MD Jacques Ameille
email: jacques.ameille@rpc.aphp.fr

MD Nicole Rosenberg

MD Mireille Matrat

PhD Alexis Descatha

MD Dominique Mompoint

MD Lounis Hamzi

MD Catherine Atassi

MD Manuela Vasile

MD Robert Garnier

PhD Jean-Claude Pairon

Asbestos-related diseases in automobile mechanics

Keywords: Asbestos, pleural plaques, automobile mechanics, HRCT

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The presence in the past of chrysotile asbestos in automobile brakes or clutches has given rise to concerns about increased risks of asbestos-related diseases among automobile mechanics [START_REF] Lemen | Asbestos in brakes: exposure and risk of disease[END_REF][START_REF] Paustenbach | Environmental and occupational health hazards associated with the presence of asbestos in brake linings and pads (1900 to present): a "state-of-the-art" review[END_REF], Welsh 2007).

A large number of automobile mechanics have been potentially exposed to asbestos.

According to the data of the 1999 census, the estimated population of automobile mechanics, aged 16 to 60 years, in France was 242,360 [START_REF] Imbernon | Quantitative assessment of the risk of lung cancer and mesothelioma among automobile mechanics[END_REF]).

Partly due to the different methodologies employed, available data concerning atmospheric measurements of asbestos fibers during brake or clutch repair are discordant [START_REF] Rodelsperger | Asbestos dust exposure during brake repair[END_REF][START_REF] Paustenbach | An evaluation of the historical exposures of mechanics to asbestos in brake dust[END_REF][START_REF] Finley | Cumulative asbestos exposure for U.S. automobile mechanics involved in brake repair (circa 1950s-2000)[END_REF][START_REF] Rohl | Asbestos exposure during brake lining maintenance and repair[END_REF], Plato et al.1995[START_REF] Blake | Evaluation of asbestos exposure within the automotive repair industry: a study involving removal of asbestos-containing body sealant and drive clutch replacement[END_REF][START_REF] Cohen | Assbestos exposures of mechanics performing clutch service on motor vehicles[END_REF][START_REF] Jiang | A study of airborne chrysotile concentrations associated with handling, unpacking, and repacking boxes of automobile clutch discs[END_REF], contributing to uncertainties about the level of risk of asbestosrelated diseases in automobile mechanics.

Cases of malignant pleural mesothelioma have been reported in automobile mechanics [START_REF] Paur | Pleural mesothelioma following asbestos exposure during brake repair inthe automobile trade: case report[END_REF][START_REF] Butnor | Exposure to brake dust and malignant mesothelioma: a study of 10 cases with mineral fiber analysis[END_REF][START_REF] Rodelsperger | Asbestos dust exposure during brake repair[END_REF]). However, published meta-analysis of the epidemiological literature that examined the risk of pleural mesothelioma [START_REF] Wong | Malignant mesothelioma and asbestos exposure among auto mechanics: appraisal of scientific evidence[END_REF][START_REF] Goodman | Mesothelioma and lung cancer among motor vehicle mechanics: a meta-analysis[END_REF]) or lung cancer [START_REF] Goodman | Mesothelioma and lung cancer among motor vehicle mechanics: a meta-analysis[END_REF] did not show any elevated meta-relative risks.

Very few data concerning non-malignant asbestos-related diseases or impairment of lung function in automobile mechanics have been published (Plato et al. 1995[START_REF] Marcus | Asbestos-associated lung effects in car mechanics[END_REF], Boilat and Lob 1973[START_REF] Dahlqvist | Lung function and exposure to asbestos among vehicle mechanics[END_REF]. Only one study, using full-size chest radiographs, investigated the prevalence of pleural plaques [START_REF] Marcus | Asbestos-associated lung effects in car mechanics[END_REF]). To our knowledge, no study using chest CT-scan for the detection of asbestos-related diseases in automobile mechanics, has been published.

The aim of the present retrospective multicenter study was to analyze the frequency of pleural and parenchymal abnormalities on chest High Resolution Computed Tomography (HRCT), in a population of automobile mechanics, referred by occupational physicians to occupational health departments in the Paris area, for systematic screening of asbestos-related diseases.

Study population and methods

Study population

The eligible population consisted of all automobile mechanics, referred to occupational health departments in university hospitals of Garches, Créteil, andParis-Fernand Widal, between 1st January 2002 and30th June 2009, for HRCT detection of asbestos-related diseases. These subjects were systematically referred by occupational physicians because of known exposure to asbestos. As social welfare benefits, such as early retirement allowance or pension, are provided in France for workers with asbestos-related diseases, including pleural plaques, when confirmed by chest CT-scan examinations, a consensus conference, held in Paris in January 1999, recommended the use of chest CT-scan, after a sufficient latency period, for the surveillance of workers occupationally exposed to asbestos (Conférence de consensus, 1999). Subjects with sources of occupational exposure to asbestos other than their work as automobile mechanics were excluded from the analysis.

Methods

For each subject, information was collected on gender, age, previous chest diseases, and smoking status. Subjects were classified into three categories according to tobacco consumption: smokers, ex-smokers and non-smokers. Ex-smokers were defined as those who had quit smoking for at least one year.

For each subject, the year of starting work as an automobile mechanic was recorded, together with the type of work (light vehicles, trucks, mixed activity), duration of exposure, and the time interval between start of asbestos exposure and the HRCT (latency). For calculation of the duration of exposure to asbestos, only work prior to 1997 (year in which asbestos was banned in France), was taken into account.

Information on the use of compressed air during brake cleaning was also collected. HRCT Spiral HRCT acquisition parameters were defined in accordance with the French Thoracic Imaging Society guidelines: full inspiration, slice thickness of 1.5 to 5 mm, pitch of 1.5 to 2.0, 120 KV, 60 to 150 mAs. All CT scans were performed without intravenous contrast agent.

Localized acquisition in the prone position was required in the case of suspected gravitational lung abnormalities.

All images were reviewed separately by two independent readers in each occupational health department (a radiologist and an occupational diseases specialist), with further consensus reading in the case of disagreement between the two readers concerning the presence of pleural or parenchymal abnormalities consistent with an asbestos-related disease.

A structured form, elaborated from a previous study [START_REF] Ameille | Computed tomography findings in urban transportation workers with low cumulative asbestos exposure[END_REF], was filled out, describing the features of pulmonary abnormalities consistent with asbestosis (thickened interstitial short lines: septal lines, non-septal lines, honeycombing, curvilinear subpleural lines, ground glass opacities), and pleural abnormalities.

