Structural and Mechanical Properties of Polycrystalline ... - IEEE Xplore

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 7, NO. 4, DECEMBER 1998

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Structural and Mechanical Properties of Polycrystalline Silicon Germanium for Micromachining Applications Sherif Sedky, Paolo Fiorini, Matty Caymax, Stefano Loreti, Kris Baert, Lou Hermans, and Robert Mertens, Fellow, IEEE

Abstract— In this paper, we propose polycrystalline silicon germanium (poly SiGe) as a material suitable for MEMS applications. Films are prepared by chemical vapor deposition (CVD) at atmospheric pressure (AP) or reduced pressure (RP). The structure of the films is investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) for different deposition and annealing conditions. The stress in the as-grown and annealed layers is measured, and the correlation between stress and structural properties is discussed. It is demonstrated that by adjusting the deposition conditions, the stress of the asgrown material can be varied from 0145 to +60 MPa. Examples of poly SiGe micromachined devices, prepared at 650 C, are presented. It is shown that by using as-grown poly SiGe, it is possible to realize surface-micromachined suspended membranes having 0.6-m-wide and 50-m-long supports. The effect of the average stress and stress gradient on the mechanical stability of surface-micromachined structures is illustrated. Finally, the strain in poly SiGe is measured, and it is found to vary, according to the deposition conditions from 06.88 2 1004 to 3.6 2 1004 . These values are compared to those measured for APCVD poly Si. [358] Index Terms—Bolometers, poly SiGe, stress, surface micromachining.

I. INTRODUCTION

S

URFACE-micromachining techniques are now used in the fabrication of a wide variety of devices. Examples are infrared (IR) detectors (namely, bolometers), gas sensors, accelerometers, microturbines, microrelays, microgyroscopes, etc. These devices are based on the fabrication of a suspended structure that performs the main function, for example, it provides thermal insulation in IR bolometers [1], generates lateral deflections in accelerometers [2], or produces vibrations in gyroscopes [3]. The magnitude and sign of stress of the material used for the fabrication of the suspended structures severely affects their mechanical stability. Some applications require stress-free or low-tensile stress materials, and others require compressive stress. Stress can be controlled either by the deposition conditions or by subsequent annealing. Manuscript received June 23, 1998; revised September 24, 1998. Subject Editor, R. T. Howe. S. Sedky, M. Caymax, K. Baert, L. Hermans, and R. Mertens are with the Interuniversity Microelectronics Center (IMEC), B3001 Leuven, Belgium. P. Fiorini is with Unita’ INFM and the Dipartimento di Fisica “E. Amaldi,” Universita’ di Roma III, 00146 Rome, Italy. S. Loreti is with C. A. Enea, Portici 80055, Italy. Publisher Item Identifier S 1057-7157(98)09434-7.

Poly SiGe alloys have a lower melting point than poly Si, hence, physical phenomena like grain growth, changes in morphology with annealing, dopant activation, and diffusion are expected to occur at a lower temperature than in poly Si. The above properties, together with the similarity of the electrical behavior of these two polycrystalline materials, makes poly SiGe a suitable material for microelectronics applications where a low-thermal budget is required [4]. In this work, it is also demonstrated that for poly SiGe, the stress can be controlled at lower temperatures than in poly Si. This feature reduces the thermal budget for the preparation of micromachined devices and minimizes the influence of the fabrication process on the functionality of any electronic circuit already present on the substrate. Also, the low-thermal conductivity of poly SiGe [5] makes it attractive for the fabrication of those micromachined devices that require highthermal insulation (as uncooled IR detectors [1]). The paper is organized as follows. In Section II, we briefly describe the deposition conditions. In Section III, the structural characteristics are presented: texture and grain size are investigated by XRD and TEM. Section IV is devoted to stress measurements. Values of stress in poly SiGe films are reported for different deposition conditions and different annealing temperatures. For comparison, data referring to poly Si film is also be reported. In Section V, some surfacemicromachined structures, based on poly SiGe, are presented and the influence of the average stress and stress gradient on their mechanical stability is discussed. The strain in poly SiGe deposited under different conditions is experimentally determined. In Section VI, conclusions are drawn. II. SAMPLE PREPARATION Poly SiGe samples were grown in an ASM EPSILON I chemical vapor deposition (CVD) reactor. It consists of a horizontal lamp-heated quartz chamber with a SiC-coated graphite susceptor. The layers were grown on 6-in silicon wafers covered on both sides by 1 m of thermal oxide. The choice of this kind of substrate is due to the fact that thermal oxide can be etched selectively with respect to poly SiGe, thus, it can be used as a sacrificial layer for surface micromachining applications. For the deposition, we have used a mixture of germane and dichlorosilane as germanium and silicon gas sources respectively. The samples have been grown

