF] Giesen | Mangrove Guidebook for Southeast Asia[END_REF]. Vietnam has a long coastline of 3,260 kilometers, providing a supporting ground for mangroves with high ecological and economic importance [START_REF] Do | Estimating wetland biodiversity values: a choice modelling application in vietnam's mekong river delta[END_REF][START_REF] Veettil | Mangroves of vietnam: Historical development, current state of research and future threats[END_REF][START_REF] Hung | Willingness to pay for mangrove preservation in xuan thuy national park, vietnam: do household knowledge and interest play a role?[END_REF]. Mangrove forests are found more extensive in the south and north than those in central Vietnam. A recent evaluation of global flood reduction benefits from mangroves in the world shows that Vietnam ranks first in terms of protected population (7.02 millions out of a total Vietnamese population of 97.09 millions of inhabitants), second in terms of averted land flooding (3,120 out of 331,210 km 2 ), and sixth in terms of property benefits that stem from protecting assets in exposed coastal areas (6.45 billions US dollar, or 3.14% of GDP) [START_REF] Menendez | The global flood protection benefits of mangroves[END_REF].

Nevertheless, it was reported that 61% of mangrove forests have been lost in recent decades in Vietnam [START_REF] Veettil | Mangroves of vietnam: Historical development, current state of research and future threats[END_REF][START_REF] Mcnally | The Potential for Mangrove Carbon Projects in Vietnam[END_REF]. In 1943, prior to the onset of the conflicts in Vietnam, mangrove forests in Vietnam were reported to cover an area of 400,000 ha. The use of herbicides during the Vietnam wars from 1945 to 1975 destroyed more than 100,000 ha of mangrove forest [START_REF] Veettil | Mangroves of vietnam: Historical development, current state of research and future threats[END_REF]. The rise of shrimp farming since the mid-80s caused further destruction of mangrove forests. In 2005, only 157,500 ha of mangrove forests were found [START_REF] Mcnally | The Potential for Mangrove Carbon Projects in Vietnam[END_REF].

The removal of mangrove forests has undoubtedly led to many undesirable consequences such as decreasing the amount of clean water and organism as well as making local residents living near the coastline more susceptible to hurricanes. Mangrove degradation also speeds up soil erosion which leads to a coastal retreat of around 20 to 40 metres per year observed in coastal regions such as Kien Giang Province, the Mekong Delta [START_REF] Mcnally | The Potential for Mangrove Carbon Projects in Vietnam[END_REF].

Non-market values of mangrove ecosystem services such as the ability to lessen the damage from storms, the prevention of coastal erosion, the protection of bird habitats play an important role in maintaining mangroves for the future. These values could provide a baseline, not only for climate change mitigation actions to decrease national carbon footprint, but also national climate change adaptation plans to reduce flooding risks and enhance the local economy's resilience. However, measuring these values is not straightforward because the market settings for environmental goods are often absent.

Revealed preference and stated preference methods are two major valuation techniques to measure the environmental goods and services. Some wetland ecosystem services such as recreation and tourism could be valued using revealed preference methods. However, our objective is to estimate the willingness to pay (WTP) for non-use values which can be accomplished at best by stated preference methods such as contingent valuation method (CV) and discrete choice experiment (DCE). Contingent valuation is a method of estimating the willingness to pay for overall changes in the ecosystem service.

For instance, [START_REF] Hung | Willingness to pay for mangrove preservation in xuan thuy national park, vietnam: do household knowledge and interest play a role?[END_REF] recently used the contingent valuation method with double-bounded dichotomous-choice to elicit WTP for mangrove ecosystem restoration in Xuan Thuy National Park, Vietnam. The study found that income, perceived knowledge of mangrove benefits and interest in conservation activities influence WTP. Discrete choice experiment with more voices at the forefront of literature reveals WTP for changes in the component attributes of mangrove ecosystem services [START_REF] Holmes | Choice Experiments[END_REF]. The existing wetland valuation studies that use discrete choice experiments include those by [START_REF] Birol | Using a choice experiment to account for preference heterogeneity in wetland attributes: The case of cheimaditida wetland in greece[END_REF] on the case of Cheimaditida wetland restoration in Greece; [START_REF] Rezende | An economic valuation of mangrove restoration in brazil[END_REF] on the restoration of mangrove forest in the northern region of Rio de Janeiro state, Brazil; [START_REF] Tan | Valuation of environmental improvements in coastal wetland restoration: A choice experiment approach[END_REF] on the coastal wetland restoration in Ximen Island Special Marine Protected Area, China.

In Vietnam, only one choice experiment was conducted by [START_REF] Do | Estimating wetland biodiversity values: a choice modelling application in vietnam's mekong river delta[END_REF] to estimate biodiversity values at Tram Chim National Park, a typical wetland ecosystem in Mekong River Delta.

The study applied conditonal logit and random parameter logit models to capture taste preference heterogeneity for different types of improved outcomes (mangrove coverage, the number of Sarus Cranes, the number of bird species, the number of households affected) associated with wetland restoration. Individual characteristics such as age, education, household income, knowledge of the study site, and perceived benefits of mangrove values for future generations were found to affect the WTP for wetland conservation program. This paper aims to trace out local preferences by examining how people responded to different types of outcomes associated with mangrove conservation programs at Xuan Thuy National Park (XTNP) at the Ba Lat estuary, Nam Dinh province, in north of Vietnam. The park was chosen as a study site because of its coastal protection capabilities and diverse wetland ecological system that are particularly important in the socio-economic development of the Red River Delta region. XTNP is also an incredible natural spectacle with thousands of wild birds migrating from the north to the park each year.

There is scant information on the values of wetlands in the Red River Delta. Hence, this study is necessary to reflect the relative values of resources in alternative wetland management in the region.

We employed direct non-market valuation approach for valuing environmental goods by creating hypothetical markets where respondents were asked to state their preferences for improved environmental quality [START_REF] Champ | A Primer on Nonmarket Valuation[END_REF]. Households valued access to mangrove ecosystem services with many benefits expressed in economic terms as household WTP in exchange for better mangrove ecosystem services. The findings will advise policy-makers to take important conservation measures to ensure wetland expansion, maintain coastal protection and preserve habitat for bird species.

The paper is structured as follows. Section presents the background of the study site, survey methodology, sampling strategy and the econometric framework used in the paper. Section summarizes the main features of data. Main findings are discussed in section . Finally, main conclusions are drawn in section .

Methodology

Study site

Xuan Thuy National Park (XTNP) is a special nature reserve with vast areas of mangrove swamps located in the coastal zone of the Red River Delta and Ba Lat estuary [START_REF] Thanh | Climate Change Challenges for Sustainable Coastal Wetland Management in Xuan Thuy Ramsar Site, Vietnam[END_REF]. The park is divided in two main areas: a core zone (7,100 hectares) where human activities are strictly prohibited and a buffer zone (8,000 hectares) where human activities are managed to mitigate adverse impacts on the core zone [START_REF] Pham Hong | Vulnerability to climate change of mangroves in Xuan Thuy National Park, Vietnam[END_REF]. Figure 1 shows the map of XTNP including the five communes in the buffer zone that were chosen as the study site: Giao Hai, Giao Xuan, Giao Lac, Giao An and Giao Thien. The Convention on Wetlands is an intergovernmental treaty that provides the policy framework for national action and international cooperation for the proper management of wetlands and their resources.

floating aquatic plants, 500 species of benthos and zooplankton (shrimp, fish, crab, oysters, etc.) have been recorded at the park. This site is also internationally-recognized for many migrating birds with 219 bird species recorded. Many of them are listed in the Red List of Endangered species such as the Black-faced Spoonbill (Platalea minors), Spoon-billed Sandpiper (Calidris pygmaea), and Spotted Green-shank (Tringa guttifer ) [START_REF] Nguyen | Analyse Wetland Economic Value: Case Study in Xuan Thuy National Park, Viet Nam[END_REF].

