Arun Bhaskar

Justine Philippe

Flavie Braud

Etienne Okada

Vanessa Avramovic

Jean-François Robillard

Cédric Durand

Daniel Gloria

Christophe Gaquière

Emmanuel Dubois

Jean- François Robillard

Large-area femtosecond laser milling of silicon employing trench analysis

Keywords: Femtosecond laser micromachining, Laser milling, Optical profilometer characterization, Silicon-oninsulator (SOI) technology, Radio-frequency (RF) integrated circuits

Introduction

Laser micromachining is a tool which is becoming increasingly relevant for micro/nanostructuring of materials. The use of ultrashort laser processing with pulse width in femtosecond range is advantageous because it can be applied on a wide range of materials like metals, semiconductors, dielectrics, alloys, and ceramics [START_REF] Nolte | Ablation of metals by ultrashort laser pulses[END_REF][START_REF] Lopez | Comparison of picosecond and femtosecond laser ablation for surface engraving of metals and semiconductors[END_REF][START_REF] Perry | Ultrashort-pulse laser machining of dielectric materials[END_REF][START_REF] Kim | Ablation characteristics of aluminum oxide and nitride ceramics during femtosecond laser micromachining[END_REF]. The range of surface processing is diverse varying from a small scale (a few nm 2 ) [START_REF] Dufft | Femtosecond laser-induced periodic surface structures revisited: a comparative study on ZnO[END_REF][START_REF] Wu | Femtosecond laser-induced periodic surface structure on diamond film[END_REF] to large scale (a few mm 2 ) [START_REF] Sikora | Picosecond laser micromachining prior to FIB milling for electronic microscopy sample preparation[END_REF][START_REF] Yong | Rapid fabrication of large-area concave microlens arrays on PDMS by a femtosecond laser[END_REF][START_REF] Huang | Large area uniform nanostructures fabricated by direct femtosecond laser ablation[END_REF]. Heat affected zone is greatly reduced for ultrashort lasers which allows for enhanced machining quality with low thermal impact of laser radiation on material.

Femtosecond micromachining of silicon has been reported previously in different studies. It has been explored for various applications like microfluidics [START_REF] Kam | Femtosecond laser machining of multi-depth microchannel networks onto silicon[END_REF], photovoltaics [START_REF] Halbwax | Micro and nano-structuration of silicon by femtosecond laser: Application to silicon photovoltaic cells fabrication[END_REF], IC characterization [START_REF] Sikora | Picosecond laser micromachining prior to FIB milling for electronic microscopy sample preparation[END_REF], hydrophobic surfaces [START_REF] Zorba | Making silicon hydrophobic: wettability control by two-lengthscale simultaneous patterning with femtosecond laser irradiation[END_REF], and laser dicing [START_REF] Domke | Ultrashort pulse laser dicing of thin Si wafers: the influence of laser-induced periodic surface structures on the backside breaking strength[END_REF][START_REF] Bärsch | Ablation and cutting of planar silicon devices using femtosecond laser pulses[END_REF]. For this work, the targeted application is the fabrication of free standing membranes of SOI-CMOS RF circuits/functions on Silicon-on-Insulator (SOI) wafers. The term SOI-CMOS refers to CMOS processing technology used in semiconductor industry in combination with SOI wafer as the host substrate. SOI wafer comprises of a thick layer of handler silicon at the bottom, typically 750 µm, topped with a thin isolating buried oxide (BOX) layer and finally a thin layer of active silicon on the top. Microelectronic devices and circuits are realized into the topmost silicon active layer. The buried oxide is a thin layer of silicon dioxide which provides electrical isolation between active and handler silicon. Handler silicon is present for mechanical support and thermal dissipation. Despite the presence of an isolating BOX layer, handler substrate degrades electrical performance because it offers a parasitic coupling path to RF signals. By locally removing handler silicon under the active area of circuits, they can be suspended on the BOX in the form of a membrane. The motivation for creation of such membranes is to attain better RF performance with reduced loss and improved linearity as it has been reported previously in numerous similar studies [START_REF] Bhaskar | Large-area femtosecond laser ablation of Silicon to create membrane with high performance CMOS-SOI RF functions[END_REF][START_REF] Bhaskar | Femtosecond laser micromachining of crystalline silicon for ablation of deep macro-sized cavities for silicon-on-insulator applications[END_REF][START_REF] Philippe | Application-oriented performance of RF CMOS technologies on flexible substrates[END_REF][START_REF] Des Etangs-Levallois | A converging route towards very high frequency, mechanically flexible, and performance stable integrated electronics[END_REF][START_REF] Des Etangs-Levallois | 150-GHz RF SOI-CMOS technology in ultrathin regime on organic substrate[END_REF][START_REF] Des Etangs-Levallois | Radio-frequency and low noise characteristics of SOI technology on plastic for flexible electronics[END_REF][START_REF] Philippe | Thermal Analysis of Ultimately-Thinnedand-Transfer-Bonded CMOS on Mechanically Flexible Foils[END_REF][START_REF] Tagro | High frequency noise potentialities of reported CMOS 65 nm SOI technology on flexible substrate[END_REF].

The technique of laser milling is used in this work to create cavities underneath RF functions implemented in SOI-CMOS technology. The laser beam is raster scanned over the area to be milled. The starting thickness of SOI wafer is usually high (~750 µm) in order to prevent wafer warping during fabrication. As a preliminary step, the wafer is thinned down using processes like grinding and chemical mechanical polishing (CMP). The final thickness of silicon is much smaller and depends on the application requirements. With this in mind, the laser milling process is studied for depths up to 200 µm. The process is adaptable for geometries of side length ranging from ~100 µm to several mm by appropriately defining the laser milling trajectory.

The objective of this study is the development of a milling methodology that enables the control of surface quality and roughness while keeping the volume removal rate high. During micromachining of silicon, a commonly identified problem is the appearance of surface corrugations [START_REF] Domke | Ultrashort pulse laser dicing of thin Si wafers: the influence of laser-induced periodic surface structures on the backside breaking strength[END_REF][START_REF] Crawford | Femtosecond laser micromachining of grooves in silicon with 800 nm pulses[END_REF][START_REF] Domke | Controlling depth and distance of the hole formations at the bottom of laser-scribed trenches in silicon using fs-pulses[END_REF][START_REF] Kautek | Femtosecond-Pulse Laser Microstructuring of Semiconducting Materials[END_REF][START_REF] Fornaroli | Dicing of thin Si wafers with a picosecond laser ablation process[END_REF][START_REF] Audouard | Optimization of surface engraving quality with ultrafast lasers[END_REF] which renders the surface cross section comb like as shown in Fig. 1. In the top view (Fig. 1c), these corrugations appear as microholes which can penetrate deep into the material depending on the process parameters used for milling. These corrugations are also observed in other materials like germanium [START_REF] Haugen | Examination of Surface Morphology and Sub-Surface Crystallographic Changes of Si, Cu, GaP and Ge After Ultrashort Laser Pulse Irradiation[END_REF]. They are not to be confused with the Laser Induced Periodic Surface Structures (LIPSS) which are ripples of the order of the wavelength of laser radiation [START_REF] Tan | A femtosecond laser-induced periodical surface structure on crystalline silicon[END_REF][START_REF] Hong | Femtosecond laser fabrication of large-area periodic surface ripple structure on Si substrate[END_REF]. In this study, this problem is tackled by choosing appropriate process parameters to minimize formation of corrugations in order to keep the roughness low while still having sufficiently high rates of removal.

In this work, a systematic approach is employed whereby step-by-step insight is gained towards best parameters for laser milling. In the first step, trenches (grooves) are scribed in silicon by linear displacement of the laser beam. Scribing allows quick estimation of impact of different process parameters on milling without performing two-dimensional surface machining. Section 3 reports different trench measures that are going to be studied and quantified by using optical profilometer measurements. The understanding of these parameters lays the foundation for high quality and high removal rate milling. Section 4 subsequently proposes the study of the surface laser milling to further understand the aspects of changing surface morphology, effect of debris, surface roughness and sidewall quality. The application of laser milling is presented in section 5 where membranes of RF circuits are fabricated. RF characterization of such circuits is performed in order to demonstrate the superior performance of the circuits on SOI membranes.