When associated with rounded atelectasis or parenchymal bands, pleural thickenings were considered to correspond to diffuse pleural thickening (fibrosis of visceral pleura) [START_REF] Gevenois | Asbestosis, pleural plaques and diffuse pleural thickening: three distinct benign responses to asbestos exposure[END_REF][START_REF] Beigelman-Aubry | Computed tomographic atlas of benign asbestos related pathology[END_REF] Pleural plaques were considered to be present in the case of circumscribed quadrangular pleural elevations, with sharp borders and tissue density, sometimes calcified, or in the case of multiple, bilateral, less typical images, with a typical topography [START_REF] Beigelman-Aubry | Computed tomographic atlas of benign asbestos related pathology[END_REF]).

Pleural abnormalities not meeting the criteria for pleural plaques or diffuse pleural thickening were considered to be nonspecific pleural thickening.

Statistical analysis

Values for continuous variables were expressed as the mean (standard deviation, SD). Bivariate analysis was performed to study factors associated with pleural plaques, using Fisher's exact test for qualitative variables, or Wilcoxon's test for continuous variables. A p-value less than 0.05 was considered statistically significant.

Multiple logistic regression models were constructed, including age, previous respiratory diseases, smoking status, latency, and duration of exposure to asbestos.

Due to the strong collinearity between latency and duration of exposure, separate models were constructed. A p-value less than 0.05 was considered statistically significant.

Statistical analysis was performed using SAS software version 9.1 (SAS Institute Inc, Cary, NC, USA).

Informed consent was obtained from all subjects. The study was approved by the French national committee for data protection (Commission Nationale de l'Informatique et des Libertés: CNIL).

Independent ethics committee approval was not necessary, as chest CT scans are recommended in France for screening purposes in asbestos-exposed workers (Conférence de consensus, 1999).

Results

During the study period, 242 automobile mechanics were referred to the occupational health departments participating in the study. Eighty-nine automobile mechanics with other possible sources of asbestos exposure, and 50 automobile mechanics with no available HRCT were excluded. The study population therefore consisted of 103 subjects.

General characteristics of the study population are given in Table 1. All subjects were male with a mean age of 54.4 (7.2) years; 69 (68%) were smokers or ex-smokers.

Mean latency was 36.8 (7.0) years (range: 19-57 years). Mean duration of asbestos exposure was 22.6 (8.1) years (range: 4-45 years). Most of the study population (83.5%) used compressed air for brake cleaning.

Interstitial abnormalities consistent with asbestosis were observed in one subject. This subject had worked for 24 years as an automobile mechanic, ensuring brake repairs on trucks for more than 8 hours a week. Interstitial abnormalities consisted of bilateral honeycombing, ground-glass opacities and non-septal lines in the posteroinferior zones of the lungs. Five automobile mechanics had pleural plaques, 4 had diffuse pleural thickening, and 9 had nonspecific pleural thickening. No case of lung cancer or pleural mesothelioma was detected. Pleural plaques were diagnosed in each occupational health department participating in the study (one in two departments, two in one department). The 5 subjects with pleural plaques had no previous chest diseases likely to explain pleural thickening.

Characteristics of the subjects according to HRCT abnormalities are presented in Table 2. Subjects with nonspecific pleural thickening had similar asbestos exposure to that of subjects without HRCT abnormalities, in terms of latency or duration of exposure. Subjects with pleural plaques and subjects with diffuse pleural thickening had longer latency and duration of asbestos exposure than subjects without HRCT abnormalities (Table 2).

In comparison with subjects without pleural plaques, subjects with pleural plaques had significantly longer latency and duration of asbestos exposure (Table 3).

After adjustment for age, smoking status, and history of previous asbestos-unrelated respiratory diseases, multiple logistic regression models showed a statistically significant association between the duration of asbestos exposure and the presence of pleural plaques (Table 4).

Discussion

In this population of automobile mechanics referred for systematic detection of asbestos-related diseases by HRCT, only one case of interstitial abnormalities consistent with asbestosis, 5 cases of pleural plaques (4.9%) and 4 cases (3.9%) of diffuse pleural thickening were observed. The 5 subjects with pleural plaques had no known history of chest disease likely to explain pleural abnormalities.

The prevalence of pleural plaques in this study is similar to that observed in the only previous published study that investigated the frequency of pleural plaques in car mechanics [START_REF] Marcus | Asbestos-associated lung effects in car mechanics[END_REF]. In that study, based on chest radiographs, pleural plaques, defined by pleural thickening at least 5 mm thick on the chest wall or 3 mm thick on the diaphragm, were observed in 41 out of 925 car mechanics (4.4%).

However, our results must be compared cautiously with those reported by Marcus et al. due to selection factors and the use of different tools for the radiological detection of asbestos-related abnormalities.

The prevalence of pleural plaques observed in this study was similar to that observed on HRCT examinations in subjects exposed to indoor asbestos pollution [START_REF] De Raeve | Observer variation in computed tomography of pleural lesions in subjects exposed to indoor asbestos[END_REF] or in urban transportation workers with low cumulative exposure to asbestos [START_REF] Ameille | Computed tomography findings in urban transportation workers with low cumulative asbestos exposure[END_REF].

The main limitations of this study are the relatively small number of subjects, the absence of an unexposed control group, and the lack of objective assessment of the level of occupational exposure to asbestos. The representativity of the study sample is also questionable. However, inclusion criteria had nothing to do with symptoms or abnormalities on chest radiographs.

One of the strengths of this study is the use of HRCT for detection of asbestosrelated diseases. HRCT has been shown to be much more sensitive and specific than chest radiographs for the detection of early pleural and parenchymal changes [START_REF] Gevenois | Conventional and high-resolution CT in asymptomatic asbestos-exposed workers[END_REF], Aberle et al. 1988 (a), Aberle et al. 1988 (b), [START_REF] Neri | Findings from highresolution computed tomography of the lung and pleura of symptom free workers exposed to amosite who had normal chest radiographs and pulmonary function tests[END_REF][START_REF] Staples | High resolution computed tomography and lung function in asbestos-exposed workers with normal chest radiographs[END_REF][START_REF] Friedman | Asbestos-related pleural disease and asbestosis: a comparison of CT and chest radiography[END_REF]).

Another strength is the standardized reading, with systematic double interpretation followed by consensus reading in the case of disagreement.

Furthermore, only automobile mechanics with no other known occupational exposure to asbestos and with a sufficiently long latency (> 20 years in 99% of cases, > 30 years in 89% of cases) to allow evaluation of asbestos-related diseases, especially pleural plaques, were included in the study [START_REF] Schwartz | New developments in asbestos-induced pleural disease[END_REF][START_REF] Paris | Pleural plaques and asbestosis: dose and time response relationships based on HRCT data[END_REF]).