Nevertheless, there are a variety of issues that threaten wetlands in the area. Firstly, mangrove forests have been cleared and converted into agricultural lands for fish and shrimp farming. Secondly, the future of mangrove forests is very sensitive to climate variability such as sea level rise, elevated levels of temperature, rainfall. The local government protects XTNP from being exploited by enforcing strict rules with respect to fishing, hunting and harvesting plants inside the reserve. International organizations are finding solutions to preserve the mangroves for future generations and for locals to make a sustainable living.

Survey methodology

Questionnaire development The questionnaire was developed through an iterative design process. We first consulted with researchers and economists in the field of natural resource valuation to review different versions of the questionnaire. We then had several meetings with the board of XTNP to identify the opportunities for mangrove management. We also conducted meetings with local representatives of villages to understand the local livelihoods in the buffer zone. We wanted to know the local representatives' view on climate change impacts and how the risk of flooding varied depending on the location.

In addition, the local representatives gave important suggestions on how community funding for mangrove rehabilitation program could be used properly to cope with climate change. Concrete missions such as the construction of dykes to protect mangroves against sea rise, ensuring the sediment supply, mangrove planting were suggested by the local representatives. The information was very useful for our selection of attributes with the levels associated in the choice design. Furthermore, a pre-test survey was conducted with 20 households in the buffer zone to get some estimates of their minimum and maximum WTP for a mangrove conservation program. This prelimi-nary survey was helpful in formulating the contribution rates for the improved scenarios in the choice sets.

Hypothetical setting In our choice experiment survey, respondents were presented with a series of scenarios or choice sets of mangrove preservation programs that described the hypothetical market for the attributes. The future climate change scenario (see Appendix) provided by the Ministry of Natural Resources and Environment (MONRE), Vietnam 2 , released in 2016, gave sea level, temperature rise trajectory, and minimum rainfall variation whereas the report by the XTNP's management board provided the estimated rate of mangrove degradation [START_REF] Tran | Climate Change and Sea Level Rise Scenarios for Vietnam[END_REF]Management Board of Xuan Thuy National Park, 2014). Our enumerators demonstrated clearly to respondents what would happen without feasible plans on restoration of mangrove ecosystem. The hypothetical market needed to be understandable and plausible to respondents so that they firmly believed that environmental change would happen.

Attribute definition

The choice of attributes and their associated levels were based on many references. First, the report "The current state of biodiversity at XTNP" (Management Board of Xuan Thuy National Park, 2014) helped us gain a basic understanding of the mangroves at the study site and their benefits in the context of climate change. Initially, a list of 15 potential attributes characterizing mangrove conservation programs was introduced. These attributes included areas of mangroves planted at intertidal zone, areas of lagoons for aquaculture, average daily income from fishing in the wetland, number of fish species, number of the black-faced Spoonbill, number of bird species, number of tourists, and ability to prevent saline water intrusion, storms and coastal erosion.

After several meetings with experts, local representatives in charge of mangrove conservation, and households, we decided to shorten the list of attributes to three (Table 1 gives their definitions and levels.). The first two are ecological attributes: the total area of tidal flat which has mangrove coverage being described in terms of hectares of mangroves protected, and biodiversity defined as the number of the black-faced Spoonbill preserved at the Ba Lat estuary. The third attribute was storm 2 The climate change scenario introduced by MONRE in 2016 was built upon the 5th assessment report of the Intergovernmental Panel on Climate Change (IPCC). MONRE developed a technique to take global and regional climate information from the scenarios defined by IPCC and then zoomed onto much finer details of small regions in Vietnam. Hence, these projections provide more specific, regional information that local authorities and researchers need for their climate adaptation action plans in the studied regions.

prevention ability measured in terms of expected economic losses from storm level, high or low. The avoided storm losses are linked to a recent event (Tropical Storm Mirinae) in order to tie an attribute of a hypothetical good to a real (consequential) event experienced by respondents.

The selected attributes were consistent with those chosen in other studies on wetland valuation [START_REF] Brouwer | A meta-analysis of wetland contingent valuation studies[END_REF][START_REF] Do | Estimating wetland biodiversity values: a choice modelling application in vietnam's mekong river delta[END_REF][START_REF] Tan | Valuation of environmental improvements in coastal wetland restoration: A choice experiment approach[END_REF]. The number of attributes were also compatible with cognitive capacities of respondents at the study site. Indeed, all attributes represented important aspects of mangrove ecosystem services at Ba Lat estuary to respondents. Each choice option is based on a combination of attributes' levels and respondents' choices indicated how they valued these attributes. Last column of Table 1 gives the values corresponding to status quo.

Table 1

Wetland Management Attributes and Levels in the CE.

Attribute Description

Management Levels Status Quo

Area

The total area of tidal flat which has mangrove coverage, providing a better habitat for animal species and increasing yields of eco-friendly shrimp ponds.

Low A payment instrument was added to the list of attributes. Income tax is the most widely used payment instrument in developed countries. Nevertheless, this payment instrument could be inappro-priate in Vietnam, and especially, in Nam Dinh province, due to a lack of transparency in tax system and low population coverage [START_REF] Do | Estimating wetland biodiversity values: a choice modelling application in vietnam's mekong river delta[END_REF]. Hence, households' donations to the Fund for Wetland Conservation of the Mangroves in the Ba Lat estuary were used as our payment instrument.

Our survey targets households because community driven projects often require household contribution in Vietnam, especially for rural households that have low budgets. In addition, we did not conduct interviews with multiple members of the household because doing so would require multiple visits that might push up the cost of the survey considerably and the enumerators might have incentives to obtain information as quickly as possible.

Choice sets elaboration. The choice of attributes and their levels resulted in 72 potential choice options. Ngene software was used to implement a fractional factorial design for defining choice sets (ChoiceMetrics, 2018). A subset of complete factorials was chosen to capture efficiently the effects of interest according to the D-optimal criterion [START_REF] Reynaud | Is there a demand for flood insurance in Vietnam? Results from a choice experiment[END_REF]. 24 choice sets were constructed and grouped into four versions of the choice experiment presented to respondents. Each version contained six choice sets with varying levels of attributes in order to get a range of environmental goods and payments.

In each choice set, respondents were presented with two options. The first option was the status quo, keeping the entire mangrove forest intact (see last column in Table 1). All attribute levels were kept at their current levels and required zero payment from respondents. The second option proposed improved levels of the restoration program but with some payments. An example in which respondents were asked to choose between the status quo and the alternative scenario with an increase in payment for more mangrove coverage given the climate change projection is provided in Table 2.

Sampling strategy

The survey was conducted in Nam Dinh Province, which is located in the North East of Vietnam. Nam Dinh province has 72-kilometers coastline and is often affected by saline intrusion and tropical storms throughout the year [START_REF] Leslie | Bringing social and cultural considerations into environmental management for vulnerable coastal communities: Responses to environmental change in Xuan Thuy National Park, Nam Dinh Province, Vietnam[END_REF].

The sampling strategy was implemented as follows. We first chose Giao Thuy district to be the study site because this is the location of Xuan Thuy Wetland Nature Reserve. Then, 5 communes including Giao Thien, Giao An, Giao Xuan, Giao Hai and Giao Lac (out of 20 communes in Giao Thuy

Discrete choice models

Random utility framework. Building upon the characteristics theory of demand [START_REF] Lancaster | A new approach to consumer theory[END_REF], random utility models, or RUMs, have become the basis for the econometric analysis of DCE.