Experimental description 2.1 Laser system

The experiments are performed under ambient conditions using a Diode Pumped Solid State (DPSS) ultrafast fiber laser from Amplitude Systèmes (Tangerine) with a fundamental output at 1030 nm and pulse width (FWHM) of 300 fs. Additional wavelength output of 343 nm is generated using a harmonic box (Amplitude-Systèmes). The 343 nm wavelength is obtained by frequency tripling realized by cascading non-linear crystals, beginning with frequency doubling of the fundamental input beam at 1030 nm and subsequent sum frequency generation of both waves.The maximum power output available at the workpiece is 1.2 W and 13 W for 343 nm and 1030 nm sources, respectively. Power attenuation is achieved by cascading a rotating birefringent half-wave plate with a polarizer which provides a continuous variation of the laser power while decoupling the power attenuation from the laser repetition rate. The pulse repetition rate for maximum efficiency of laser output is 200 kHz and can go up to 2 MHz with reduced efficiency. The laser beam displacement is effected by a galvanometric scanner (Canon) followed by a flat-field telecentric lens (Sill Optics) with a focal length of 100 mm. The maximum galvo speed for milling without path distortions is 20 mm s -1 and 100 mm s -1 for 1030 nm source and 343 nm source respectively [START_REF] Bhaskar | Large-area femtosecond laser ablation of Silicon to create membrane with high performance CMOS-SOI RF functions[END_REF]. The beam waist (radius) is determined experimentally using the regression of squared diameter on fluence as first proposed by Liu et al. [START_REF] Liu | Simple technique for measurements of pulsed Gaussian-beam spot sizes[END_REF]. For 343 nm and 1030 nm sources, the measured beam waist on silicon is 5.8 ± 0.3 µm and 8.2 ± 0.2 µm, respectively. A quarter wave (𝜆/4) plate is placed before the lens to obtain a circular polarization of the laser beam.

Laser micromachining is performed on High-Resistivity (HR) SOI substrate with resistivity > 1 kΩ.cm. This value of resistivity is 2-3 orders of magnitude higher than normal silicon wafer. However, the interpretations in this study is applicable for any type of silicon as it has been shown that substrate resistivity does not affect the ablation threshold for multi-pulse ablation [START_REF] Oosterbeek | Effects of dopant type and concentration on the femtosecond laser ablation threshold and incubation behaviour of silicon[END_REF]. The laser machined lines and cavities are characterized using optical profilometry, optical microscopy and scanning electron microscopy (SEM). The optical profilometer (Bruker ContourGT-X) is a non-contact profiler based on vertical scanning interferometry using a broadband white light source. The SEM (Zeiss Supra55VP) scans are taken at an acceleration voltage of 10 kV.

For SOI-CMOS application demonstration, gas phase XeF2 etching is used to selectively remove silicon with respect to silicon dioxide. An etch-protect layer (Ajinomoto GX-T31) is laminated on the back side of the SOI die. This ensures that silicon is removed completely only in the areas where cavities are milled leaving the BOX layer intact. The etching is performed in a Xactix XeF2 etcher.

Choice of parameters for trench scribing

Scribing of trenches is the first stage of our analysis to study the different ablation regimes and to determine the fluence and repetition rate suitable for milling application. The parameters that are varied are the pulse repetition rate (𝑓 ), pulse energy (𝐸 ) and scan speed (𝑣 ). From the chosen parameters, the average power can be given by the expression:

𝑃 = 𝐸 𝑓 (1)
To obtain high removal rates, a maximum average power of >1 W up to ~2.5 W is used for removal of trenches. Although higher power is supported in the laser system for the 1030 nm line, it is not utilized because surface roughness and volume removal rate are degraded at high power as it shall be discussed in section 3.2. In order to obtain a given average power, the pulse repetition rate and the pulse energy can be independently chosen. When the pulse energy is varied, correspondingly the peak fluence of the laser pulse changes as given by the expression:

𝜙 = (2)
Here, 𝑤 is the beam waist radius, the radial distance measured from beam cross section at focal plane where the intensity reduces to 1/𝑒 times the value at the centre. All fluence values reported hereinafter corresponds to the peak fluence. Three cases of trench scribing have been studied as shown in Table 1. For the first two cases, 200 kHz repetition rate is used as this is the repetition rate at which laser efficiency peaks. For the third case repetition rate is reduced to 30 kHz so that power corresponding to maximum pulse energy is sufficiently high (~1.8 W). While these cases have been briefly treated in [START_REF] Bhaskar | Femtosecond laser micromachining of crystalline silicon for ablation of deep macro-sized cavities for silicon-on-insulator applications[END_REF], this paper outlines a detailed analysis into each case. Three scan speeds are studied per case. Each scribed trench is characterized using optical profilometry. The different analyses that are performed for each scribed trench are: average trench depth, recast layer volume, average profile roughness, average trench width and volume removal rate. A graphical illustration of measured parameters can be found in Appendix A.1. Each analysis is presented separately in section 3. The uncertainty on measured depths is less than 1% owing to the high accuracy of vertical scanning interferometry. The accuracy of volume measurements is primarily governed by the evaluation of distances in the plane of the sample surface. This accuracy is a function of the spatial sampling determined by the magnification of the lens and the pixel capacity of the observation camera.

In this study, we estimate that lateral distances are affected by a maximum fixed error of 1 µm corresponding to 2 spatial sampling pixels. The power meter accuracy is less than 3%. Based on these figures, the uncertainty in fluence is estimated as < 13%.

Scribing of trenches

This section presents the discussion regarding the different experimental observations made in the scribing of trenches utilizing the parameters detailed in section 2.2.

Ablation regimes in micromachining of silicon

Laser micromachining is essentially multi-pulse ablation wherein different ablation regimes can be identified. Each ablation regime is characterized by a certain dominant physical mechanism of ablation and the regimes are separated by a threshold fluence, which changes as a function of the process parameters that are employed. In this section, the average trench depth and recast layer volume are studied to distinguish the ablation regimes.

Average trench depth (𝑑 )

The optical profilometer renders a 3D profile of the trench. A 1D height profile is taken approximately along the central axis of the trench. Average trench depth is determined by summing the different height values comprising the axial trench profile and dividing by the number of points as given by the equation:

𝑑 = ∑ 𝑧 (3)
For the 3 cases, the plots of average trench depth as a function of fluence is shown in Fig. 2. The depth plotted on a semi-logarithmic scale shows two ablation regimes with different ablation thresholds. This is a well-known phenomenon which has been reported in metals [START_REF] Nolte | Ablation of metals by ultrashort laser pulses[END_REF][START_REF] Furusawa | Ablation characteristics of Au, Ag, and Cu metals using a femtosecond Ti: sapphire laser[END_REF], semiconductors [START_REF] Crawford | Femtosecond laser micromachining of grooves in silicon with 800 nm pulses[END_REF][START_REF] Borowiec | Femtosecond laser micromachining of grooves in indium phosphide[END_REF] and dielectrics [START_REF] Ashkenasi | Laser processing of sapphire with picosecond and sub-picosecond pulses[END_REF][START_REF] Ali | Analysis of nanosecond and femtosecond laser ablation of rear dielectrics of silicon wafer solar cells[END_REF]. The depths in the two regimes can be given by

𝑑 , = 𝑙 , 𝑙𝑛 , (4)
where 𝑑 , is the average trench depth, 𝜙 , the threshold fluence and 𝑙 , the characteristic ablation depth with the subscript/superscript index 'g' and 's' standing for gentle and strong ablation regimes. The transition fluence between these two regimes is determined by the scan speed when all other laser parameters are fixed. Higher scan speeds allow utilization of higher fluence (consequently a higher average power) in the gentle ablation regime. Thus, higher removal rates can be obtained by using higher scan speeds while still operating in the gentle ablation regime. The ablation regime is important as it determines several aspects like recast layer volume, roughness, trench width narrowing and a detailed discussion of this is presented in sections 3.3 -3.5.

From the plots of average depth as a function of fluence, curve fitting is performed to associate data points with the two regimes. The extracted values of characteristic depth and threshold fluence for the 3 cases at different speeds are tabulated in Table 2. Sample trench profiles for the two regimes have been provided in appendix A.2. With increase in wavelength, both the characteristic depths (𝑙 , ) and threshold fluences (𝜙 , ) increase at a given speed. In the gentle ablation regime, at repetition rate of 200 kHz, the difference in ablation thresholds between 343 nm and 1030 nm source is small. For the 1030 nm source at a repetition rate of 200 kHz, a good point of comparison with nearly the same laser system as ours is provided in the work of Shaheen et al [START_REF] Shaheen | Femtosecond laser ablation behavior of gold, crystalline silicon, and fused silica: a comparative study[END_REF]. They reported a threshold fluence of 0.54 and 2.4 J cm -2 in the two regimes for 20 pulses. This correlates excellently with values obtained for scan speed of 50 mm s -1 where pulse overlap is small (0.25 µm) . For the 1030 nm source, at a repetition rate of 30 kHz, the threshold fluence for gentle ablation is much larger as compared to other two cases. The characteristic depths in the gentle ablation regime are smaller at same speed for this case. Higher threshold fluence and lower characteristic depths do not necessarily mean that ablation rate is reduced. This is because the width of the ablated trench also needs to be considered to calculate the volume removal rate. The width of trench is dependent on the experimental parameters and is discussed in section 3.5. The material removal rates for the 3 cases are presented in section 3.6. For the 200 kHz repetition rate cases, average depth measured from axial trench profile starts to decrease at a high fluence. This is observable at speeds of 50 and 70 mm s -1 for 343 nm source and at 50 mm s -1 for 1030 nm source. This is mostly likely due the formation of debris at the bottom of the trench as observed in [START_REF] Crawford | Femtosecond laser micromachining of grooves in silicon with 800 nm pulses[END_REF] and supported by explanation given in Appendix A.3. The higher ablation threshold seen in the gentle ablation regime for 1030 nm source at a repetition rate of 30 kHz is attributed to the higher fluence used. It has been reported that for fluence > 4 Jcm -2 , the ablation enters a different regime and is dominated by hydrodynamic motion of silicon [START_REF] Moser | Single pulse femtosecond laser ablation of silicon -a comparison between experimental and simulated two-dimensional ablation profiles[END_REF]. In all cases the increasing threshold fluence at higher speed is because of the fact that effective number of pulses hitting a spot reduces at higher speed. In the work of Ashkenasi et al., it was shown that when number of pulses are high, the transition from gentle to strong regime happens at a lower fluence and this explains the threshold fluence trend with respect to speed [START_REF] Ashkenasi | Laser processing of sapphire with picosecond and sub-picosecond pulses[END_REF].