In the absence of a control group, demonstration of a significant relationship between pleural plaques and duration of exposure (used as a surrogate of cumulative exposure to asbestos) is an argument in favor of the causal role of asbestos [START_REF] Paris | Pleural plaques and asbestosis: dose and time response relationships based on HRCT data[END_REF].

The low prevalence of pleural plaques and interstitial abnormalities in this study is an indirect argument in favor of low cumulative exposure to asbestos for most automobile mechanics. These data are consistent with the meta-analysis of epidemiologic studies that investigated the incidence of lung cancer and pleural mesothelioma in automobile mechanics and failed to demonstrate increased metarelative risks [START_REF] Wong | Malignant mesothelioma and asbestos exposure among auto mechanics: appraisal of scientific evidence[END_REF][START_REF] Goodman | Mesothelioma and lung cancer among motor vehicle mechanics: a meta-analysis[END_REF]. Our data are also consistent with the results of a recently published population-based case-control study [START_REF] Rolland | Occupations and industries in France at high risk of pleural mesothelioma: a population based-study (1998-2002)[END_REF]) that reported a slight but not significant increased risk of pleural mesothelioma in male car mechanics (OR=1.50; 95% CI: 0.76 -2.95).

Many measurements in car repair workshops have shown that occupational exposure of car mechanics to asbestos is generally fairly low. [START_REF] Rohl | Asbestos exposure during brake lining maintenance and repair[END_REF] reported airborne chrysotile concentrations of up to 30 f/ml. However, these were peak exposure concentrations measured during brake cleaning with the use of compressed air. According to [START_REF] Paustenbach | An evaluation of the historical exposures of mechanics to asbestos in brake dust[END_REF], analysis of the available published data indicates that 90% of 8 hours time weighted average (TWA) concentrations, experienced by mechanics servicing brakes in the 1970s, was less than 0.1 f/ml (mean 0.04 f/ml). In Sweden, Plato et al (1995), using a model based on data from the international literature and quantitative asbestos measurements performed in 1976-1978 in Swedish car repair workshops, estimated the mean cumulative exposure to be 2.6 f/ml x years, in a group of 103 car and bus mechanics, with more than 15 years of asbestos exposure. Car mechanics had a TWA exposure range of 0.11-0.41 f/ml (mean 0.21 f/ml) in 1965, and 0.003-0.08 f/ml in 1985 (mean 0.021 f/ml). In Germany, based on dust measurements in 76 service stations, and the occupational histories of 210 vehicle mechanics with an average duration of employment of 21±10 years, a mean cumulative exposure to asbestos was estimated to be 0.54 ±1.1 f/ml x years for car mechanics [START_REF] Rodelsperger | Asbestos dust exposure during brake repair[END_REF]). [START_REF] Finley | Cumulative asbestos exposure for U.S. automobile mechanics involved in brake repair (circa 1950s-2000)[END_REF] recently developed estimates of cumulative chrysotile exposures experienced by automobile mechanics working with brakes and manual clutches. They reported that the 95 th percentile and 99 th percentile cumulative exposures for vehicles mechanics in the 1970s, were 2.0 and 5.7 f/ml years, respectively.

Another explanation for the low prevalence of asbestos-related diseases in automobile mechanics might be that decomposition of asbestos occurs during normal use of brakes due to thermal decomposition into forsterite, a non-fibrous material [START_REF] Lemen | Asbestos in brakes: exposure and risk of disease[END_REF]).

Conclusion

The asbestos exposure experienced by French car mechanics may lead to pleural plaques. The low prevalence of non-malignant asbestos-related diseases, despite the use of HRCT which is a very sensitive diagnostic tool, is in favor of low cumulative exposure to asbestos in the study population.

Acknowledgments

This work was supported by a grant from the Caisse nationale d'assurance maladie des travailleurs salariés (Département des risques professionnels), and the Caisse régionale d'assurance maladie d'Ile de France.

The authors declare that they have no conflict of interest


Bassem Mouawad

Maher Soueidan

Damien Fabrègue

Member, IEEE Cyril Buttay

Senior Member, IEEE Bruno Allard

Hervé Morel Senior Member, IEEE Vincent Bley

Member, IEEE Christian Martin

Mr Mouawad

Buttay, Allard Morel Soueidan

Mr Fabrègue

Mr Bley

Mr Martin

Application of the Spark Plasma Sintering Technique to Low-Temperature Copper Bonding

Keywords: semiconductor device packaging, power electronics, Spark Plasma Sintering, Three-dimensional packaging. I

Planar structures, in which a power die is soldered on a substrate and wirebonds are used to connect the top of the die with the substrate, are limited in terms of thermal management and power density. Three-dimensional packaging techniques have been proposed to overcome these limits. Here, an innovative copper-to-copper bonding solution is presented, that can be used for 3-D packaging. The bonding process is described and the effect of the bonding parameters is investigated. It is found that this technique is compatible with the requirements of power electronic packaging. A test assembly including a silicon power die and ceramic substrates is presented.

system that could remove the heat through both sides of the die would be much more efficient.

Replacing the planar structure and its wirebond interconnects is not an easy task, as it is a very mature solution. Many research groups are working on alternative architectures, some focusing on replacing the wirebonds by a flex substrate [START_REF] Dieckerhoff | Electric Characteristics of Planar Interconnect Technologies for Power MOSFETs[END_REF], or with a screen-printed paste [START_REF] Ning | A Novel High-Temperature Planar Package for SiC Multi-Chip Phase-Leg Power Module[END_REF]. Other researchers have worked on so-called "sandwich structures", which use two similar substrates with the dies in-between. These solutions are designed to offer double-side cooling of the devices [START_REF] Gillot | Double-Sided Cooling for High Power IGBT Modules Using Flip Chip Technology[END_REF], [START_REF] Buttay | Compact Inverter Designed for High-Temperature Operation[END_REF]. Several papers have been dedicated to the comparison of the various 3-D structures [START_REF] Markus | Study on Advanced cooling with focus on double-sided cooling[END_REF]- [START_REF] Schulz-Harder | Review on Highly Integrated Solutions for Power Electronic Devices[END_REF].