These models assume that the utility for an alternative j in in a particular choice task k perceived by respondent i, or U ijk , can be decomposed into observed, or V ijk , and unobserved, or ε ijk , parts.

Assuming that those parts are additive, RUM ends up with the following utility specification:

U ijk = V ijk + ε ijk (1)
Most RUMs assume then that the observed part of utility V ijk can be expressed as a weighted sum of observed attribute levels of each alternative.

Conditional logit model. The most basic RUM is the conditional logit model, or CL, which assumes constant (homogeneous) weights for each attribute over all respondents [START_REF] Mcfadden | Conditional logit analysis of qualitative choice behavior[END_REF].

CL model assumes that the random part of utility, i.e. ε ijk , is identically and independently extreme type 1 distributed. Omitting indexes for simplicity, the specification for the deterministic utility function for the CL model is:

V = ASC • (α) + Area • (β) + Loss • (ζ) + Biodiversity • (µ) + P ayment • (κ) (2)
where Area is a dummy variable equal to 1 for the area of tidal flat with average or high mangrove coverage respectively (reference category is low mangrove coverage), Loss is a dummy variable equal to 1 if a potential storm level 10 yields low economic losses (300 billion VND) in comparison to the high losses for the status quo, Biodiversity is a dummy variable equal to 1 if there are 80 or 100 individuals of endemic species preserved at Ba Lat estuary respectively (reference category is 60 individuals of endemic species), P ayment is the one-off payment of each household (in thousand VND) for different conservation management levels (no payment is made for the status quo). An alternative specific constant (ASC), which is a dummy variable that equals to one when the alternative scenario is chosen and to zero when the status quo is chosen, is also added to the model [START_REF] Hoyos | The state of the art of environmental valuation with discrete choice experiments[END_REF][START_REF] Reynaud | Is there a demand for flood insurance in Vietnam? Results from a choice experiment[END_REF][START_REF] Adamowicz | Stated preference approaches for measuring passive use values: Choice experiments and contingent valuation[END_REF].

Random parameter logit model. More flexible models, such as random parameter logit model, or RPL, have been developed to overcome some restrictive assumptions of CL model: no random taste variation, restrictive substitution patterns and no correlation of unobserved factors as captured by ε ij [START_REF] Train | Discrete Choice Methods with Simulation[END_REF]. RPL model introduces taste heterogeneity by allowing for some or all utility weights to be individual specific, i.e. β i = β + η i where η i is a vector of individual i-specific deviations from the mean utility parameters β. These deviations vary with density f (η i |θ), where θ is a vector of unknown parameters describing the distribution of β i (for example their covariance matrix). RPL model allows modeling unobserved preference heterogeneity across respondents. The utility-specification of an RPL model can be written as

U ijk = β i x ijk + ε ijk (3)
The specifications of the indirect utility function for the RPL model, after omitting indexes for simplicity, are:

V i =ASC • (α + η α,i ) + Area • (β + η β,i ) + Loss • (ζ + η ζ,i ) + Biodiversity • (µ + η µ,i ) + Payment • (κ) (4)
For each attribute and for the ASC, we have a mean preference parameter, for example, α for ASC, and a standard deviation coefficient, such as the standard deviation of η α,i for ASC, to represent random taste variation for mangrove restoration services. Each of the individual utility parameters is assumed to be normally distributed. But, for the sake of ease and accuracy of WTP computation, individual payment utility parameter is supposed to be be fixed in the RPL model.

Willingness-to-pay. Coefficient estimates in the utility function do not have any real meaning, except for their significance and sign [START_REF] Reynaud | Is there a demand for flood insurance in Vietnam? Results from a choice experiment[END_REF]. To get a more concrete, interpretable result, it is important to compute the marginal WTP for a change in the attribute. The marginal WTP for an attribute is described as the marginal rate of substitution between this attribute and the monetary cost to maintain the same level of utility [START_REF] Train | Discrete Choice Methods with Simulation[END_REF]. This is done by dividing the vector of non-price parameters by the price parameter, or a Mangrove dependency refers to households that were direct beneficiaries of mangroves for their livelihood activities such as fish and wood. b Household agricultural activities, and income affected by natural disasters in the past 5 years.

W T P attribute-level,i = - β attribute-level,i β price,i (5)

Local awareness about mangrove restoration in XTNP

Table 4 indicates the local subjective views of mangrove benefits. Over 67% of respondents believed that the mangroves in the XTNP played an important role in controlling flood, storms and soil erosion.

The results also indicate that a major number of the respondents (37.9%) realized that mangroves have been a vital source of aquatic products, raw material for their production and consumption. Table 6 shows perceived motives for mangrove protection in the Ba Lat estuary. In order to answer the questions, respondents were clearly explained how the mangroves in the Ba Lat estuary have been degraded overtime. Given various scenarios of the mangroves in the context of climate change, respondents were asked Likert scale questions to rate the importance of reasons to protect the mangroves, on a scale from 1 to 5, with "1 = Not at all important", "2 = Not so important", "3= Neutral" , 4= Important", and "5= Very important". Respondents could also choose "not able to evaluate" option. These results suggest preventing the coastal zone against floods, erosion, salinization and providing benefits for future uses were the two most important motives. Biodiversity preservation was the third most important motive for mangrove rehabilitation. We excluded respondents who reported a zero-probability belief in all proposed options in most choice sets. It was not worth studying how much these respondents are willing to pay because they believed that the proposed conservation programs were not going to take place. That turned out to be about 13.7% of the sample excluded from our statistical analysis. The remaining 125 households were given 6 choice sets, making up a total of 750 observations (choice occasions) in the statistical analysis. In addition, the random sampling guarantees that the sample is representative of Nam Dinh population that lived near mangrove forests and had some exposure to climate change impacts.

There are around 13,000 households in the five surveyed communes, given the desired margin of error at 5%, a statistically viable sample size is about 380 observations 4 . By using experimental design theory, we have increased the statistical efficiency of the parameters estimated so that smaller samples may be used [START_REF] Holmes | Choice Experiments[END_REF]. The minimum sample size necessary for choice studies is suggested about 50 decision makers for each alternative or about 300 observations (choice occasions)

for 6 choice sets (Hensher et al., 2015, p. 194).

4

The sample size n =

N 1+N * ε 2 .

Results

Different versions of the models presented in section were estimated using a set of variables with descriptions given in Table 7. 5 The alternative specific constant (ASC), which is a dummy variable that equals to one when the alternative scenario was chosen and to zero when the status quo was chosen, was also added to the models [START_REF] Hoyos | The state of the art of environmental valuation with discrete choice experiments[END_REF][START_REF] Reynaud | Is there a demand for flood insurance in Vietnam? Results from a choice experiment[END_REF][START_REF] Adamowicz | Stated preference approaches for measuring passive use values: Choice experiments and contingent valuation[END_REF].

In addition to a CL, a classical RPL with only individual parameters heterogeneity was considered.

Individual parameters heterogeneity was captured using normal distributions. Conditional Logit. For the CL model, there were consistent preferences in the sample. The insignificant coefficient on the ASC means respondents had no preference for which program was chosen on average. The statistically significant, positive coefficients on mangrove coverage attributes indicate that respondents wanted to have more mangroves preserved either at the medium (1991 ha) or high management level (2100 ha). There are positive and significant coefficients on the total loss incurred by a storm at low levels (losses of 300 billion VND or less presumably). Therefore, if respondents were given a choice with a high flood prevention ability, everything else being held constant, they were more likely to say yes to the alternative program. Interestingly, there was no preference for the medium or high management level of biodiversity (80 or 100 individuals of endemic species, respectively). Hence, the lack of interest in biodiversity benefits indicating that biodiversity conservation plan might not be prioritized by households in the coastal area since it might only affect their livelihoods in the short-run. The negative coefficient on payment makes general sense. As respondents disliked the higher cost, they were less likely to pick the alternative program.