In the detailed study of grooves (trenches) in silicon by Crawford et al., it is argued that scribing of trenches may not be equivalent to stationary multi-pulse ablation [START_REF] Crawford | Femtosecond laser micromachining of grooves in silicon with 800 nm pulses[END_REF]. For scribing of trenches, there is a spatial displacement of laser beam with respect to the sample from one pulse to the next. Each pulse causes ablation and modification of the surface. Thus, for subsequent pulses, the laser energy is incident on an uneven and changing surface which gives rise to local variation of deposited energy. While this is also true for stationary ablation, the difference is that geometry of the surface encountered during ablation involving translation is much more skewed making the local energy distribution more uneven. Additionally, they found that at low translation speeds, a nearly linear dependence of trench depths as a function of pulse energy. At the same time, on a logarithmic scale, they could fit the data points into two regimes. The authors preferred to use the two regimes model to interpret their data. However, they expressed the view that linear dependence of ablation depths cannot be ruled out and that two different regimes may not be present.

In this study, the same question had to be addressed: is a linear or two-regime model better to explain ablation data? In order to answer this question, several analyses of the recast layer are performed. It will become clear in next subsection that two ablation regimes are indeed present for all 3 cases. Recast layer is defined as the aggregate of debris/redeposited material in the vicinity of the ablated trench. In the profilometer scan, the points representing the recast layer form a 2D surface extending above the substrate. The recast layer volume is calculated as the volume enclosed by this surface and the reference level of the substrate (zero level). The recast layer volume obtained for the three cases is shown in Fig. 3. The unit µm 3 mm -1 refers to the volume of recast layer in µm 3 when scribing a trench of length 1 mm. This measure is a key indicator of the presence of two ablation regimes. For all 3 cases, the volume of recast layer is low in the gentle ablation regime followed by an abrupt increase which shows the onset of strong ablation. At a given speed, there is only a very small increase in ablation volume when going from lowest to highest fluence within the gentle ablation regime. Higher fluences out of this regime result in strong ablation and this is accompanied by an increase of volume of recast layer. In the strong ablation regime, the volume of recast layer is much more sensitive to the fluence. A large difference in volume is observed between the highest and lowest fluence conditions within the strong ablation regime at a given speed. For the 1030 nm source and repetition rate of 30 kHz, at speed of 30 mm s -1 , the volume of recast layer starts to decrease at very high fluences. When the profilometer scan is observed, it is seen that some parts of the recast layer is missing along the sides of the trench. This can be attributed to strong expulsion of ablated material at such fluences which possibly impedes the formation of recast layer at certain places along the sides of the trench.

Recast layer volume (𝑉 )

A threshold fluence is also extracted using the recast layer volume data. An assumption is made that the recast layer volume (𝑉 ) is directly proportional to the ablated volume of the silicon. The threshold fluence for gentle ablation regime has not been reported because the measured volume is close to the lower limit of the profilometer measurement of volume and hence it is difficult to obtain a reliable curve fit. The ablated volume of silicon follows a squared logarithm dependence with respect to the fluence [START_REF] Sikora | Wavelength scaling of silicon laser ablation in picosecond regime[END_REF]. Accordingly, the equation for recast layer volume data can be given by:

𝑉 = 𝑘. 𝑙𝑛 (5)
Least squares curve fit has been performed and the threshold fluence in strong ablation regime is depicted as 𝜙 in Table 2. It can be seen that the threshold fluence for 200 kHz repetition rate for both cases are slightly higher than those obtained by trench scribing. However, it is vastly different for 1030 nm source with repetition rate of 30 kHz. The reason could be that the assumption of recast layer volume being proportional to ablated volume of silicon is not valid at high fluences.

Impact of ablation regime on micromachining

In the previous section, ablation regimes have been studied. This section outlines how the ablation regime affects the micromachining in terms of three important parameters: average roughness, trench width and volume removal rate.

Average roughness of trench profile (𝑅 )

One of the key aims in this study is to determine the fluence and repetition rate which minimize corrugations. Trench profiles have been presented and corrugations have been studied for different experimental conditions in our previous work in [START_REF] Bhaskar | Femtosecond laser micromachining of crystalline silicon for ablation of deep macro-sized cavities for silicon-on-insulator applications[END_REF]. Here, corrugations have been quantified by using average roughness of axial trench profile. Higher roughness gives trench morphology with microholes that go deeper into the material. The expression for average roughness can be given by:

𝑅 = ∑ |𝑧 -𝑑 | (6)
The average roughness measured from the axial trench profile using equation 6 is plotted in Fig. 4. It can be seen that for the two 200 kHz repetition rate cases, very high roughness values of 2 -3 µm can be reached. The profile is highly corrugated at high fluences. For the two cases, the increase in trench profile roughness shows the same behaviour as the recast layer volume. A nearly constant roughness is seen in the gentle ablation regime followed by an increase in roughness with respect to fluence in the strong ablation regime. Interestingly, for the 1030 source at a repetition rate of 30 kHz, the surface roughness shows a different behaviour. It is nearly constant over both regimes with only a small increase at high fluences and the maximum value is < 1 µm.

The formation of a periodic structure of holes with a spatial separation well above the laser firing repetition pitch has been reported in the literature. This effect has been observed on silicon where the holes thus formed are called craters or cones [START_REF] Domke | Controlling depth and distance of the hole formations at the bottom of laser-scribed trenches in silicon using fs-pulses[END_REF][START_REF] Neuenschwander | Optimization of the volume ablation rate for metals at different laser pulse-durations from ps to fs[END_REF] .The formation of such holes has been used for the fabrication of superhydrophobic surfaces in the work of Yong et al. [START_REF] Yong | Stable superhydrophobic surface with hierarchical meshporous structure fabricated by a femtosecond laser[END_REF]. They attribute the appearance of holes to the dynamic accumulation of recast material along the machining path. The hypothesis is that initial few laser pulses cause slow build-up of recast material ahead of the current laser position. In this region, depth of removal of silicon is high. For the next few pulses, the laser spot encounters the recast material projecting over the silicon surface. In this region, overall depth of removal of silicon remains low because part of energy is lost to removal of accumulated recast material. This cycle repeats and results in the formation of a periodic structure. We find that this hypothesis is applicable in our experiments as well and there are two supporting arguments. For high repetition rate and low fluence cases (Fig. 4a and4b), the roughness increases abruptly in a fluence regime where recast layer volume also increases abruptly (Fig. 3a and3b). Additionally, the inner trench width also reduces for these two cases suggesting a build-up of recast material. For the case with low repetition rate and higher fluences, the roughness (Fig. 3c) does not change abruptly even when the recast layer volume does. This can be due to two reasons. The first is that when the repetition rate is 30 kHz, the spacing between pulses is one order of magnitude higher as compared to 200 kHz which could prevent localized accumulation of recast silicon. The second is that higher fluence results in more vigorous expulsion of ablated material [START_REF] Lee | Femtosecond laser micromilling of Si wafers[END_REF], which again possibly prevents local accumulation of recast material .

The periodic nature of the profile can be easily noticed in the SEM scans depicted for a limited set of processing parameters in Appendix A.4. It is found that the period of the profile is proportional to the scan speed. For the 343 nm source, a Fourier analysis of profiles revealed periods of 6.9, 9.6, and 13.8 µm for speeds of 50, 70 and 100 mm s -1 , respectively. The periodicity is found not to depend on the fluence. However, the amplitude of the dominant period increases with fluence. While average roughness is one measure of roughness that has been presented, Root Mean Squared (RMS) roughness may also be used. Same trends can be observed by plotting the RMS roughness as a function of fluence and is not separately shown here.