II. PRESENTATION OF THE 3-D STRUCTURE

A. Description

As mentioned in the previous section, a three-dimensional (as opposed to planar) structure is needed to take full advantage of the power devices. A possible solution for such a package is depicted in figure 1(b) [START_REF] Ménager | A Lab-Scale Alternative Interconnection Solution of Semiconductor Dice Compatible with Power Modules 3D Integration[END_REF]. The originality of the presented technique is that copper posts (or pillars) are grown electrochemically on the die. These posts are then bonded to a Direct Bonded Copper (DBC) substrates on both sides. DBC is basically a ceramic tile with two layers of copper, one of them being patterned to form an electrical circuit.

Such a structure offers double-side cooling capability. Furthermore, using copper posts instead of a single, massive interconnect should give more freedom for the system to accommodate the thermal expansion of copper (which is 17 ppm/K, as compared to less than 4 for Si or SiC). One of the advantages of this technique is that we can set the height of the posts to insure sufficient spacing between the DBC tiles and the die. This is required to sustain the required voltage (1200 V or more).

Another advantage of the post structure is that it can accept dies of different thicknesses in the same package, by varying the height of the posts.

However, this "micro-posts" structure has a drawback: the effective interconnect area is smaller than in the case of a single, massive post. With the current design, the copper posts only represent 25 % of the die contact area (the remaining 75 % is the space around the posts). From an electrical point of view, this area is sufficient as it is already more than what is attainable using wirebonds. From a thermal point of view, however, a special care must be given to achieve a low thermal resistance. In particular, only materials with a very high thermal conductivity should be used for the interconnections, Fig. 1. A planar module (a) and the proposed "3D" structure based on micro-posts, which offers double-side cooling including the bonding between the posts and the DBCs. There is an interest to remove the inner bonding material. For this reason, it was decided to develop a special direct copper-tocopper bonding technique.

B. Fabrication

The first step to achieve a structure such as described in figure 1(b) is to grow copper posts on the semiconductor die. This is described in details in [START_REF] Ménager | A Lab-Scale Alternative Interconnection Solution of Semiconductor Dice Compatible with Power Modules 3D Integration[END_REF]. Thin Ti and Cu layers (20/100 nm respectively) are deposited on the aluminium pad of a die (to make it compatible with the following precess steps). Then, a thick photoresist layer (in our case a 75 µm-thick Riston PM275 dry film) is applied on the die and patterned. Finally, the copper posts are electrochemically grown through the openings of the photoresist film, polished to obtain a flat surface, and the photoresist is removed.

In the technique described in [START_REF] Ménager | A Lab-Scale Alternative Interconnection Solution of Semiconductor Dice Compatible with Power Modules 3D Integration[END_REF], a layer of tin (0.5 µm) is deposited on top of the posts. This layer is used as a solder between the posts and the copper of the DBC substrate. The die-to-DBC assembly is performed in vacuum, under a pressure of 10 MPa and at a temperature of 200 °C. This results in the diffusion of the tin layer in the copper parts.

Another assembly approach, described in this paper, is to get rid of the tin layer, and to achieve a direct copper-to-copper bonding (that is, to bond the micro-posts to the DBC substrate without any additional material). This technique is promising, as it reduces the number of processing steps. Furthermore, a good thermal performance can be expected, as only copper is used.

C. Review of the low-temperature direct bonding techniques for Copper

Packaging techniques used to bond conductive parts usually rely on soldering. Many soldering alloys exist, with various properties (such as their melting temperature), but they all offer medium to low thermal conductivity (less than 50 W/K.m) [START_REF] Sheng | Power Electronic Modules: Design and Manufacture[END_REF]. The melting temperature of the solder alloys, which is usually between 200 °C and 350 °C also sets a limit to the operating temperature of the whole power module [START_REF] Guth | New assembly and interconnects beyond sintering methods[END_REF]. Furthermore, a power module comprises many layers (figure 1). Such a stack of materials with different CTEs is prone to failure when submitted to temperature cycling.

Therefore, we have tried to get rid of the not-so-conductive layers, as well as to reduce the materials count by bonding directly the copper posts onto the copper metallizations of the substrate. Such a solution is also expected to operate satisfactorily at elevated temperatures, as the melting point of copper (1084 °C) is much higher than that of the classical solder alloys. Copper has attracted much attention in the area of very large scale integration (VLSI) interconnection technology as the most prospective candidate for the conducting material due to its high electrical conductivity and high electromigration resistance compared with aluminium [START_REF] Nitta | Evaluating the Large Electromigration Resistance of Copper Interconnects Employing a Newly Developed Accelerated Life-Test Method[END_REF], [START_REF] Pai | Selective electroless copper for VLSI interconnection[END_REF]. It is also a material compatible with industrial processes.

Several experiments on Cu/Cu direct bonding have been reported [START_REF] Fan | Copper Wafer Bonding[END_REF]- [START_REF] Li | Low Temperature Copper to Copper Direct Bonding[END_REF]. Although the presented methods are simple, they require either high external pressure and annealing or toxic chemical cleaning processes to achieve a reasonable bonding strength. For example, Cu wafers exhibit good bonding properties when Cu/Cu contacts are carried out at 400 °C and 4 bar for 30 min, followed by an annealing cure at 400 °C for 30 min in N 2 atmosphere [START_REF] Chen | Microstructure examination of copper wafer bonding[END_REF].

Another solution, the so-called Surface Activated Bonding "SAB" process was used to bond Cu-coated wafers at room temperature without any wet chemical process. SAB is a process that joins similar or dissimilar materials by means of the adhesion force between atoms of two atomically clean surfaces under an ultrahigh vacuum (UHV) at room temperature [START_REF] Takagi | Surface activated bonding of silicon wafers at room temperature[END_REF], [START_REF] Takagi | Transmission Electron Microscope Observations of Si/Si Interface Bonded at Room Temperature by Ar Beam Surface Activation[END_REF]. The same authors also reported that the bonding strength of Cu/Cu interface is approximately 6.47 MPa [START_REF] Kim | Room temperature Cu-Cu direct bonding using surface activated bonding method[END_REF]. J. W. Elmer et al. [START_REF] Elmer | Diffusion bonding and brazing of high purity copper for linear collider accelerator structures[END_REF] have presented a diffusion bonding of high purity copper using a conventional furnace. A series of diffusion bonds was performed to determine the relationship between bond strength and bonding parameters in order to determine a range of bonding conditions. These tests were made over a wide range of temperature (from 400 °C to 1000 °C), with different bonding pressures and surface states. In these studies, the bonding time was held constant (60 min) at the peak temperature, following a 30 min warming ramp. Bonding strengths of 65 MPa and 190 MPa were found using uniaxial tensile test for samples bonded at 400 °C and 800 °C respectively.