Random parameter logit. The RPL adds unobserved random variation in preferences between individuals to the classical CL specification. Random effect is demonstrated by normal distribution, the most commonly used distribution within RPL literature [START_REF] Train | Discrete Choice Methods with Simulation[END_REF]. By knowing the mean and standard deviation for each attribute level, it is then possible to know what percentage of respondents likes or dislikes it.

It should be noted firstly that the RPL model nests the CL model. Then, the null hypothesis that the standard deviation coefficients are all equal to zero in the RPL model can be tested using classical log-likelihood test comparing log-likelihoods of the two models. The null hypothesis is then rejected at a 95% confidence interval, implying that improvement in the model fit is achieved with the inclusion of unobserved individual heterogeneity when using the RPL model.

Secondly, taking into account unobserved individual heterogeneity in preferences does not modify the direction of the effects of attribute levels. Indeed, the signs and significance of the mean utility parameters in the RPL model are similar to those in the CL model. But now, the analysis of these mean effects is enriched by the knowledge of the standard deviations associated with them.

Standard deviations are not statistically significant in the effects of storm prevention capacity as well as mangrove coverage and biodiversity at the average management levels. Nonetheless, there was a considerable preference heterogeneity among respondents in the effects of mangrove coverage and biodiversity at the high management levels. Some people prefer a high management level of biodiversity and high level of mangrove coverage because they perceived a great deal of positive benefits. Some people, on the other hand, might dislike high levels of management as they prefer more mangroves to be converted into short-term profitable alternatives like shrimp ponds, or they simply could not perceive the ecological and social benefits of having high management levels for these attributes. It is noted that the mean coefficient of having high management level for biodiversity is negative and not statistically different from zero. For the CL model without accounting the variances of tastes, we would conclude that having biodiversity at high level of management is irrelevant to people's choices. However, it is still relevant to respondents' choices in the presence of taste variation.

Some respondents really wanted biodiversity while other respondents did not see the benefits of having more biodiversity. On net, the negative and positive effects average each other out and the mean coefficient of high biodiversity level was close to zero. The same case is for the coefficient on the ASC given its significant standard deviation but insignificant mean coefficient. The statistically insignificant ASC means that WTP is not statistically different from zero in the status quo scenario. In addition, this is another instance where the random parameters model finds heterogeneity across respondents since the ASC standard deviation is greater than the mean.

Willingness-to-pay estimates. Table 9 displays the marginal WTPs for conservation attributes for the two specifications, i.e. the WTPs to change from the status quo to the improved scenario given a change in each of the attribute level.

The estimation results for the attributes were found to be highly consistent for the two specifications. Furthermore, the results demonstrate fairly significant WTPs for households for mangrove coverage and flood prevention across the two specifications. In the CL specification, for instance, to move from low mangrove coverage (1661 ha) to medium coverage (1900 ha), households were willing to pay an additional 213,126 VND (9.19 USD) per household. The confidence interval for this is the range of marginal WTP between 123,539 VND to 319,080 VND per household. This confidence interval potentially containing the true population parameter is fairly tight around the estimate, which is a good sign.

For a greater extension of mangroves, households on average were willing to pay 157,816 VND (6.81 USD) per household to have from 1661 ha to 2100 ha of mangroves preserved in the park.

Surprisingly, household WTP for the highest level of mangrove coverage was lower than that for the medium level of mangrove coverage. Respondents would be likely to vote against the proposed program for the greater coverage of mangroves due to higher payment. For protection against storms, the level of household WTP tied around 151,803 VND (6.55 USD).

However, it should be stressed that there was no statistically significant effect from biodiversity, whether it was just the medium or the high management level across the two specifications. Confidence intervals that range from some negative numbers to large positive numbers indicate that there is no confidence about how precisely these estimates are. This could be explained by the fact that respondents were not aware of the biodiversity benefits on their livelihoods or they paid more attention to other aspects of the ecosystem services.

In addition, the confidence intervals in the RPL model shows a wider range around the mean WTP for all attributes than those in the CL model, implying a relatively inconsistent preference across the sample. For instance, in the RPL model specification, the confidence interval indicates much variation of marginal WTP for the highest level of mangrove coverage around the mean estimate from -347,245 -132.494, 52.516] [-544.966 , 383.357] Note: For the CL,CIs are computed with 1000 bootstrap replications [START_REF] Krinsky | On approximating the statistical properties of elasticities[END_REF].

The negative values mean willingness to accept to abandon the proposed scenario.

VND to 593,129 VND. This is due to the preference heterogeneity among respondents over mangrove coverage obtained at a larger scale. The negative value implies some respondents were even willing to accept to abandon the proposed scenario, due to the higher cost associated with the higher attribute level.

It is worth noting that the ranking of attributes with regards to household WTP was fairly consistent in the two specifications. The highest marginal WTP was found for medium level of mangrove coverage based on the benefits and the costs associated with the attributes. There was a slight variation in the ranking of the marginal WTP for the highest level of mangrove coverage and storm protection attribute, reflecting how taste preferences were modeled differently across the specifications.

Attribute scope. This section considers how attribute scope in a choice experiment affects preferences for mangrove management policy. Insensitivity of respondents to the scope of environmental goods has been a concern in the non-market valuation literature [START_REF] Smith | Arbitrary values, good causes, and premature verdicts[END_REF][START_REF] Kahneman | Valuing public goods: The purchase of moral satisfaction[END_REF][START_REF] Czajkowski | Using Labels to Investigate Scope Effects in Stated Preference Methods[END_REF][START_REF] Whitehead | Plausible responsiveness to scope in contingent valuation[END_REF]. There is a conflict of scope insensitivity in real world settings and economic theory which states that WTP is increasing with more quantity or quality of a good. The scope test is important to determine economic significance in addition to statistical significance for an efficient level of policy [START_REF] Whitehead | Plausible responsiveness to scope in contingent valuation[END_REF]. Various types of scope tests have been proposed to infer how elicited values are sensitive to changes in quantity or quality of environmental area attribute. This might reflect a scope effect in which the more mangrove areas are offered to respondents, the more they are willing to pay for those areas. The results from table 9 confirm that mangrove forests are valued by local residents, and we would expect a positive marginal WTP for 1900 ha relative to the status quo. Nonetheless, scope sensitivity is not observed because marginal WTP for 2100 ha is lower than that for 1900 ha. The difference in findings from table 9 and 10 could be explained by policy bias in which some respondents valued the whole policy package differently from the sum of individual parts [START_REF] Spencer-Cotton | Spatial and scope effects: Valuations of coastal management practices[END_REF]. The standard deviation on the high hectares coefficient confirms that there is a wide range of individual coefficients. If individual WTP is considered, some respondents would pass the scope test while others would see the additional hectares as a bad. Individuals that are insensitive to area levels had "preference satiation" in which marginal WTP for additional area diminishes rapidly to zero given the higher payment [START_REF] Rollins | The Case for Diminishing Marginal Existence Values 1[END_REF][START_REF] Whitehead | Plausible responsiveness to scope in contingent valuation[END_REF].