Trench width (𝑊 )

For the measurement of trench width, the 1D cross-sectional profiles are taken perpendicular to the axis of the trench. The width of the recast layer can vary considerably along the length of the trench. Hence, in order to obtain an average width, 10 cross-sectional profiles are taken at different points along the length of the axis. The average widths calculated from these profiles are plotted in Fig. 5.

Only the inner trench widths are shown. When the outer trench widths were plotted, a large scatter is observed and no clear trend can be seen. Hence, these data are not presented here. The 343 nm source features a smaller beam waist of as compared to 1030 nm. The width of cut scales directly with the beam waist as given by the equation:

𝑊 = 2𝑤 ln (7)
Hence a bigger width of cut can be expected for 1030 nm source. However, it is also determined additionally by the threshold fluence which is smaller for 343 nm source. On comparing the two 200 kHz repetition rate cases, an overall observation indicates higher width of cut is indeed seen for 1030 nm source. However, the scatter of data is not negligible for these data points, which makes it difficult to compare the two cases. Nevertheless, it is seen for both cases that in the gentle ablation regime, the width of trench increases. An additional important observation is made for the two cases.

With the onset of strong ablation regime, the trench width starts to decrease. This is most likely due to accumulation of recast material on the sidewalls of the trench as reported in [START_REF] Crawford | Femtosecond laser micromachining of grooves in silicon with 800 nm pulses[END_REF]. The reduction in trench width makes the milling process less efficient. This is because for each subsequent line traversed in the milling trajectory, a considerable amount of energy is deposited on the recast layer which does not result in any net ablation from the substrate. For the last case of the 1030 nm source at a repetition rate of 30 kHz, the trench widths in the gentle ablation regime is more sensitive to the fluence especially at high scanner speeds. A very low width of cut of ~3 µm can be obtained at a scanner speed of 90 mm s -1 . The scatter of data is smaller as compared to the other two cases indicating the presence of a more uniform recast layer. The narrowing of trench width in the strong ablation regime is not seen for this case. This would be useful in preventing loss of laser energy to recast layer and improving milling efficiency. The volume removal rate is an important factor which determines how fast material can be removed from the substrate. It is computed based on the empirical observation that the trench cross section can be well approximated to be a triangle using the formula:

Volume removal rate (𝑉𝑅𝑅)

𝑉𝑅𝑅 = (8)
The volume removal rates are compared as a function of average power and the plots for the 3 cases are shown in Fig. 6. An overall comparison in Fig. 6 shows that the high repetition rate cases (a) and (b) have higher material removal rates as compared to case (c) within the range of experimental parameters chosen. This difference is more distinct by observing the slopes at low laser power. For cases (a) and (b), with increasing power, the removal rate tends to flatten out. This is because of formation of debris which reduces both trench width and depth. The maximum value is higher for higher scan speeds. This phenomenon is not seen for case (c) and the removal rate steadily rises even at high power. The trench narrowing effect and corresponding reduction of VRR for cases (a) and (b) can be potentially reduced by using a larger beam waist which gives a wider cut and helps easier ejection of debris. This also provides an added advantage of even higher removal rate because threshold fluence reduces with increasing beam waist [START_REF] Armbruster | Spot size and pulse number dependence of femtosecond laser ablation thresholds of silicon and stainless steel[END_REF]. However, this compromises the resolution of the laser milling which is more important in our application. In summary, laser scribing of trenches has been studied for 3 different cases leading to the following important observations: i) the use of the 1030 nm source at a lower repetition rate and higher fluence gives a lower material removal rate, ii) however, other advantages, such as reduction of debris, low profile roughness for deep trenches, absence of trench narrowing effect, make this case suitable for usage in laser milling. Thus, the multiple analyses based on i) the ablation average depth, ii) the volume of recast material, iii) the average longitudinal profile roughness, iv) the inner trench width and v) the volume removal rate made it possible to identify the ablation regimes and to determine the impact of the fluence and repetition rate on these parameters. Additional factors come into play during laser milling. This is discussed in the following section.

Milling of cavities

Section 3 provided an overview of trench scribing with different parameters and the optimal values have been highlighted. In this section, further analysis is presented for milling over a surface area using the chosen parameters and how process parameters are further refined in order to avoid high surface roughness.

Laser milling is performed by raster scanning the laser beam with line to line spacing (pitch) of 10 µm. Square cavities are milled with side length ranging from 100 µm to 1 mm. The parameters of interest are surface morphology, roughness and quality of sidewall. The repetition rate of 30 kHz is chosen to avoid microhole formation. The scan speed is set to 20 mm s -1 and the fluence is varied. The first study is made by a single scan over the milling area. A trench is also scribed using the same parameters to make a comparison between 1D machining and 2D milling. The SEM scans of the surface of the cavities for one scan of laser beam are shown in Fig. 7. It can be seen that overall morphology looks good with presence of few microholes notably at a fluence of 4.7 J cm -2 . Several parameters are computed for single scan milling from the profilometer scans of the milled cavity and the scribed trench which is summarized in Table 3. It can be seen that depth of cavity after ultrasonic cleaning increases in all cases. With increase in fluence, the difference is larger. This means that a significant quantity of material at the bottom of the cavity is loosely bound suggesting a redeposition of ablated material. At the lowest fluence of 2.8 J cm -2 , the roughness and depth values for the cavity and trench are excellently correlated. With increasing fluence, the difference in depth and roughness parameters between cavity and trench profile becomes larger. This increase in roughness can be attributed to presence of the ablation plume and redeposited material in the vicinity of the ablation area which interacts with the laser beam [START_REF] Bhaskar | Substrate engineering using laser micromachining for improvement of RF devices and systems integrated in SOI-CMOS technology[END_REF][START_REF] Preuss | Sub-picosecond UV laser ablation of metals[END_REF]. Ablation plume is the collection of atoms, clusters, ions and electrons that is emitted during ablation which can dynamically interact with the laser beam during the milling process. For the chosen set of parameters, high removal rates are obtained in the range of 10 5 -10 6 µm 3 s -1 . Having studied milling for a single pass, the number of passes is increased to mill cavities with a depth of ~100 μm. The morphology of the surface of the cavity is shown in Fig. 8. It is drastically different from that observed for single pass. A large number of microholes are seen on the cavity surface. The number of holes decreases while the size of the holes increases at a higher fluence. At the lowest fluence of 2.8 J cm -2 , a regular array of microstructures approximately in the form of pyramids is seen.

The appearance of such self-organized microstructures has been reported for different metals like Cu, Al, Ti etc. [START_REF] Nayak | Self-organized micro/nano structures in metal surfaces by ultrafast laser irradiation[END_REF]. Same has been observed for silicon in numerous studies [START_REF] Halbwax | Micro and nano-structuration of silicon by femtosecond laser: Application to silicon photovoltaic cells fabrication[END_REF][START_REF] Lee | Femtosecond laser micromilling of Si wafers[END_REF][START_REF] Tull | Silicon Surface Morphologies after Femtosecond Laser Irradiation[END_REF]. The reason for formation of such ordered structures is formation of regular array of bumps in the initial few passes of laser in the milled area. This gradually evolves into a surface morphology containing pillars due to local concentration of laser energy in the valleys and accumulation of ablated matter on the top of the pillars. A visual depiction of this phenomenon can be observed in the work of Lee et al. where SEM images for different number of passes has been taken [START_REF] Lee | Femtosecond laser micromilling of Si wafers[END_REF]. They also found that the ordered microstructures disappear at higher fluence which is consistent with this work. The reason has been attributed to strong expulsion of ablated matter which prevents formation of ordered structures. Additionally, it is observed that for the low fluence of 2.8 J cm -2 , a lot of debris are formed and the smaller cavities are almost fully covered in particle debris. For higher fluences, the quantity of debris is much lower and mostly found in the regions surrounding the ablated cavity.