In general, a high temperature annealing cure (>400 °C) is required to increase the bonding strength. However, semiconductor devices have been designed to meet the requirement of the existing soldering technology (a few seconds above 300 °C). Therefore, an increase in the process temperature could require heavy modification in the dies manufacturing parameters to avoid possible degradation. This is especially true for longer bonding process such as those involving chemical species diffusion, as they last from minutes to hours. In addition, a low temperature process is advantageous in terms of manufacturing costs. Finally, keeping the process temperature to a low level reduces the residual mechanical stresses of the assembly. Therefore, lowering the temperature for Cu/Cu direct bonding is a key issue in various future device applications. An alternative solution, which uses an electroless deposit of copper to fill the gap between two copper posts, and thus create a low-temperature bond, is presented in [START_REF] He | Low-Temperature Bonding of Copper Pillars for All-Copper Chipto-Substrate Interconnections[END_REF]. This technique offers a very good bonding strength (>148 MPa).

The solution presented in this paper is based on a different approach, as no chemical solution is involved during the assembly. To the best of our knowledge, Cu/Cu direct bonding using Spark Plasma Sintering (SPS) has never been reported. In this paper, we focus on the bonding issue by evaluating the bonding strength as a function of temperature, pressure and bonding time.

III. BONDING OF COPPER

A. Sample Preparation

Commercial copper rods (Goodfellow, Cambridge, UK, 99.99+% purity) with a diameter of 19 mm and a length of 400mm were used as the base material in the bonding experiment. These rods were cut into 25 mm-long cylindrical pieces with one flat side manually polished. The polishing procedure starts with 220-grit silicon carbide paper, down to 3 µm diamond paste and 0.05 µm silica.

The samples were then cleaned to remove the lubricant. The cleaning process found to be successful consisted of (i) ultrasonic cleaning in acetone to remove the lubricant, (ii) ultrasonic cleaning in trichloroethylene, (iii) ultrasonic cleaning in microelectronic grade ethanol, (iv) immersion in 5% HF for 1 min at room temperature for oxide elimination (v) followed by rinsing in DI water and blow drying with nitrogen gas.

B. Bonding Process

The Cu/Cu bonding was carried out with a Spark Plasma Sintering (SPS) system (FCT HPD 25). As with a classical sintering press, a SPS system applies both pressure and heat to the samples. The difference is that the heat is provided by a pulsed DC-current circulating through the graphite die (or through the sample if it is conductive), whereas the conventional sintering press uses external heating elements. As a result in the case of the SPS, the heat is generated much closer to the sample, offering very fast temperature dynamics: more than 100 K/min for the system used in this study, using current pulses of several kA.

Although not initially designed for the assembly of electronic systems, the SPS technology offers interesting features: its cycle time is comparable to that of a reflow oven (a few minutes) and it operates under vacuum (avoiding oxidation of the copper). SPS systems with platens as large as 30 cm (12 in) in diameter are available, enabling the assembly of several modules at once.

In each experiment, two copper pieces were introduced in a graphite die of 20 mm inner diameter (figure 3). The bonding was performed in vacuum (10 -2 torr) with temperatures ranging from 200 °C to 300 °C. The uniaxial force , applied during the bonding and maintained during the cooling down to room temperature, was varied from 5 kN to 24 kN (corresponding to 16 and 77 MPa on the sample respectively). The temperature was measured on the surface of the graphite die using a thermocouple.

The heating rate was fixed to 100 K/min and the holding time was varied from 5 to 20 minutes (Figure 2).

C. Characterization

After bonding the samples were machined for the mechanical test (tensile test), as described in figure 4. Mechanical testing of the bonded samples was performed at room temperature using a Lloyd EZ20 tensile machine with a 20 kN cell force. The displacement rate was set up to 1 mm/min.

Copper being ductile, the sample quickly enters plastic deformation during the tensile test (figure 5). This results in a reduction of the actual section of the sample as it elongates. In this document, a nominal stress value is calculated using the initial surface of the tensile sample. It will thus constitute a lower bound of the true stress experimented on the sample, as plastic deformation and then necking occur before the actual rupture of the sample (hence reducing the surface of the bond). It should be mentioned that all the samples fractured along the bond line (due to the design of the sample).

Surface microstructure observations were studied by a Leitz optical microscope and a scanning electron microscope (SEM Philips XL20).

IV. PARAMETRIC ANALYSIS

A. Design of experiment approach

The cost of the experiments and the availability of the SPS equipment resulted in a limited set of assemblies. A classical 2 3 factorial plan [START_REF] Dagnelie | Principes d'expérimentation -Planification des expériences et analyse de leurs résultats[END_REF], [START_REF] Dascalescu | Using Design of Experiments and Virtual Instrumentation to Evaluate the Tribocharging of Pulverulent Materials in Compressed-Air Devices[END_REF] was therefore used to study the effect of three factors (time, temperature, pressure) without making any assumption on which factors or factor combinations are significant.

This factorial plan requires 2 values to be chosen for each factor (one low and one high), resulting in 8 ( 23 ) distinct experimental conditions. The position of these 8 points in the (t, T , P ) space is presented in figure 6. The low and high values for t have been chosen to be different enough, yet compatible with a microelectronic assembly process. Values lower than 5 minutes would not have been sufficiently accurate. We considered that 300 °C is the maximum a semiconductor die can safely withstand. As for the pressure, 16 MPa corresponds to the pressure applied on a 20 mm-diameter sample at the minimum force setting of the press (5 kN). Note that bonding of semiconductor devices at a comparable pressure (or even higher, 40 MPa) has already been demonstrated successfully [START_REF] Göbl | Low temperature sinter technology Die attachment for automotive power electronic applications[END_REF].

Finally, 5 additional experimental values have been acquired on a different point (I in figure 6). These points are used to evaluate the accuracy and dispersion of the experimental data. A list of the experiments, with the corresponding tensile strengths measured is given in table I A simple linear model that takes into account all the interactions between the factors can be expressed as:

y = a 0 + n i=0 a i x i + n i=0 n j=i+1 a i,j x i x j + n i=0 n j=i+1 n k=j+1 a i,j,k x i x j x k + . . . (1)
For a 3-factor case, this can be simplified to

y = a 0 + a 1 x 1 + a 2 x 2 + a 3 x 3 + a 1,2 x 1 x 2 + a 1,3 x 1 x 3 + a 2,3 x 2 x 3 + a 1,2,3 x 1 x 2 x 3 (2)
Where x 1 , x 2 and x 3 are the normalized factors, corresponding respectively to t, T and P , and y the tensile stress experienced by the sample at the breaking point. The normalization of a factor (or parameter) p is calculated as:

x i = p -pmax+pmin 2 pmax-pmin 2 (3)
With p min and p max the low and high values of the factor p respectively. The normalized values (1, -1, or 0) of t, T and P are given for each point in figure 6.