Concluding remark and policy implication

A choice experiment was employed for eliciting households' preferences for mangrove rehabilitation program at the Ba Lat estuary, Nam Dinh province. The survey consisted of choice sets in which respondents had to choose between two options: a status quo option and an improvement option based on different combinations of attributes such as the area of the restored mangrove forest, the number of preserved "Black-faced Spoonbill", the total damages caused by storms, and the contribution rate.

In addition to questions on socio-demographic characteristics of the respondents, various questions were asked about the awareness of the respondents with respect to mangrove ecosystem services in a climate change context.

Two econometric specifications were used to capture households' preferences: a) Conditional multinomial logit (CL model) which assumes homogenous preferences across respondents; b) Random parameter logit (RPL model) which accounts for unobserved preference heterogeneity .

We found that mangrove coverage extension (at the medium or high management level), storm prevention ability, and contribution rate were all statistically significant in explaining household WTP across the two specifications. Surprisingly, biodiversity conservation either at the medium or high management level was not statistically significant in explaining household WTP. The unobserved heterogeneity of tastes in the sample was found for the high mangrove coverage, and high management level of biodiversity. Some respondents might have incomplete understanding of the way biodiversity maintains the value of ecosystem services and were not willing to pay for this management attribute. If these respondents knew more about biodiversity aspects, they would value differently. Our results thus suggest a scope for training and education in order to raise local awareness of biodiversity benefits.

Marginal WTP per household for each attribute was estimated according to each specification.

Our results reveal some different levels of WTP for mangrove conservation depending on the attributes and the management levels. The findings shed light on non-market values of wetland management attributes at Ba Lat estuary in the context of climate change, taking with caution the fact that people are different in their choice preferences.

We acknowledge that our study is subject to several shortcomings. Firstly, in order to expand the scope of the study, future studies could include additional improvement options in each choice set or incorporate other wetland management attributes such as the improvement of water quality, ecotourism facilities. Another interesting avenue for further research is to examine whether these social norms or morals play an important role in determining households' WTP for mangrove conservation [START_REF] Czajkowski | Social norms, morals and self-interest as determinants of pro-environment behaviours: The case of household recycling[END_REF]. Secondly, more information on the project costs should be included in the current valuation so that policy makers can have a comprehensive cost benefit analysis of the coastal wetland restoration projects. Thirdly, our scope test points out a scope sensitivity of households to the area of the forest, however, the reported marginal WTP estimates for this attribute do not indicate this. This implies taste variation of individuals with respect to the scope of mangrove areas that requires further assessment. The small sample size could reduce parameter precision. A split-sample design survey with a larger sample size could be conducted to assess how respondents value different segments of the forest with different characteristics, for instance, a variation of mangrove degradation rate, speed of coastal erosion, vulnerability to natural disasters. This design allows for testing some hypotheses which deepen the scope effects for mangrove management policy implications and improve the statistical analysis of the WTP.

Figure 1

 1 Figure 1 Map of Xuan Thuy National Park and the study site

  Multi-stage sampling was used to select villages and households. At the first stage, two villages were selected by random sampling from the list of villages in each commune. At the second stage, 145 households were surveyed by convenience sampling, i.e. surveying any household in each village without any prior notice given their proximity to enumerators.Face-to-face interviews were carried out in March and April 2017. All enumerators were students from Hanoi University of Natural Resources and Environment (HUNRE) and Vietnam National University, University of Economics and Business (VNU-UEB), Hanoi, Vietnam. Before reaching the households, we contacted the head of the XTNP's management board for a formal approval for the survey. Our enumerators mentioned clearly the official approval of the survey at the beginning of each interview with the households. Most respondents in the survey were household heads or their spouses.

Table 2

 2 Example of a Choice SetComparing the current scenario with the improved scenario, what is your selection?

	Attribute	Current Scenario A Proposed Scenario B
	Mangrove coverage area	1661 ha	1900 ha
	Biodiversity	62 species	62 species
	Loss	622 billion VND	622 billion VND
	One-off payment	0 VND	200,000 VND
	I would prefer	2 Choice A	2 Choice B

District) were selected because these communes constitute mostly the buffer zone at XTNP. Then, 145 households were chosen from the village listing of registered households. These representative households came from 74 hamlets in the Giao Thuy District.

Table 4

 4 Perceived Benefits from Mangrove Forests

	Benefits from Mangrove Forest	Percentage
	Aquatic products, raw material for production and consumption 37.9
	Recreation, tourism	0.7
	Prevention of storms, floods, tides, and coastal erosion	67.6
	Underground water protection, preventing salinization	7.6
	Climate regulation, carbon dioxide absorption	18.6
	Preserving silt, sea encroachment	6.9
	Habitat for fish and animals	29.7
	Biodiversity	2.1
	Other	6.9
	Do not know	10.3

Table 5

 5

displays respondents' perceived reasons for mangrove degradation. 41.4% of the respondents perceived human activities such as aquaculture, fishery, etc. as the major threat to mangrove forests. 20% of them perceived that climate change posed threat to the survival of mangroves.

Table 5

 5 Perceived Causes of Mangrove Degradation

	Reasons	Percentage
	Human activities: aquaculture, fishery, etc. 41.4
	Pollution	12.4
	Climate change	20.0
	Other	11.0
	Do not know	1.4

Table 6

 6 Perceived Motives for Mangrove ConservationOur multi-stage sampling ensures a statistically viable sample size for the choice experiment study.

	Reason	Not at	Not so	Neutral Important Very
		all Im-	Impor-			Impor-
		portant	tant			tant
	Provides wood, fish, and raw materials	6.2	21.5	30.6	22.2	19.4
	Provides recreation	1.4	8.3	20.1	40.3	29.9
	Prevents floods, erosion, salinization	0	1.4	2.1	9.7	86.9
	Conserves biodiversity	1.4	4.2	19.4	33.3	41.7
	Benefits for future uses	0.7	2.8	4.1	17.2	75.2

Table 7

 7 Description of Variable.Estimation results are reported in Table8. These results are discussed starting from the most commonly used specification in the literature, namely the CL model, and moving towards a richer content model in terms of analysis of choice behaviors (the RPL model).

	Variable	Description

Table 8

 8 Maximum Likelihood Results

	Variable	CL Model	RPL Model
	Coefficients
	ASC	0.167(0.25)	0.378(0.297)
	AreaAverage	0.767(0.194)*** 0.876(0.211)***
	AreaHigh	0.565(0.197)*** 0.569(0.254)**
	BioAverage	0.068(0.191)	0.076(0.203)
	BioHigh	-0.131(0.195)	-0.311(0.267)
	LossLow	0.534(0.158)*** 0.676(0.179)***
	Payment	-0.004(0.001)*** -0.005(0.001)***
	Standard deviations
	ASC	NA	0.456(0.2)**
	AreaAverage	NA	0.011(0.778)
	AreaHigh	NA	1.213(0.403)***
	BioAverage	NA	0.005(0.437)
	BioHigh	NA	1.172(0.448)***
	LossLow	NA	0.015(0.369)
	Observations	750	750
	Log likelihood	-464.50	-457.51
	AIC	943.00	941.03
	BIC	975.34	1000.29
	Note: Standard errors are in parentheses. * (p<0.1); * * (p<0.05); * * * (p<0.01).

Table 9

 9 Marginal WTP (in thousand VND) for Conservation Attributes

	Variable	CL Model	RPL Model
	AreaAverage 213.126	190.884
		[123.539, 319.080] [-76.133, 474.889]
	AreaHigh	157.816	122.123
		[64.873, 258.865]	[-347.245, 593.129]
	LossLow	151.803	149.120
		[74.836, 248.224]	[7.520, 300.556]
	BioAverage	20.222	16.759
		[-65.734, 110.495] [-148.485, 184.129]
	BioHigh	-36.680	-67.060
		[

  Note: p values are in parentheses. p < 0.1,* p < 0.05, ** p < 0.01, *** p < 0.001. Vuong test[START_REF] Vuong | Likelihood ratio tests for model selection and non-nested hypotheses[END_REF] is applied for comparing non-nested models (ID versus LIN model). Log-Likelihood Ratio test is applied for comparing nested models (ID versus DIFF model).