The average depth is plotted in Fig. 9a for all cavities before and after ultrasonic cleaning to quantify the amount of redeposited material. The x-axis in this graph represents the side length of square cavity being milled. It can be seen that in all cases redeposited matter exists at the bottom of the cavity for all cases. For fluences of 2.8 J cm -2 and 4.7 J cm -2 , milled depth is less for smaller cavities. The average difference in depths before and after ultrasonic cleaning are 3, 1.7 and 1.5 μm for fluences of 4.7, 12.7, and 17.4 J cm -2 respectively. For fluence of 2.8 J cm -2 , the data before ultrasonic cleaning is not taken because many cavities are covered fully in particle debris. In Fig. 9b, the average depths are found in a similar way but the number of passes is doubled to have nearly twice the depth. The average difference in depths before and after ultrasonic cleaning are 5.2 and 3.7 μm for fluences of 12.7 and 17.4 J cm -2 , respectively. Thus, when the cavity becomes deeper, the quantity of redeposited material increases. An efficiency factor is determined using the following formula which is one way to quantify how efficiently laser energy is utilized in the milling process:

𝜂 = ( 9
)
where 𝑑 is the average depth of cavity, 𝑁 the number of laser scans and 𝑑 the average depth obtained from the 1D trench profile using same milling parameters as described in section 3.2. The different parameters computed for 1mm square cavity are summarized in Table 4. The efficiency factors at fluences of 12.7 and 17.4 J cm -2 are found to be 0.91 and 0.92, respectively, when the cavity depth is ~100 μm. On doubling the number of passes, the efficiency factor reduces to 0.85. Thus, it can be said that with increasing depth of the cavities, ablation plume and redeposited material play important role in determining ablated depth and roughness. With the reduction in efficiency factor, the surface roughness also increases. This is clearly noticed for the fluence of 2.8 J cm -2 , where the efficiency factor is much smaller than other cases. The volume removal rate (VRR) is increased as compared to that obtained single pass (in Table 3). This is because the first pass is on a pristine surface with good crystalline order and material removal is more difficult. In the subsequent passes, the structure has been already been significantly damaged or in other words there is incubation effect which increases the material removal rate [START_REF] Bonse | Femtosecond laser ablation of silicon-modification thresholds and morphology[END_REF]. The only exception again is fluence of 2.8 J cm-2 because high volume of ablation plume in the form of debris impedes material removal. When the quantity of redeposited debris is small, the quality of side walls is good as seen in Fig. 10.

It has been shown in this section that choice of fluence is important in milling. Low fluence results in particle debris and surfaces which have high roughness. High fluence reduces particle debris to a great extent. However, with increasing fluence roughness also increases. Hence, it is beneficial to use the lowest possible fluence which does not give rise to particle debris. Also, when all other parameters are kept constant, the roughness increases with number of laser scans due to the build-up of ablation plume and redeposited material. Having discussed the different aspects of milling process, its application to SOI-CMOS technology is elaborated in the next section.

SOI-CMOS Applications

The previous sections have dealt with the fabrication of cavities. For application in SOI-CMOS technology, an etch-protect layer is first laminated onto the handler side of the SOI die. Milling is then performed in the area where membranes need to be fabricated. In order to speed up the process, a two-step milling strategy has been used in the applications. In the first step, a cavity with larger area is created at a higher removal rate (fast step). In the second step, the smaller cavity of required dimensions of membrane is created at a slower removal rate (slow step). The bigger cavity allows more space for the diffusion of ablation plume generated during milling of smaller cavity which helps lower surface roughness. The die is then subject to XeF2 etching to complete the removal of remaining few tens of microns of silicon remaining under smaller cavities and suspended membranes are obtained. The membranes of 3 different circuit components are studied: RF switch, RF isolation structures and Low-noise amplifier (LNA). The RF switch is a module in transceiver systems which contain large transistors for switching between transmission and reception of RF signals. RF isolation structures are specially designed structures to quantify RF coupling through the substrate. LNA is present in RF receivers to have a good overall noise performance of the receiver chain. A detailed treatment of these circuits can be found in [START_REF] Bhaskar | Substrate engineering using laser micromachining for improvement of RF devices and systems integrated in SOI-CMOS technology[END_REF][START_REF] Nguyen | Radio-frequency integrated-circuit engineering[END_REF].

Two different application demonstrations employing two-steps ablation are shown in Fig. 11. The milling parameters used for each case is shown in Table 5. The first case demonstrates possibility of milling close to the full thickness of SOI die (~750 µm). The mm sized cavity is shown in Fig. 11a is for the fabrication of membranes of RF switch circuit. For the RF switch, a single small cavity is milled within a large cavity and total depth of etching is 635 µm. The second case where two-step milling can be used for multiple membranes is shown in Fig. 11b applied to the study of RF isolation structures. Here, milling is performed on reduced starting thickness of the SOI die. The SOI die is grinded to have a thickness of 250 µm prior to the laser milling process. For RF isolation structures, four smaller cavities are milled within a large cavity and total depth of milling is 185 µm. It can be seen that by using a slower removal step within the larger cavity, roughness parameters are better for the smaller cavity as compared to larger one. Hence, a two-step strategy enables to obtain a good balance between removal rate and roughness. These two examples show that laser milling supports fabrication of membranes on varying die sizes and x-y dimensions. By using XeF2 etching, the silicon inside the inner cavities can be completely etched and the BOX remains intact because it does not react with XeF2. The etch-protect layer leaves the rest of the die intact. Additional details of XeF2 etching are provided in [START_REF] Bhaskar | Femtosecond laser micromachining of crystalline silicon for ablation of deep macro-sized cavities for silicon-on-insulator applications[END_REF]. After XeF2 etch, free standing membranes of circuits are obtained. Since, the circuits are suspended on a transparent layer of BOX, light from the backside can be viewed on the front side. This allows to create an image of the suspended membrane of the RF functions. The membranes of different circuits are shown in Fig. 12. RF characterization of these circuits have been performed to quantify the performance improvement. Two types of SOI substrates are used for comparison: i) high resistivity SOI (HR-SOI) substrate and ii) trap rich SOI (TR-SOI) substrates. These are the most advanced substrates for RF applications on SOI-CMOS and are described in detail in [START_REF] Maleville | Engineered substrates for Moore and more than Moore's law: Device scaling: Entering the substrate era[END_REF]. Many performance improvements are observed for different RF circuits as outlined in Table 6. For isolation structures, there is improvement in isolation (measured by ∆S21). A reduction of S21 by 7 dB and 5 dB at a frequency of 5 GHz for HR-SOI and TR-SOI respectively signifies that the stray coupling through the substrate is reduced after substrate removal. For RF switch, there is improvement in large signal linearity (2 nd and 3 rd harmonics) and also small signal losses (insertion and return loss) on both substrate types. The 2 nd Harmonic improvement of ~23 dB and insertion loss improvement of 0.4 dB on HR-SOI substrate are the most significant results. Better performance of the switch can be attributed to elimination of substrate coupling in the OFF branches of the switch, which is the source of performance degradation. The Q-factor of inductors is improved significantly on membranes. A near doubling of Q-factor is observed for single turn inductor on HR-SOI substrate indicating amelioration of substrate losses. Such inductors with improved Q factor can help boost the performance of different RF circuits. For instance, inductor membranes have been used in LNA circuits for improvement of noise figure and linearity (IIP3). A 0.1 dB improvement in noise figure and 0.5 dB in IIP3 as a result of improved quality factor of inductors successfully demonstrates the applicability of membranes in circuits of practical interest like LNA. Further details about electrical characterization of different circuits can be found in [START_REF] Bhaskar | Substrate engineering using laser micromachining for improvement of RF devices and systems integrated in SOI-CMOS technology[END_REF].

Conclusion

Large-area femtosecond laser milling of silicon has been reported which employs trench analysis. Five measures have been analysed for the trenches: i) average depth ii) recast layer volume (iii) average roughness (iv) trench width and (v) volume removal rate. Optical profilometry has been systematically used to obtain accurate and reliable measurements of all the parameters under study. Using these analyses, fluence and repetition rate conditions have been found suitable for laser milling. Further study on laser milling of cavities have shown additional factors to be taken into consideration. It has been found that presence of debris and material redeposition affects the quality of milling and suitable process parameters have been outlined. Application of large area milling has been demonstrated for RF circuits in SOI-CMOS technology. Suspended membranes of different circuits like RF switch, isolation structures, inductors and LNA have been successfully fabricated. The obtained performance is better than state-of-the-art SOI wafers for RF technology demonstrating the capability of laser processing for microelectronics applications.

A.3: Debris along the trench axis

The profile taken for a 200 kHz repetition rate and 1030 nm source at a scan speed of 50 mm s -1 and fluence of 12.1 J cm -2 is shown in Fig. A.1. The axial trench profile shows an indication of presence of debris. There are locally deep regions of depth ~25 µm. This is likely the depth at which there is penetration of laser energy and ablation occurs. However, the presence of debris along the axis of the trench can be seen in the form of lower local depths and protrusions above the trench. This causes the measured average depth to be much smaller (~5 µm). It can be also seen that the trench opening is really narrow because of a prominent recast layer. Etched profiles with a periodic distribution of holes are clearly visible for both 343 nm and 1030 nm source at a repetition rate of 200 kHz. It can also be seen that with increase in scan speed, the period increases. For the 1030 nm source at a repetition rate of 30 kHz, no distinct periodic hole pattern is observed.

Fig. 1 .

 1 Fig. 1. (a) Linear translation of laser (b) Cross section of machined line and (c) Top view of machined line

Fig. 2 .

 2 Fig. 2. Average trench depth as a function of fluence for 3 cases at different scan speeds (a) source -343 nm, frep -200 kHz (b) source -1030 nm, frep -200 kHz and (c) source -1030 nm, frep -30 kHz. The dashed lines represent the fitting of points into two different regimes with x-intercept being the threshold fluence

Fig. 3 .