The model in eq. ( 2) requires 8 parameters (a 0 . . . a 1,2,3 ) to be identified. This model can be expressed as:

y = M • a ( 4
)
Where y is the column matrix that contains the tensile strengths for points A to H (see table I), a the column matrix that contains the parameters a 0 . . . a 1,2,3 to be calculated and M a matrix that contains the values of the 3 factors and their combinations, for each of the 8 experimental points.

Each row of M corresponds to the coefficients of (2): x 0 (constant factor associated with a 0 in eq. ( 2), always equal to 1), x 1 (normalized factor t), x 2 (T ), x 3 (P ),

x 1 x 2 , x 1 x 3 , x 2 x 3 , x 1 x 2 x 3 :             y A y B y C y D y E y F y G y H             =             1 -1 1 1 -1 -1 1 -1 1 1 1 1 1 1 1 1 1 1 1 -1 1 -1 -1 -1 1 -1 1 -1 -1 1 -1 1 1 -1 -1 1 1 -1 -1 1 1 1 -1 1 -1 1 -1 -1 1 1 -1 -1 -1 -1 1 1 1 -1 -1 -1 1 1 1 -1             •             a 0 a 1 a 2 a 3 a 1,2 a 1,3 a 2,3 a 1,2,3             (5)
As M T • M = nI, with n the size of the matrix M (in our case, n = 8), a can be calculated as:

a = 1 n M T y (6)
An expanded version of (6) for n = 8 is given in [START_REF] Ménager | Industrial and lab-scale power module technologies : A review[END_REF], with y A . . . y H the tensile stress measured for points A . . . H respectively. The calculated values of a 0 to a 1,2,3 are listed in table II.

B. Model accuracy

5 samples were assembled at the same experimental point I (see table I) in order to characterize the reproducibility and accuracy of the bonding process and of the tensile tests. From these measurements, a tensile strength value of 164.4±20% was calculated for point I. This fairly large tolerance is due to the low sample count, as well as to the low reproducibility of the tensile tests: a much bigger number of samples is usually required but this was unpractical in our case.

A diagram of the influence of each factor is given in figure 7. This diagram is obtained by using the data in table I (points A to H). For each sub-graph, two data set are formed (one for both values of the corresponding factor, i.e 1 or -1), and the average value of y is calculated for each set. As an example, for the first sub-plot (t), the two dataset correspond to t =5 min (points A, D, E, H) and 20 min respectively (points B, C, F, G).

Considering the ±20% accuracy, only few parameters in table II are significant: a 0 (the average tensile strength for all samples), a 1 (effect of time), a 3 (effect of the pressure) and a 1,2 (conjugated effect of time and temperature).

Therefore, the model ( 2) can be simplified as:

y = a 0 + a 1 x 1 + a 3 x 3 + a 1,2 x 1 x 2 = 166 + 65x 1 + 29x 3 -41x 1 x 2 (8)
Applying this model to the parameters of point I in table I (t = 20 min, T = 250°C, P = 16 MPa, or x 1 = 1, x 2 = 0, x 3 = -1 respectively), yields a tensile strength of 202±40 MPa. This is compatible with the tensile strength measured at point I (164.4±20%), but the precision of the results is too low to conclude about the suitability of our linear model. More data points would be needed to improve the accuracy.

V. DISCUSSION

Even with the limited variation range for all three factors (5-20 minutes, 200-300 °C, and 16-77 MPa), substantial differences were found in tensile strength. In most cases, however, a very good bond was formed: the tensile strength of the used bulk copper-rod was found to be 365 MPa, while most results in table I are over 122 MPa (a third of the bulk value).

Another evidence of the good bond quality is visible in figure 8: the fracture surface is characteristic of ductile fracture with the presence of dimples. This means that the sample has been heavily deformed, which is also exhibited by the large strain value attained during the tensile tests (as can be seen for example in figure 5). Similar patterns were found over the entire fracture surface and for all samples. From the model in ( 2), it can be seen that time (x 1 ) and pressure (x 3 ) have the expected effect: an increase in either of these parameters results in a stronger bond.

           a 0 = 1 8 [y A + y B + y C + y D + y E + y F + y G + y H ] a 1 = 1 8 [-y A + y B + y C -y D -y E + y F + y G -y H ] . . . a 1,2,3 = 1 8 [-y A + y B -y C + y D + y E -y F + y G -y H ] (7)
The negligible influence of temperature and the negative effect of the simultaneous increase of time and temperature can firstly be surprising, but can actually be explained by the following considerations: in these tests, a good bond could be achieved for any temperature or time value. However, the application of temperature to the samples also constitutes a heat treatment of the copper. The effect of such treatment on work hardened copper is a change in its microstructure. For example, increasing the temperature from 200 °C to 300 °C would involve further annealing and recrystallization of the sample, decreasing its overall mechanical properties. This point is beyond the scope of this paper, but the Electron Backscatter Diffraction (EBSD) images shown in figure 9 Fig. 10. Example of a micropost bonded onto a DBC substrate. This post is part of a diode-to-DBC bonding experiment in which the interface between the posts and the diode was found to be defective. The copper-to-copper bond, however, is of very good quality. The bond line only visible at the edges of the assembly. demonstrate that the crystallographic structure of the copper changes with the bonding parameters. Therefore, the decrease in mechanical strength is mainly caused by the reduction in the intrinsic mechanical properties of the copper rather than by the bond itself. This is confirmed by the ductile fracture surfaces observed on all samples.

Finally, a test structure comprising a silicon power diode (12.7×12.7 mm die size) with copper posts electroplated on one side, placed between two DBC substrates, was assembled. This test structure comprises two insulating layers (the ceramic layers of the DBCs). The voltage applied by the SPS equipment is in the order of 4 V, much lower than the dielectric strength of the ceramic layers (several kilovolts). Therefore, it was expected that the path of the heating current in the SPS press would differ dramatically from the conductive (copper) samples in figure 4(b). As a result, this could have a strong effect on the bonding.

The copper posts were successfully bonded on the copper layer of the DBC (a close-up cross-section is shown in figure 10). However, the post-to-die interface was found to be weak and failed, which is why no diode is visible in the figure. This issue is related to the processes prior to the copper plating, and is therefore beyond the scope of this paper. It is important to note that the electrical performances of the diode were not degraded in the assembly process. This test demonstrates that a complex structure including conductive layers, but also insulating layers and semiconductor devices, can be successfully bonded using the technique presented here.