	Table 10
	The atribute scope test results
		CL model	RPL model
	Estimation models
	Log likelihood	-465.50	-457.51
	LIN model
	Log likelihood	-468.29	-463.92
	Test N(0,1)	0.1768 (0.8596) -0.4949 (0.6206)
	DIFF model
	Log likelihood	-465.05	-459.64
	LRT χ 2 (2)	1.097 (0.2948)	4.2461 (0.1197)

The comprehension or engagement in face-to-face interviews was independent of respondents' low education level. Respondents were intrinsically connected to the sea and their lives depend on it. Hence, they were keen on expressing personal concerns about climate change and their answers were important to our survey.

The gmnl package in R was used to estimate the models[START_REF] Sarrias | Multinomial Logit Models with Continuous and Discrete Individual Heterogeneity in R: The gmnl Package[END_REF]. Following[START_REF] Train | Discrete Choice Methods with Simulation[END_REF], the distribution simulations in the RPL model were based on 1000 sequence Halton draws.

grateful for the constructive comments by anonymous reviewers, the Associate Editor and the Editor-in-Chief. This research is funded by Vietnam National University, Hanoi (VNU) under project number QG.17.35.

Confidence intervals of the estimated marginal WTP can be computed by applying the non-parametric bootstrap technique proposed by [START_REF] Krinsky | On approximating the statistical properties of elasticities[END_REF] with a large number of simulations from the chosen distributions in the different models presented above.

Data

Individual characteristics

Table 3 displays the respondents' socio-demographic characteristics. In this sample, the number of female participants (50.3% of the 145 respondents) was almost equal to the number of male participants, ensuring that gender balance was represented in the survey. Almost 60% of the total sample aged over 45. This suggests that the middle-aged and elderly (over 45) people tend to work in the villages and were available for the interviews while younger people might commute to work outside of the village and did not show up in the interviews. Married individuals accounted for 91.72% of the sample. 76.6% of the respondents could not complete high school, reflecting the fact that most people in rural areas obtain low education level 3 . In this study, farmers or fishermen constituted 57% of the total sample. The other main categories of jobs such as business owners, and hired employees from the public or private sectors, comprised about 29% of the sample. Students, retirees, housewives, and unemployed people represented the remaining sample.

One-third of the respondents (33.33%) reported having a monthly household income lower than 3 million VND (about 129 USD). 39.58% of the respondents reported receiving a monthly household income between 3 and 6 million VND (about 129-258 USD). 13.89 % of respondents said that they received a monthly income of over 10 million VND (about 430 USD). There were about 1 to 8 people in the surveyed households. The mean household size of the sample was around 4.08, representing normal family size in Nam Dinh Province. Interestingly, most respondents (84.83%) were very passionate about environmental conservation activities. Finally, 76.6% of the respondents reported that their agricultural activities were affected to a certain degree by natural disasters in the past 5 years.

goods, but they yielded mixed empirical evidences. Here, we adopted the internal test proposed by [START_REF] Spencer-Cotton | Spatial and scope effects: Valuations of coastal management practices[END_REF] for the scope effect on the forest area attribute. This approach allows us to test whether WTP could be derived from changes in quantity of valued mangrove coverage (the LIN model) or be considered as a reflection of motivation for having the public goods (the DIFF model). The deterministic part of the utility for the three model specifications for the CL model is as follows:

For Eq. 6 (ID model), mangrove area is presented as 'individual dummies' whereas mangrove area (with three attribute levels: 1661, 1900, 2100) enters Eq. 7 (LIN model) as a continuous variable so that utility is a linear function of the attribute. For Eq. 8 (DIFF model), area attributes are included as dummy variables, but their coefficients are restricted to be equal. The specifications used for internal scope tests for the RPL model are similar to those of the CL, but incorporating η i as individual deviations from the mean coefficients on most variables, except for payment (see Eq. 4). A summary of these tests is presented in Table 10. For both CL and RPL models, the restriction in the LIN model is not possible to reject (p values > 0.1 in both cases). It indicates that there is no statistical difference between the LIN model and the ID model; hence, marginal utility is constant.


Real-time dynamics of the anti-parallel to parallel switching in STT-MRAM

Keywords: Magnetic switching, MRAM devices, perpendicular magnetic tunnel junction, spin transfer torque

In this paper a new experimental technique for measuring the switching dynamics and extracting the energy consumption of Spin Transfer Torque MRAM (STT-MRAM) device is presented. This technique is performed by a real-time current reading while a pulsed bias is applied. The switching from a high resistive state, anti-parallel (AP) alignment, to a low resistive state, parallel (P) alignment, is investigated as well as the impact of the cell diameter on the switching parameters. We demonstrate that preswitching and switching times and energies have a log-linear relationship with the applied voltage. Increasing the applied voltage leads to a higher spin torque on the free layer in a shorter time. This decreases the time needed to change the magnetization orientation of this layer, thus the time required before the switching occurs. We have also shown that for a given applied voltage, the smaller the cell the longer the time before switching. For low applied voltages, the preswitching time increases exponentially dominating the whole reversal time. The longer switching times can be explained by a lower Joule heating not sufficient to induce the thermally activated reversal process. This phenomenon is accentuated for smaller cells, where the heating is more significant and the time before switching is shorter than for larger cells.

I. INTRODUCTION

AGNETIC Random Access Memory (MRAM) has been developed in several generations so far, in particular STT-MRAM which is one of the most attractive emerging memories candidates [START_REF] Kharbouche-Harrari | MRAM: from STT to SOT, for security and memory[END_REF][START_REF] Kryder | After Hard Drives-What Comes Next[END_REF]. For high-density STT-MRAM integration, perpendicular magnetic tunnel junctions (p-MTJs) are more energy efficient due to stronger perpendicular anisotropy and reduced energy levels needed to perform switching are expected to achieve lower critical current density and greater thermal stability, thus lower bias voltage. A CoFeB/MgO/CoFeB based tunnel junction represents the core component of this p-MTJ stack, where spin transport phenomena such as tunneling magnetoresistance and magnetization reversal of the storage layer by STT occur [START_REF] Chatterjee | Physicochemical origin of improvement of magnetic and transport properties of STT-MRAM cells using tungsten on FeCoB storage layer[END_REF][START_REF] Chang | Perpendicular magnetic tunnel junctions with synthetic antiferromagnetic pinned layers based on [Co/Pd] multilayers[END_REF][START_REF] Apalkov | Magnetoresistive Random Access Memory[END_REF]. Fig. 1.

a shows a High-Resolution Transmission Electron

This work was supported by the grant n°ANR-19-CE39-0010 overseen by the French National Research Agency (ANR).

N. Yazigy, J. Postel-Pellerin, V. Della Marca, K. Terzyian and P. Canet are with Aix-Marseille University, CNRS, IM2NP, 13451 Marseille, France (e-mail: nicole.yazigy@im2np.fr).