 3 Fig. 3. Recast layer volume as a function of fluence for 3 cases at different scan speeds (a) source -343 nm, frep -200 kHz (b) source -1030 nm, frep -200 kHz and (c) source -1030 nm, frep -30 kHz. . The dashed lines represent the fitting of data points in strong ablation region with x-intercept being the threshold fluence

Fig. 4 .

 4 Fig. 4. Average roughness of trench as a function of fluence for 3 cases at different scan speeds (a) source -343 nm, frep -200 kHz (b) source -1030 nm, frep -200 kHz and (c) source -1030 nm, frep -30 kHz.

Fig. 5 .

 5 Fig. 5. Inner trench width as a function of fluence for 3 cases at different scan speeds (a) source -343 nm, frep -200 kHz (b) source -1030 nm, frep -200 kHz (c) source -1030 nm, frep -30 kHz. The error bars represent symmetric error of one standard deviation.

Fig. 6 .

 6 Fig. 6. Volume removal rate as a function of average power for 3 cases at different scan speeds (a) source -343 nm, frep -200 kHz (b) source -1030 nm, frep -200 kHz (c) source -1030 nm, frep -30 kHz.

Fig. 7 .

 7 Fig. 7. SEM image of cavity surface after ultrasonic cleaning. Milling parameters: 1030 nm; Nscans = 1; Scan speed -20 mm s -1 ; Repetition rate -30 kHz; Pitch -10 μm; Fluence (a) 2.8 J cm -2 (b) 4.7 J cm -2 (c) 12.7 J cm -2 (d) 17.4 J cm -2

Fig. 8 .

 8 Fig. 8. SEM image of the bottom of the trench after cleaning in ultrasonic bath as seen on SEM. Multipass milling (Depth ~ 100 μm) using 1030 nm source with parameters: Scan speed -20 mm s -1 ; Repetition rate -30 kHz; Pitch -10 μm; Fluence/ Number of scans (a) 2.8 J cm -2 /65 (b) 4.7 J cm -2 /28 (c) 12.7 J cm -2 /17 (d) 17.4 J cm -2 /10

Fig. 9 .

 9 Fig. 9. Average depth of cavities milled on the 1030 nm source at repetition rate of 30 kHz for different fluences before and after ultrasonic cleaning (a) Cavities with depth ~100 µm (b) Cavities with depth ~ 200 µm

Fig. 10 .

 10 Fig. 10. SEM image of cavity side walls after ultrasonic cleaning. Milling parameters are: Scan speed -20 mm s -1 , Repetition rate -30 kHz, Fluence/ Number of scans (a) 2.8 J cm -2 /65 (b) 4.7 J cm -2 /28 (c) 12.7 J cm -2 /17 (d) 17.4 J cm -2 /10

Fig. 11 .

 11 Fig. 11. Optical microscope images of two step milling for SOI-CMOS application for (a) RF switch and (b) Isolation structures

Fig. 12 .

 12 Fig. 12. Optical microscope pictures of membranes on SOI wafers obtained by passing backlight through the circuits (a) Isolation structure (b) Inductor (c) RF switch

Fig. A. 3 :

 3 Fig. A.3: Axial trench profile showing narrowing of trench and possible presence of debris along trench axis

Fig. A. 4 :

 4 Fig. A.4: SEM scans of trenches of average depth ~5 µm with different process parameters

Table 1 .

 1 Different cases in trench scribing study

	Case	Repetition rate (kHz)	Average Power (W)	Fluence ( J cm -2 )	Wavelength (nm)
	1	200	0.08 -1.21	0.76 -11.67	343
	2	200	0.18 -2.56	0.86 -12.11	1030
	3	30	0.09 -1.79	2.59 -56.5	1030

Table 2 .

 2 Characteristic depth and corresponding threshold fluence in the two regimes for 3 sets of process parameters (repetition rate and wavelength) at 3 scan speeds. The final column contains threshold fluence extracted from recast layer volume in the strong ablation regime

	Parameter	Speed (mm s -1 )	𝒍 𝒈 𝝁𝒎)	𝝓 𝒕𝒉 𝒈 𝑱 𝒄𝒎 𝟐 )	𝒍 𝒔 𝝁𝒎)	𝝓 𝒕𝒉 𝒔 𝑱 𝒄𝒎 𝟐 )	𝝓 𝒕𝒉 𝑽𝒔 𝑱 𝒄𝒎 𝟐 )
		50	2.34	0.45	8.60	1.62	1.86
	=343 nm frep=200 kHz	70	1.63	0.43	7.72	2.18	2.88
		100	1.26	0.45	6.75	2.74	3.61
		50	2.53	0.5	11.71	2.58	2.12
	=1030 nm frep=200 kHz	70	2.26	0.55	10.81	3.24	4.11
		90	1.91	0.55	9.83	4	5.26
		30	1.91	1.94	11.12	11.73	6.08
	=1030 nm frep=30 kHz	50	1.29	2	5.38	12.25	5.94
		90	0.68	2.34	3.07	12.6	2.96

Table 3 .

 3 Calculated

quantities for milled cavities (single scan) on 1030 nm source at repetition rate of 30 kHz and scan speed of 20 mm s

-1

Parameter Fluence (J cm -2 ) 2.8 4.7 12.7 17.4 Before ultrasonic cleaning

	Average depth of cavity (μm)	1.3	1.5	2.3	4.1
	After ultrasonic cleaning (in IPA for 5 minutes)
	Average depth of cavity (μm)	1.9	2.9	6	8.9
	Average depth of trench profile (μm)	1.9	4.2	6.9	10.4
	Average roughness of trench profile (μm)	0.2	0.4	0.4	0.6
	Average roughness of cavity (μm)	0.2	0.5	0.8	1.2
	RMS roughness of trench profile (μm)	0.2	0.5	0.6	0.7
	RMS roughness of cavity (μm)	0.2	0.7	1	1.5
	Volume removal rate of cavity (μm -3 s -1 )	0.36	0.55	1.14	1.69

Table 4 .

 4 Calculated quantities for milled cavities (multiple scans) on 1030 nm source at repetition rate of 30 kHz and scan speed of 20 mm s-1

	Fluence
	(J

cm -2 ) No of scans Depth (μm) Average roughness (μm) RMS roughness (μm) Efficiency factor VRR* (x10 6 μm 3 s -1 )

	2.8	65	90	9.9	11.3	0.68	0.26
	4.7	28	121.8	3.9	5.1	0.94	0.83
	12.7	17	120.1	4.3	5.5	0.91	1.35
	12.7	34	224.5	5.1	6.4	0.85	1.26
	17.4	10	108.9	4.8	6.1	0.92	2.07
	17.4	20	202.5	5.9	7.3	0.85	1.93
	*VRR -Volume removal rate

Table 5 .

 5 Different parameters in the 2-step fabrication of cavities on SOI wafer for switch and isolation structures

		Switch		Isolation structure
	Parameter	Fast	Slow	Fast	Slow
		step	step	step	step
	Scanner speed (mm s -1 )	30	20	20	5
	Pulse repetition rate (kHz)	25	40	30	2
	Laser power (W)	1.45	0.26	1.006	0.013
	Number of passes	10	30	4	180
	Fluence (J cm -2 )	34.7	8.3	32.1	6.4
	Focus change per pass (μm)	50	-	-	0.6
	Cavity Depth (μm)	450	185	100	85
	RMS roughness (μm)	15.5	12.8	8.3	6.1
	Removal rate (x 10 6 μm 3 s -1 )	6.87	0.35	4.75	0.02

Table 6 .

 6 RF performance improvements obtained on membranes on two different types of SOI wafers

			Improvement on membrane
	Component	Parameter
			HR-SOI	TR-SOI
	Isolation structures	∆S21 @ 5 GHz	7	5
		2nd Harmonic (dBm)	22.7	5.6
	Switch	3rd Harmonic (dBm) Return Loss (dB)	7.8 4	NI 4
		Insertion Loss (dB)	0.4	0.3
	Inductors	Q-factor (1 turn) (%) Q-factor (2 turns) (%)	90 35 -60	80 75 -85
	LNA (using membranes of inductors)	Noise Figure (dB) IIP3 (dBm) P1dB (dBm)	0.1 0.5 NI	---
	NI -No improvement

Acknowledgements

This work was supported by the French government through the National Research Agency (ANR) under program PIA EQUIPEX LEAF ANR-11-EQPX-0025, the STMicroelectronics-IEMN common laboratory and the French RENATECH network on micro and nanotechnologies. The authors also thank Oxford Lasers Ltd for helpful discussions on instrumental developments and laser processing techniques.

The trench profile roughness is determined from the axial trench profile using the formula [START_REF] Wu | Femtosecond laser-induced periodic surface structure on diamond film[END_REF]. The inner and outer trench width correspond to the measured excluding and including recast layer width respectively. The recast layer volume is the entire volume projecting over the substrate level and normalized to unit length of scribed trench.