VI. CONCLUSION

The copper-to-copper bonding technique that is presented is compatible with the requirements of the application: a semiconductor die can sustain temperatures up to 300°C for a few minutes and pressures in the order of a few tens of MPa.

A bonding strength higher than previously reported in the literature was found (between a third and more than two thirds of the bulk copper tensile strength). Furthermore, this good bond was obtained for temperatures as low as 200°C. A study is currently being carried out to understand the physics of this bonding process. In particular, the effect of the intense electrical current used for the SPS heating is under investigation.

Based on this bonding technique, a "sandwich" structure comprising a silicon die and two DBC substrates was assembled. A bond was observed between the copper posts of the die and the copper of the DBC, even with the ceramic layers of the substrates forming an electrical insulation around the parts to be bonded. This proves the feasibility of the technique for 3D packaging applications.

Fig. 2 .

 2 Fig. 2. Temperature profile during the bonding process. Pressure (16 to 76 MPa) and vacuum (10 -2 Torr) are applied during the entire cycle.

Fig. 3 .Fig. 4 .

 34 Fig. 3. The graphite die used to bond the copper rods, with the two 20 mmdiameter pressing cylinders.

Fig. 5 .

 5 Fig. 5. Tensile test result for one of the sample (sample B in tableI).

Fig. 6 .

 6 Fig. 6. Position of the experimental points in the t,T,P space

Fig. 8 .

 8 Fig. 8. SEM image of the fracture surface (after tensile test), for a sample bonded at 300 °C for 20 minutes and with a pressure of 16 MPa.

Fig. 7 .Fig. 9 .

 79 Fig. 7. Influence of the factors time (t), temperature (T ), pressure (P ) and their interactions on the tensile strength y. This shows that the most important factors are the time, pressure, and the interaction between time and temperature (t,T).

ACKNOWLEDGMENT

The authors would like to thank the Agence Nationale de la Recherche for their financial support of this work under the project grant ECLIPSE.

They are also pleased to acknowledge important contributions used in the body of this work from G. Bonnefont and F. Mercier from MATEIS for SPS processing, as well as to thank M. Perez from MATEIS for fruitful discussions.


M K Nguyen

Clémence Driol

T T Truong
email: truong@u-cergy.fr

NEW CONCEPT FOR HIGH SENSITIVITY SCATTERED GAMMA-RAY IMAGING

Keywords: Gamma-ray imaging, sensitivity, photon Compton scattering, image formation, object reconstruction

Towards a new concept for high sensitivity Compton scatter emission imaging

INTRODUCTION

Emission imaging with gamma rays is widely used in numerous fields such as medical imaging, non-destructive testing, gamma astronomy and environmental survey. In conventional nuclear imaging, a collimated gamma camera rotates in space to collect primary radiation emitted by an object under investigation. In this case Compton scatter radiation behaves generally as noise hindering image quality and consequently correction to scatter should be applied.

However recently an interesting new imaging concept, which precisely uses, as imaging agent, scattered radiation by the object medium (instead of primary radiation), has been proposed. A spatially fixed collimated gamma camera records now images labeled by the energy of scattered radiation (or equivalently its scattering angle). It is then shown that the reconstruction of a three-dimensional object is feasible using this data [START_REF] Nguyen | On an integral transform and its inverse in nuclear imaging[END_REF][START_REF] Nguyen | A novel inverse problem in gamma-ray emission imaging[END_REF][START_REF] Nguyen | Novel approach to stationary transmission scanning using compton scattered radiation[END_REF]. However in this situation, the image sensitivity is considerably affected due to the presence of the collimator. Only about one out of 10 4 scattered photons reaches the detector. Therefore in order to record a much larger amount of scattered radiation, we propose to extend the working principle of this gamma-ray imaging to a functioning modality without collimator, as depicted by Fig. 1. However, as can be seen, this procedure differs from other proposals on scattered radiation imaging processes [START_REF] Clarke | The use of compton scattered gamma rays for tomography[END_REF][START_REF] Evans | Nondestructive inspection using compton scatter tomography[END_REF][START_REF] Hussein | Compton scatter imaging systems[END_REF][START_REF] Johns | Medical x-ray imaging with scattered photons[END_REF], in particular:

-Compton tomography [START_REF] Norton | Compton scattering tomography[END_REF], which reconstructs the electron density of the object (instead of its activity density), and uses a moving point-like detector collecting scattered radiation from an external radiation source, -Compton camera [START_REF] Singh | An electronically collimated gamma camera for single photon emission computed tomography[END_REF], which reconstructs the activity density of an object from scattered radiation using coincidence measurements between a site on a scatter layer-detector and another site on an absorption-detector.

As the true three-dimensional problem implies involved space geometry of scattered rays, we shall first study its twodimensional counterpart to test the viability of this idea. To this end, we give a careful analysis of the image formation. The corresponding PSF is derived and shall be compared with the previous case where a collimator is present. Next we perform numerical reconstruction of the object radiation activity from simulated scattered radiation (recorded by a camera without collimator) to illustrate the working of this concept.

IMAGE FORMATION

To understand image formation by scattered radiation, we follow radiation propagation in a two-dimensional (2D) slice (or thin section) of a scattering medium with an electronic density n e assumed to be approximately constant. This can be in principle implemented with a slit placed above a detector without collimator, see Fig. 2. Moreover, to concentrate chiefly on scattering effects radiation, attenuation will be left out as working hypothesis.

Object detector medium Scattering Slit Fig. 2. Two dimensional working with a slit.

Let -S = (x S , y S ) be a radiation emitting point source of an object described by its activity density f (S) (number of photons emitted per unit time isotropically in all directions, -M = (x M , y M ), a scattering site in the medium, -n e (M), the electron density at site M, -D = (x D , y D ), a detection site on a linear detector which collects the photon flux density at a photon energy E ω (see Eq. ( 1)).

The energy of scattered photon is related to the scattering angle ω by the Compton relation:

E ω = E 0 1 1 + ε(1 -cos ω) , ( 1
)
where E 0 is the emitted photon energy, ε = E 0 /mc 2 and mc 2 the rest energy of the electron.