Microscopy (HRTEM) of the whole stack used in our study. The resistance of an MTJ device depends on the relative orientation of the magnetization of the free and fixed ferromagnetic (FM) layers (Fig. 1.b and Fig. 1.c) due to spin-dependent tunneling and is quantified by tunnel magnetoresistance ratio (TMR). The free FM's magnetization direction can be changed either by applying a static magnetic field higher than the coercivity of the FM layer or by forcing a spin-polarized current through the device. The transverse component of spin current gets absorbed in the FM, implying a torque on the magnetization of the FM known as the Spin Transfer Torque (STT) effect [START_REF] Sharma | Proposal for energy efficient spin transfer torque-magnetoresistive random access memory device[END_REF][START_REF] Brataas | Current-induced torques in magnetic materials[END_REF][START_REF] Butler | Spin-dependent tunneling conductance of Fe/MgO/Fe sandwiches[END_REF]. Fig. 1.b and Fig. 1.c also show the two possible states: the high-resistive AP state and the low-resistive P state that usually represent "1" and "0" logical states respectively. The time required for this change of state of the STT-MRAM cell and its dependence on the voltage used during programming are key criteria. Recent studies have simulated this behavior but very few real-time measurements studies exist to confirm the proposed reversal mechanisms [START_REF] Sharma | Proposal for energy efficient spin transfer torque-magnetoresistive random access memory device[END_REF][START_REF] Hu | Reliable 5 ns writing of spin-transfer torque MRAM[END_REF][START_REF] Shi | Fast low-current spin-orbit-torque switching of magnetic tunnel junctions through atomic modifications of the free-layer interfaces[END_REF]. This article proposes an experimental evaluation of the switching dynamics thereby we chose one of the two STT-MRAM transitions which is the passage from AP to the P state of the STT-MRAM cell. Using a simple method where a pulsed bias voltage is applied while reading the current at the same time will allow the extraction of the real-time dynamics of the switching phenomenon. The novelty of our work lies in the distinction between two phases called "preswitching" and R. C. Sousa and G. Di Pendina are with University Grenoble Alpes, CNRS, CEA-SPINTEC, CEA, 38000 Grenoble, France. "switching" while in previous works these two phases were combined in a single global phase called "switching" [START_REF] Devolder | Size dependence of nanosecond-scale spin-torque switching in perpendicularly magnetized tunnel junctions[END_REF]. The separation is important because in reality two successive phases occur as demonstrated in [START_REF] Devolder | Size dependence of nanosecond-scale spin-torque switching in perpendicularly magnetized tunnel junctions[END_REF][START_REF] Guenole | Achieving Sub-ns switching of STT-MRAM for future embedded LLC applications through improvement of nucleation and propagation switching mechanisms[END_REF][START_REF] Bouquin | Size dependence of spin-torque switching in perpendicular magnetic tunnel junctions[END_REF][START_REF] Alzate | 2 MB Array-Level Demonstration of STT-MRAM Process and Performance Towards L4 Cache Applications[END_REF]. In the first phase, the nucleation of the domain wall, during what we call in this paper the "Preswitching time", spins are accumulated in the free layer and the nucleation takes place. Then, during the second phase, the propagation phase of the domain wall of the entire device, called "Switching" in this paper, all the spins reverse. We want to study the behavior of each of these mechanisms with applied voltage and diameter. Fig. 2.a. presents the shape of the pulse which consists of an initial reading phase at 0.1V to confirm the AP state followed by a rise time. This is followed by a pulse up to the maximum voltage value (VTOPmax). This phase has the role of measuring and detecting the dynamic current transition. A second reading phase at 0.1V, following a fall time, enables the extraction of the resistance value after switching. The time between the start of the pulse rise time and the beginning of the current induced switching is called preswitching time (𝑡 " ). On the other hand, the switching time (𝑡 # ) is defined as the time required for the cell to pass from a low-current level to a high-current level, as described in Fig. 2.b.

II. EXPERIMENTAL SETUP

Our samples are integrated in a 100mm wafers we fabricated in an academic cleanroom within the CEA facilities. They are characterized using a Keysight B1500A semiconductor device analyzer to apply signals for Quasi-Static Measurements (QSM) through Source Monitor Units (SMU) while Pulsed Measurements (PM) are carried out by Waveform Generator Fast Measurement Units (WGFMU in a B1530 module), as shown in Fig. 3. and explained in a previous work [START_REF] Della Marca | Quantitative correlation between Flash and equivalent transistor for endurance electrical parameters extraction[END_REF]. The Remote-sense and Switch Unit (RSU) modules enable the link with the probes and the device under test [START_REF] Aoyama | B1500A Semiconductor Device Analyzer[END_REF]. The Bottom Electrode (BE) current (IBOT) is measured instead of the Top Electrode (TE) current (ITOP) to avoid drift current due to the capacitance of the connecting wires. We applied the bias voltage (VTOP) on the TE of the MTJ while the BE is grounded. In order to study the size effect on the switching phenomenon, we investigated three different cells with their respective diameters, TMR and Resistance-Area (RA) products shown in Table I. A. Quasistatic Measurement (QSM) Fig. 4. presents the quasi-static current-voltage characteristics where the voltage applied VTOP is swept from zero to a positive switching bias and then swept to a negative switching bias, measuring the IBOT current, for cells with different diameters.

The switching voltages are affected by the sweep ramp speed that cannot be exactly controlled in QSM. This limitation implies to develop a dynamic measurement mode based on pulsed measurements (PM). This quasi-static study provides a benchmark for further work based on pulsed measurements from which we could extract preswitching and switching time.

The reading of the AP or P state of the cell is performed using a 0.1V QSM.

B. Pulsed Measurements (PM)

In this paper we focus on the switching from AP to P state. The method consists in applying the pattern, described in Fig. 2.a., on the top electrode after ensuring that the cell is initially in AP state. The total number of measurement points is limited by the measurement setup to 10 & sampling points of which in all our measurements we applied the rise-fall time equals to 10 sampling points (which is short enough not to impact the measurement and fast enough not to have an overshoot). Note that sampling of PM method limits the precision of switching time measurement. The switching from high to a low resistive state can be observed Fig. 5. We can notice that the switch takes place gradually because the initial nucleation of a magnetic domain is followed by a phase of propagation of the wall through the rest of the free layer [START_REF] Devolder | Size dependence of nanosecond-scale spin-torque switching in perpendicularly magnetized tunnel junctions[END_REF][START_REF] Guenole | Achieving Sub-ns switching of STT-MRAM for future embedded LLC applications through improvement of nucleation and propagation switching mechanisms[END_REF][START_REF] Bouquin | Size dependence of spin-torque switching in perpendicular magnetic tunnel junctions[END_REF][START_REF] Alzate | 2 MB Array-Level Demonstration of STT-MRAM Process and Performance Towards L4 Cache Applications[END_REF]. Note that with the arbitrary pulse generators we are able to modify the pulse duration (width time), VTOPmax as well as the rise and fall times. The experimental results are presented in the next section.

III. RESULTS

In order to compare quasi-static and pulse switching results we show in Fig. 6 values of the calculated resistance values from Fig. 4. The extraction of low resistance state values in QSM is possible for every positive voltage before the cell breakdown while it is impossible for the high resistance state, because of the switching occurrence. Moreover, we carried out a set of dynamic characterizations demonstrating that the AP to P transition in pulsed mode leads to higher switching voltages because of the shorter pulse width compared to the QSM (symbols in Fig. 6.). Fig. 6. Resistance as a function of the applied voltage in quasi-static mode (Fig. 4), as well as their corresponding average pulse mode resistances over 30 measurements.