Sign. Wh>W Wh=W Dry matter (%) N.S. 52

Mode

Sign. Control 41 g/kg 32 g/kg

Level

Electrostatic repulsion between caseins ↗

A cheese variety is partly characterized by its functional properties, such as its rheological and cooking properties. Semi-hard cheese varieties like Raclette present a soft and homogenous matrix with good melting properties. Curd washing is a crucial manufacturing stage for the development of these properties because it allows to manage the early composition of the aqueous phase of the cheese curd, mainly the lactose content. Nevertheless, it is an expensive stage of the manufacture because it needs addition of water, which increase the amount of water consumed and the amount of whey to manage for the factory, whose management is expensive in energy. This stage of the cheese manufacture could be optimized through two parameters, among others, we chose to study. The first is the level of curd washing (the intensity of dilution) that could be adjusted according to the desired properties of the cheese. The other is the mode of curd washing, i.e. the volumes of whey withdrawn and water added to the vat to reach the level that could be adjusted to reduce water consumption and the volume of whey produced. However, the optimization of these parameters requires a better understanding of their impacts, which are poorly described in the literature.

The aim of this study was to determine the impact of the level and mode of curd washing on the functional properties of cheese and to describe the mechanisms by which these parameters contribute to the development of these properties. These parameters have been studied in the context of cheeses with similar dry matter contents to specifically study the impact of the modification of the aqueous phase by curd washing.

Impacts of the level of the curd washing only

The increase of the level of curd washing led to a matrix with:

• A higher mechanical resistance (𝝈f)

• A higher deformability (𝜺f)

• A lower stiffness (MD)

• A greater elasticity • A greater stretchability • A lower meltability

In the specific framework of our experiment (similar dry matter contents), these 4 variables were affected by the level of curd washing and can explain the impact on the functional properties of the ripened cheese.

The 4 variables reflect 4 different types of interactions structuring the cheese matrix. Changing the number and strength of each type of interaction by changing the level of curd washing affects the microstructure of the cheese and therefore its functional properties.

N o t m ea su re d : h yp o th et ic al re su lt

Conclusion

At a determined dry matter content, the modification of the composition of the aqueous phase of the curd by the level of curd washing affects the microstructure of the ripened cheese through variables such as pH, the content of INSOL Ca, the proportion of degraded caseins and the content of hydrophobic peptides. Hence, the increase of the curd washing level leads to the development of a more deformable matrix with a higher mechanical resistance. Since the curd washing mode has no impact on all these variables, our study opens up the prospect of controlling the development of the functional properties of cheese while economically optimizing this stage of production with the mode.

Impacts of the level of the curd washing only

IDF International Cheese Science and Technology Symposium 2021 (June 7 th -11 th 2021)

  Fig.1 : Evolution of the pH of cheeses during the ripening

a

  pH increased during the ripening in cheese due to: • Deacidification by ripening microflora • Alcaline molecules (NH3) released by proteolysis pH increased faster in the 32 g/kg cheeses (Fig. 1) because of: • Lower [lactates] • Faster proteolysis (higher pH enhanced the enzymatic activities) • Faster release of NH3 • Lower capacity to buffer the NH3 26.1 c 37.8 b 80.0 a N.S. 59.6 a 58.2 a 𝝈f (2 a 30.5 ab 44.9 b N.S. 45.0 a 30.Elasticity (kgF) *** 0.7 b 0.9 b 1.3 a N.S. 1.1 a 1.0 a Impacts of curd washing on the development of the functional properties of Raclette cheese 3 levels of curd washing (Targets of residual lactose content in the curd/whey mixture before the drainage of the vat) 2 modes of curd washing (Wh = % of whey removed W = % of water added )

  kg (Wh = W) N.S: Not Significant ; *: p<0.05 ; **: p<0.01 ; ***: p<0.001

Fig. 2 :

 2 Fig.2 : Multifactorial analysis of the rheological (•) and cooking properties (+) of cheese

Table 1 :

 1 Composition on day 1 according to the curd washing parameters

		During the manufacture
	[Lactose] in curd ↘	Increasing level of curd washing leads to: • An increase in osmotic pressure differences between the aqueous phase of the curd and the whey
		è Curd's lactose content decreased because of its diffusion in the whey
		During the acidification
		• Acidification stopped when lactose was limiting (in the 32 g/kg cheeses on
		day 1; Table 1)
	pH	• pH stopped decreasing
	↗	• INSOL Ca stopped solubilizing
		• Removal of solutes (whey prot., organic acids, mineral…) decreased the
		buffering capacity of the cheese (Table 1)

Increased level of curd washing Control è 41 g/kg è 32 g/kg

On day 1 • The earlier the acidification stopped:

èThe higher the pH of the cheese and the higher its [INSOL Ca] (Table

1)

of the stirring time to reach similar drainage levels Analyses

		Experimental cheesemaking
	18 vats of 300 L of milk	Control (50 g/kg) (without curd washing) None Technological goal : similar dry matter content (52 %) at the beginning of the ripening 41 g/kg (« Moderate » curd washing) 32 g/kg (« Extreme » curd washing) Wh>W Wh=W Wh>W Wh=W è Adjustement Analyses Lactose Lactates INSOL Ca pH Buffering capacity SN/TN, NPN/TN Hydrophobic peptides Rheology (15 °C) Meltability (225 °C) Threading ability (60 °C) Elasticity (60 °C)	Methods HPAEC-amperometric detection HPLC-UV detection Ionic chromatography-conductivity detection penetrometry Acid/base titration ISO 27871 standard RP-HPLC (relative area of the peaks at the end of elution) Unaxial compression tests Schreiber test Threads length measurements Extrusion resistance force
		Number of
		interactions
		between
		caseins
		↗

Table 2 :

 2 Physico-chemichal caracteristics of ripened cheeses (60 days of ripening)

	N.S: Not Significant ; *: p<0.05 ; **: p<0.01 ; ***: p<0.001

Table 3 :

 3 Rheological and cooking properties of the ripened cheeses

	Introduction
	Materials & methods

Results Discussion Émilien Delorme a,c , Benoît Paysant a , Philippe Trossat a , Yves Gaüzère b , Éric Beuvier c , Christine Achilleos c

  ACTALIA Pôle Expertise Analytique, 39800, Poligny, France ; b ISBA Franche-Comté, ENILBio, 39800, Poligny, France ; c INRAE, URTAL, 39800, Poligny, France

a


Centre Île-de-France -Versailles-Grignon

The oxidoreduction potential (Eh) of cheeses may affect the balance of volatile compounds that determine their flavour, via an effect on cheese microbiota. However, only few studies are available on this topic. We explored the effect of Eh on the production of volatile compounds by several lactic acid bacteria (LAB) in cheese-like conditions, via inoculations in a curdbased medium under O 2 or N 2 .

19 LAB strains inoculated in a curd-based medium. 2 Eh conditions applied for each strain: 1 inoculation in the presence of O 2 (O2), 1 inoculation in an anaerobic chamber filled with N 2 (N2). 4 vials / strain and Eh condition, incubated 14 days at 26°C. 2 vials analysed at 0 and 14 days for microbial counts and volatile compounds composition (SPME/GC/MS).

Curd-based medium: 20h-semi-hard cheese ground in water (1:2) with NaCl 18g/L, peptone, lactose 1.2g/L, sterilized 15 min at 110°C [1].

Material and methods Introduction

Results

C

Principal Component Analysis of 14 days-volatile compounds affected by oxygen concentration (positively by O2 in red, N2 in blue) according to ANOVA

FHL Lc

Oxidoreduction effect on aldehydes, acids and sulphur compounds was already previously observed for Lc in a synthetic amino acids-based medium [2]. The present study in a curd-based medium revealed an additional effect, in particular on FHL, on alcohols and ketones. The present results on the intra-species diversity are complementary to results of [3] that showed the effect of a longer incubation time and of O2/N2 on oxidoreduction potential values.

[1]. Pogacic et al. (2015). Food Microbiol., 46, 145. [2]. Kieronczyk et al. (2006). J. Appl. Microbiol., 101, 1114. [3]. Buchin et al. (2021). 16th Weurman Flavour Research Symposium, 4-6 May, On-line, poster 3.21

Conclusion References

Study funded by Bourgogne-Franche-Comté region

Lcl09

Lc counts at 0 and 14 days (in ufc/mL) revealed an inoculation at 10 7 -10 8 cells/mL, then a decrease, faster with O2 (apart Lcl07 and Lcl08)

In red: O2, in blue: N2 Oxygen concentration showed globally similar effects on 14 days-volatile compounds according to the species with :

A total of 47 and 49 volatile compounds were extracted for Lc and FHL respectively. The first PCA axes show the diversity between strains and the second axes the differences between N2 and O2.