From Fig. 3, one can see that the photon flux density reaching a site M is the number of photons emitted into the angular fan dΩ per unit length and per unit time:

f (S)dS S 2π 2 arctan σ 2 |SM | 1 σ , ( 2
)
where |SM | is the distance between sites S and M and dS S the area element around S. The fraction of photons scattered in the direction making an angle ω with the incident direction depends on the Compton differential cross section σ C S (ω) (which has the dimension of a length in 2D) and on the number of electrons n e (M) dS M at site M, dS M being the integration area element around M. Hence the scattering photon flux density received at the detection site D is given by :

dφ(D, ω|S) = f (S)dS S 2π 2 arctan σ 2 |SM | 1 σ × n e (M) dS M σ C S (ω) 2 arctan σ 2 |M D| 1 σ cos θ, (3)
where θ is the angle between the outgoing photon unit vector with the detector normal unit vector, |M D| the distance from scattering site M to detection site D. In fact, for a given point source S, there will be two scattering sites M 1 et M 2 located on two arcs of circle subtending a scattering angle (π -ω), as shown in Fig. 4.

The total photon flux density at a site D is g(D, ω), the integral over all source sites and all scattering sites such that the scattering angle is ω. This last constraint is expressed by a δ-function in the integration as

g(D, ω) = dφ(D, ω|S) δ( SM D -(π -ω)), ( 4
)
where SM D is the angle at vertex M of the triangle SM D (see Fig. 4). Eq. ( 4) describes the basic image formation process from scattered radiation measured on a detector without collimator.

COMPUTATION OF THE IMAGE OF A POINT SOURCE

We now compute and study the essential object in this new imaging concept: the Point Spread Function (PSF). It is, by definition, the image of a single point source at site S. Eq. ( 4) can be rewritten in terms of the PSF as follows :

g(D, ω) = dS S f (S) P SF (D, ω|S).

(

) 5
As shown before for a camera without collimator, the scattering sites due to a single point source S are located on two circular arcs subtending an angle (π -ω) (Eq. ( 4)). The photon flux density received at site D is then given by an integration over these two circular arcs (see Fig. 4). The circular arcs have polar equations:

r = d sin (ω -γ) sin ω and r = d sin (ω + γ) sin ω , ( 6
)
where γ is the angle between -→ DS and --→ DM . The distance |SM | can be extracted from a simple identity in the triangle DSM :

|SM | = d sin γ sin ω . ( 7
)
And the integration area dS M is now reduced to the arc element:

dr 2 + r 2 dγ 2 = d sin ω dγ. ( 8
)
Hence the PSF is given by the sum of the two integrations on γ:

P SF (D, ω|S) = K(ω) d σ σ sin ω 2 Arcs γ l (ω) 0 dγ cos θ(γ) arctan σ 2d sin ω sin γ arctan σ 2d sin ω sin(ω ± γ) , ( 9
)
where K(ω) = 4n e σ C S (ω)f 0 /2π, σ C S (ω) the differential Compton cross section at scattering angle ω, f 0 the intensity of the single point source and cos θ = sin(α -γ) if the detector lies along the 0x axis and l is the distance between the line detector and the linear lower boundary of the medium L. The integration is carried out over the points inside the scattering medium. Therefore when the medium is of finite extent, the limit of the integration γ l (ω), which corresponds to the intersection of the arcs of circle with the scattering medium, should be calculated beforehand, see Fig. 4. Now if the collimator is mounted on the detector, then only one scattering site M, located on the perpendicular to the detector at site D, will contribute to detection site D, (see Fig. 5). Thus the integration on γ is restricted by a delta function which picks out only the corresponding value of γ, i.e.:

γ col = π 2 -α. ( 10
)
The resulting PSF expression for a collimated detector is:

P SF col (D, ω|S) = K(ω) d σ σ sin ω arctan dx 2d sin ω cos α arctan σ 2d sin ω ± cos(α ∓ ω) . ( 11
)
Now at a fixed scattering angle ω, the PSF curve as function of the detector position with collimator has a Mexican hat shape (see Fig. 6) For example at a scattering angle of 30 degrees, Fig. 8 shows that the PSF without collimator is about 10 times stronger than the PSF with collimator.

NUMERICAL RECONSTRUCTION RESULTS

As an illustration of this new imaging concept, we carried out numerical reconstructions of a two-dimensional Shepp-Logan medical phantom from data computed with our model. The 2D original object (see Fig. 9) is placed at the center of the scattering medium and a unit distance above the detector. A line detector of 55 pixels of 1 unit length, placed on 11 show the reconstruction results with and without collimator. One observes a better agreement with the original object when the collimator is removed. In Fig. 10, we can see that the part of the object near the detector is better reconstructed than the upper part of the object. The three small structures are invisible. The reconstructive relative error is about 9.13 %. But without collimator (see Fig. 11), the whole object is correctly reconstructed. All structures are visible and the relative error is about 1.17 × 10 -4 %.

These results have been subsequently validated by Montecarlo simulations [START_REF] Driol | Modelling and simulation results on high sensitivity scattered γ-ray imaging[END_REF].

CONCLUSION

The feasibility of image reconstruction using Compton scattered rays detected by a gamma-camera without collimator, operating in a fixed position, is demonstrated in this study. This is the essence of a new concept of high sensitivity imaging, which takes advantage of scattering rays instead of rejecting them as done usually. The main point in this imaging process by Compton scattered radiation is the fact that data acquisition is performed without the usual motion of the detector. This is a major advantage compared to existing imaging systems which require a heavy, bulky and costly mechanical rotation mechanism to move the detector around in space. Work towards an extension to three-dimensional imaging is in progress. The modeling and simulations of multiple scattered radiation in this context will be also subjects of future investigations. These promising results may open the way to new high sensitivity imaging devices which will have applications in nuclear medicine, non-destructive industrial control, high energy astrophysics, environmental survey, etc.

Fig. 1 .

 1 Fig. 1. Two modalities in emission scattered radiation imaging.

Fig. 3 .

 3 Fig. 3. Geometry of Compton scattering: source site S, scattering site M and detection site D of width σ .

Fig. 4 .

 4 Fig. 4. Locus of scattering sites due to single point source For computation ease, we shall assume a uniform electron density n e and use polar coordinates (r, γ), such that S = (d, α) with DS = d and M = (r, γ), with DM = r and --→ DM • -→ DS = r d cos γ.The circular arcs have polar equations:

Fig. 5 .

 5 Fig. 5. Scattering site in the presence of a collimator from single point source

Fig. 6 .

 6 Fig. 6. PSF shapes with collimator

Fig. 7 .

 7 Fig. 7. PSF shapes without collimator

Fig. 8 .

 8 Fig. 8. Comparison of PSF with (lower blue line) and without collimator (upper red line)

Fig. 9 .

 9 Fig. 9. Original Shepp-Logan phantom

Fig. 10 .Fig. 11 .

 1011 Fig. 10. Shepp-Logan phantom reconstruction with collimator