For lower applied voltages, the pulse width time is long enough and thus comparable for the two experimental methods. The dynamic characterizations have been repeated thirty times for each VTOPmax to limit the cycle-to-cycle variability. The preswitching time (𝑡 " ) and the switching time (𝑡 # ) have been extracted and their mean values are plotted in Fig. 7 and Fig. 8 respectively. We can notice that the results obtained for 100 and 150 nm are quite close. The similar switching behaviors can be explained by the fact that the Parallel resistance 𝑅 " value is neither proportional to the diameter nor to the device area as shown in Fig. 6. as well as in [START_REF] Devolder | Size dependence of nanosecond-scale spin-torque switching in perpendicularly magnetized tunnel junctions[END_REF]. The switching behavior is led by the 𝑅 " value, rather than the diameter or area.

IV. DISCUSSION

The preswitching and switching times showed a stochastic behavior. The preswitching time is reduced when increasing the applied bias due to a larger spin current passing through the magnetic tunnel junction device. For a 100 nm diameter cell the preswitching time decreases by a ratio of 10 ( when the applied voltage increases from 0.2 to 0.8V, while the switching time slightly decreases with the applied voltage increases. The preswitching and the switching times decrease with the current increase which corresponds to the result obtained in [START_REF] De Rose | Impact of voltage scaling on STT-MRAMs through a variability-aware simulation framework[END_REF] where the mean value of the switching time of the MTJ decreased with the current increase.

On the other hand, for the same applied voltage the preswitching time decreases when the diameter increases, which can be explained by lower retention in larger diameter sizes [START_REF] Kishi | Lower-current and fast switching of a perpendicular TMR for high speed and high-density spin-transfer-torque MRAM[END_REF][START_REF] Devolder | Material Developments and Domain Wall-Based Nanosecond-Scale Switching Process in Perpendicularly Magnetized STT-MRAM Cells[END_REF][START_REF] Santos | Ultrathin perpendicular free layers for lowering the switching current in STT-MRAM[END_REF], likely because the probability of defects and reversal nucleation sites increases for larger diameters. The observed exponential dependence, for tP and tS with the pulse voltage, is expressed in (1) and [START_REF] Kryder | After Hard Drives-What Comes Next[END_REF].

𝑡 " = 𝑡 +" . 𝑒 ./ 012 / 32 ⁄ (1)

𝑡 # = 𝑡 +# . 𝑒 ./ 012 / 35 ⁄ (2)
Where "P" stands for preswitching and "S" stands for switching. The model parameters t0P, t0S, V0P and V0S are shown in Table II. Finally, in this section, we use the dynamic results to represent the electrical energy of the MTJ based STT-MRAM. The preswitching energy is plotted in Fig. 9. The energy is the integral of the electric power over time. The model of preswitching and switching energies (EP) and (Es) are expressed in ( 3) and ( 4).

𝐸 " = 𝐸 +" . 𝑒 ./ 012 / 32 ⁄ (3)

𝐸 # = 𝐸 +# . 𝑒 ./ 012 / 35 ⁄ (4)
The model parameters E0P, E0S, V0P and V0P are shown in Table III. The preswitching energy that corresponds to the energy dissipated before AP to P transition occurs is significantly reduced when the cell diameter is increased [START_REF] Lu | Fully functional perpendicular STT-MRAM macro embedded in 40 nm logic for energy-efficient IOT applications[END_REF]. This result is consistent with the increase of the resistance when the cell's size decreases for biases higher than 0.3V. The current flow in the MTJ is thus decreased and a longer preswitching time is required to switch to the parallel state. For an applied voltage lower than 0.3V, the dissipated preswitching energy rises for larger diameters, which is caused by thermal dissipation in the cell as a result of the long pulse width times [START_REF] Munira | A quasi-analytical model for energy-delay-reliability tradeoff studies during write operations in a perpendicular STT-RAM cell[END_REF]. This observation is consistent with the assumption of a thermally activated reversal mechanism. In the Fig. 10. the switching energy also shows a downward log-linear behavior. We remind that this paper aims at presenting a new methodology for the extraction of preswitching and switching times in the AP to P state transition but it is fully applicable to the P to AP state transition since times involved in both transitions are comparable and reachable with our equipment despite the switching voltage asymmetry. In another study we propose the investigation of both transitions, also considering the impact of temperature [START_REF] Yazigy | Experimental analysis on stochastic behavior of preswitching time in STT-MRAM[END_REF]. V. CONCLUSION

In this paper we propose an experimental technique to measure the real-time reversal of AP to P state switching time in STT-MRAM cells. The time required to program the MTJ is greater when the diameter of the latter enlarges. The reversal can be decomposed in a preswitching time leading to the cell commutation and the switching time itself. We have also investigated the impact of the magnitude of the applied voltage on the tP, as well as the tS that is almost constant of the order of a few hundred of nanoseconds for the three different cell sizes.

The switching energy has a log-linear relationship with the applied voltage. The cell switching event can be divided into two regimes, the thermally activated regime and the processional switching regime. For long write pulses and low applied voltage, the switching is dominated by thermal activation. As a result, the switching time was less than or equal to the sampling time. For short pulses, at high applied voltages, the switching is governed by the conservation of the spin angular momentum of the electrons. In this regime, the switching energy is inversely proportional to the pulse width and increases sharply with decreasing pulse widths. Finally, this innovative experimental study enabled to correlate fabricated device behavior with state-of-the-art theoretical results.

Fig. 1 .

 1 Fig. 1. (a) HRTEM images of p-MTJ of STT-MRAM stack [3], (b) P state and (c) AP state configurations.

Fig. 2 .

 2 Fig. 2. (a) Applied bias voltage pattern showing the rise, width and fall times and the reading phase at 0.1 V, (b) read current and the extraction of the preswitching (𝑡 " ) and the switching (𝑡 # ) times.

Fig. 3 .

 3 Fig.3. Measurement setup a prober and an Analyzer B1500 of Keysight Technologies brand.

Fig. 4 .

 4 Fig. 4. Quasi-static measured current (IBOT) as a function of the applied voltage (VTOP) for cells with diameters 80 nm, 100 nm and 150 nm.

Fig. 5 .

 5 Fig. 5. (a) Experimental result showing applied voltage and measured current for VTOPmax = 0.6V, width time = 500µs, sampling time= 10 ns and the rise-fall time= 100 ns, as well as definition of preswitching time. (b) reading voltage and (c) switching time.

Fig. 7 .

 7 Fig. 7. The average over 30 measurements of the preswitching time for each of the 3 cells of diameter 80, 100 and 150 nm.

  This article has been accepted for publication in IEEE Journal of the Electron Devices Society. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/JEDS.2022.3185324This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/

Fig. 8 .

 8 Fig. 8. The average over 30 measurements of the switching time for each of the 3 cells of diameter 80, 100 and 150 nm.

Fig. 9 .

 9 Fig. 9. Average over 30 measurements of the preswitching energy for each of the three cell diameters 80, 100 and 150 nm.

Fig. 10 .

 10 Fig. 10. Average over 30 measurements of the switching energy for each of the three cell diameters 80, 100 and 150 nm.

  This article has been accepted for publication in IEEE Journal of the Electron Devices Society. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/JEDS.2022.3185324

	Real-time switching dynamics
	in STT-MRAM
	N. Yazigy, J. Postel-Pellerin, V. Della Marca, K. Terziyan,
	R. C. Sousa, P. Canet, G. Di Pendina
	M

This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/

TABLE I

 I Main characteristics of the three considered samples

	Diameter (nm)	TMR %	RA (Ω. m 2 )
	80	35.4	43.73
	100	32.5	34.55
	150	29.7	53.00

  TABLE II Values of the model parameters in equations (1) and (2)

This article has been accepted for publication in IEEE Journal of the Electron Devices Society. This is the author's version which has not been fully edited and content may change prior to final publication. Citation information: DOI 10.1109/JEDS.2022.3185324This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/