Apart Lc07-O2 that showed a peculiar profile with large amounts of volatile fatty acids and low contents of aldehydes, volatile profiles were less diversified between strains in Lc than in FHL. This may be linked to different growth dynamics.

-Globally, for Lc, 5 compounds were higher with N2 (2 alcohols, 1 ketone, 2 reduced sulphur compounds ) and 7 with O2 (3 ketones, 2 oxidized sulphur compounds, 1 volatile fatty acid, 1 aldehyde, 1 primary alcohol). -For FHL, 6 compounds were higher with N2 (4 alcohols, 1 aldehyde, 1 reduced sulphur compound) and 10 with O2 (6 ketones, 2 volatile fatty acids, 2 oxidized sulphur compounds).

9

  strains of Lactococcus lactis (Lc): 3 subsp. cremoris (Lcc01, Lcc02, Lcc03) + 6 subsp. lactis (Lcl04, Lcl05, Lcl06, Lcl07, Lcl09) 10 strains of facultative heterofermentative lactobacilli (FHL): 3 Lb casei (C01, C03, C05), 3 Lb paracasei (PC02, PC04, PC07), 2 Lb plantarum (PL06, PL09), 2 Lb rhamnosus (R08, R10)

  at 0 and 14 days (in ufc/mL) revealed an inoculation at 10 6 -10 7 cells/mL, then a trend to increase, but diverse dynamics between strainsIn red: O2, in blue:


Sylvie Pochet
email: sylvie.pochet@inra.fr

Céline Arnould

Perrine Debournoux

Jocelyne Guinard-Flament

Eric Beuvier

A simple method to quantify lactoferrin in milk by Micro-Batch Resin Extraction (MBRE) before RP-HPLC

Comparison with immunological methods

Separative techniques (HPLC, capillary or SDS-PAGE) were rarely used for the quantification of lactoferrin (Lf) in milk compared to immunochemical methods (ELISA, RID, biosensing) which were found more specific and sensitive, but also more expensive and moderately reproducible. We present here a new HPLC-based method validated for the quantification of lactoferrin in bovine milk. Lactoferrin, due to its cationic character (pI > 8), is positively charged at the pH of native milk (6,8-7) whereas almost all caseins and whey proteins (pI < 5,6) are negatively charged. This difference is used to specifically and quantitatively adsorb Lf on a strong cationexchange resin assay developed at a micro-batch scale. Lf is then desorbed increasing ionic strength without any dilution as compared to its concentration in milk. Finally, Lf is fully separated from other co-extracted minor cationic substances and quantified by conventional RP-HPLC with UV detection. The extraction using resin replaced the acid precipitation of caseins which was used in most HPLC-based methods causing loss of lactoferrin and keeping whey proteins in sample extract that interfered with Lf in HPLC.

Lf Adsorption

Rinses

Lf Desorption RP-HPLC Weigh 100 mg resin 1 + 1 mL skim milk 2 60 rev.min -1 , 90 min, RT 3 20 800 g, 5 min, RT Discard supernatant + 1 mL water Vortex until dispersion 10 µL supernatant (then 10 µL NaCl 2M) C4, 5µ, 300 Å, 150 x 4,6 mm (+ precol.) A: TFA 0,1% in water B: TFA 0,1% 80% MeCn 20% water T 0min =20% B, T 10 =66%, T 12 =90%, T 13 =90%, T 14 =20%, T 19 =20%. 1 mL.min -1 , 30°C UV: 220 nm (Lf and Phe) Fluo : Calibration : 11 pts 10-500µg.mL -1 NaCl 2M 60 rev.min -1 ,30 min, RT 20 800 g, Up and down, RT Keep supernatant + 1 mL NaCl 2 M with 200 µg.mL -1 phenylalanine

In-lab validation by accuracy profile ISO 5725

The method competes well with ELISA-based method considering especially its lower cost and better precision. The method was developed using bovine milk (50-350 μg.mL-1 Lf in the literature) but could probably be applied to other milk species and other fractions such as colostrum or acid serums or of cheese by means of an adaptation of the resin / sample ratio and some checks.

Fortified

  SPEC 70 SLS (Pall Biosepra) : semi-rigid microbeads of acrylic copolymer (260 to 600 µm) grafted with sulfopropyl residues, binding capacity: 20 mg Lf. mL -1 . Equilibrated in 20 mM phosphate buffer pH 6,8. See also Liang et al., 2011. Int. J. Env. Res. and Public Health, 8 3764-3776. 2 -1000 g, 30 min, 4-5°C 3 -RT: Room Temperature (22,5°C ± 1) 98 %), precision (intra-day : <3%, inter-day: <4%) LOD SN = 2 µg/mL, LOQ SN = 6,7 µg/mL but taking account of precision (accuracy profile) : LOQ = 25 µg/mL (λ=20%, β=95%) Fluorimetric detection is not recommended because of poor reproducibility

-

  All combinations of concentration of NaCl (2, 2.25 and 2.5 M) in desorbing solution and time of desorption (30, 60 and 90 min) gave non statistically different results. -Thirty samples can be extracted each day (limited by the centrifuge capacity) and HPLC analyses are then performed automatically (Lf in desorption solution was stable at least 10 h on the autosampler at the 12°C).ConclusionsThe proposed method is simple, flexible, cheap, specific, sensitive and accurate. It requires no particular skill for good precision. It is an improvement over previously published HPLC-based methods as being more accurate (no loss of lactoferrin due to sample preparation and no interference with whey proteins) and is more sensitive (no dilution during sample preparation).

9 104.9 103.0 101.1 100.3 98.1

	20 800 g, 1 min, RT
	Discard supernatant
	Repeat 4 times
							Levels
					K1	K2	K3	K4	K5	K6
		Concentration (target)	µg.mL -1	28.4	56.4	103.3 207.1 306.6 514.4
		Accuracy	Concentration (measured)	µg.mL -1	31.5	59.1	106.3 209.3 307.5 504.9
	UV	Recovery rate 110.Precision % Within-day RSD (S r ) % 1.16 Between-days RSD (S b ) % 3.35	1.97 1.93	2.67 0.00	1.33 1.59	1.78 1.67	0.91 1.47
			Intermediate RSD (S I )	%	3.55	2.76	2.67	2.08	2.44	1.73
			Expanded uncertainty (U)	%	8.57	6.15	5.68	4.49	5.20	3.65
	Fluorimetry	Accuracy Precision	Concentration (measured) Recovery rate Within-day RSD (S r ) Between-days RSD (S b ) Intermediate RSD (S I ) Expanded uncertainty (U)	µg.mL -1 % % % % %	31.7 111.6 106.2 103.6 100.7 59.9 107.0 208.6 304.5 503.6 99.3 97.9 1.31 4.61 2.57 3.13 2.66 0.80 13.7 5.98 4.64 2.86 6.40 7.84 13.7 7.55 5.31 4.24 6.93 7.88 33.5 17.2 11.9 9.07 15.0 16.9


HAL is

Conclusion

Laboratory curd yield is better predicted than milk coagulation properties whatever the dataset used. For all parameters, the best performances were obtained on individual milks with equations developed on the entire dataset. For herd milks and dairy vat milks, specific datasets must be used to have the best equation. However, performances were always lower than those established for individual milks. Cheeses will be manufactured to compare CMAP in laboratory conditions or estimated by MIR and actual CMAP.

RPD (Viscarra Rossel et al., 2006): Excellent : > 2.5, Very good : between 2 and 2.5, Good : between 1.8 and 2, Fair : between 1.4 and 1.8, Poor : RPD between 1 and 1.4

R

  Prediction of cheese-making properties of Montbéliarde milks used for PDO/PGI cheeses production in Franche-Comté by mid-infrared spectrometry Prediction of cheese-making properties of Montbéliarde milks used for PDO/PGI cheeses production in Franche-Comté by mid-infrared spectrometry

FROM'

  MIR study aimed to examine the feasibility of predicting, by mid-infrared spectroscopy (MIRS), cheese making properties (CMAP) of French Montbéliarde milks from the Franche-Comté cheese production area (4 PDO + 2 PGI cheeses).Method: Samples collection and analyses ; calibration equationsResults: Fitting statistics of prediction models in external validation (not shown: equations with R 2 <=0.5)

  ENILBio de Poligny, France ; 6 Centre Technique des Fromages Comtois, France ; 7 GABI, INRA, Jouy-en-Josas, France

C. Laithier 1 , M. El Jabri 1 , V. Wolf 2 , P. Trossat 3 , S. Gavoye 3 , D. Pourchet 1 , P. Grosperrin 1 , E. Beuvier 4 , O. Rolet-Répécaud 4 , Y. Gaüzère 5 , O. Belysheva 5 , E. Notz 6 , A. Delacroix-Buchet 7 1 Institut de l'Elevage, France ; 2 Conseil Elevage 25-90, France ; 3 Actalia, France ; 4 URTAL, INRA, Poligny, France ; 5