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Fig. 1. X-ray diffraction pattern of as-grown 1-m-thick poly SiGe. (a) APCVD poly SiGe deposited at 650 C. (b) RPCVD poly SiGe deposited at 625 C:

under two different pressures: atmospheric pressure (AP) and reduced pressure (RP) of 40 Torr. The deposition temperature for APCVD poly SiGe is 650 C and 625 C for RPCVD poly SiGe. The growth rate under the above conditions was 37 and 17 nm/min for APCVD and RPCVD, respectively, and 1- m-thick films were deposited. The amount of germanium incorporated into the poly SiGe film has been determined using Rutherford Backscattering Spectroscopy (RBS), and it has been found to be 26% in APCVD poly SiGe and 22% in RPCVD poly SiGe. Due to the presence of chlorine containing compounds in the gas mixture, nucleation on an oxide surface is rather difficult, resulting in a noncontinuous layer composed of isolated big grains. Therefore, a thin fine-grained Si nucleation layer has been used to favor the deposition. To study a possible impact of the nucleation layer on the structural properties of the poly SiGe films, it have been prepared in two different ways: 1) in situ from silane at 600 C (10 nm thick) and 2) in a lowpressure chemical vapor deposition system (LPCVD) at 590 C (50 nm thick). For comparison, poly Si films have been also prepared from dichlorosilane at atmospheric pressure and at a temperature of 850 C using a nucleation layer as 1). III. STRUCTURAL PROPERTIES OF POLY SIGE FILMS To study the crystal structure of both as-grown and annealed APCVD and RPCVD poly SiGe films, X-ray diffraction configuration, (XRD) has been performed in a standard radiation. In Fig. 1, the XRD pattern of using the Cu as-grown samples is reported. Both peaks due to the C-Si substrate and peaks characteristic of a material with a diamond structure can be seen. Also, there are no dual peaks, indicating that the deposited film is a homogeneous SiGe alloy, rather than clusters of Ge or a Ge rich material embedded in a Si matrix. From the position of the peaks, lattice parameters ˚ can be derived for APCVD and of 5.4864 and 5.4829 A RPCVD, respectively. Both of them lie between those of Si and Ge and correspond to a Ge concentration of 25% and 23%, respectively, in good agreement with RBS data. The pattern of the APCVD sample is dominated by the line, whereas in the RPCVD pattern the

and lines are of considerable intensity. The strong texturing of the APCVD samples, derived from XRD, points to a columnar structure of the film. This will be confirmed by TEM analysis. For RPCVD samples, the and lines are intensity ratio of close to those of a randomly oriented powder, indicating the coexistence of a columnar structure and of randomly oriented grains. The presence of randomly oriented grains can be ascribed to the lower deposition temperature used for RPCVD samples, which is close to the amorphous-polycrystalline transition temperature. Thus, the grains grow in the amorphous phase and recrystallize in the furnace. From the full width at half maximum (FWHM) of the XRD peak, the length of the scattering domain can be determined line, we using the Debye Scherrer formula. For the found 120 and 70 nm for APCVD and RPCVD samples, respectively. As XRD is sensitive to the grain dimension perpendicular to the surface, the above evaluation confirms that the columnar structure is more developed in APCVD samples. For the other lines, we found a scattering domain of 16 nm for APCVD and of 50 nm for RPCVD. These values suggest that the randomly oriented part in APCVD is basically an interconnective tissue in between columns, with very small grains, while in RPCVD samples, columnar and random grains are of comparable size. Transmission electron microscopy (TEM) has been used to determine the grain size and the crystal microstructure. In Fig. 2, a TEM cross section of as-grown APCVD and RPCVD poly SiGe is presented. The electron diffraction pattern in the inset reveals a ring-and-dot pattern, which is a characteristic of a polycrystalline structure. The dots along the inner ring are clustered, which confirms the existence of a dominant orientation in APCVD poly SiGe, as detected by XRD. From the TEM cross sections, it is obvious that the grain structure of APCVD poly SiGe is more columnar than that of RPCVD poly SiGe, which is in correspondence with the large orientation (cf. Fig. 1). Such columnar intensity of the structure results in a relatively high-compressive stress [6], as we illustrate later. The crystallites of APCVD poly SiGe have a V appearance, starting from the SiO interface and ending at the film surface. On the other hand, the crystallites of RPCVD poly SiGe have a tooth-like shape, and the grain size does not vary significantly along the growth direction. The average grain size was determined using the grain-boundary-crossing technique [7], and it was found to be 220 and 340 nm for asgrown APCVD and RPCVD poly SiGe, respectively. These results do not contradict those found by XRD. As a matter of fact, they refer to grain dimensions in a direction parallel to the growth surface, whereas XRD explores mainly a direction normal to the growth surface. The TEM cross sections presented in Fig. 3 and the XRD patterns displayed in Fig. 4 illustrate the effect of annealing at 1050 C for 30 min on the structural properties of RPCVD and APCVD poly SiGe. From Fig. 3, we notice that the crystal structure of RPCVD poly SiGe is nearly unaffected by annealing temperatures up to 1050 C, which is consistent with the larger grains observed in the as-grown material. Meanwhile, the structure of APCVD poly SiGe is significantly

SEDKY et al.: STRUCTURAL AND MECHANICAL PROPERTIES OF GERMANIUM

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(b) Fig. 2. Cross-sectional TEM micrograph of as-grown 1-m-thick poly SiGe. (a) APCVD poly SiGe deposited at 650 C. (b) RPCVD poly SiGe deposited at 625 C: The inset represents the electron diffraction pattern for as-grown APCVD poly SiGe.

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(b) Fig. 3. Cross-sectional TEM micrograph of 1-m-thick poly SiGe annealed at 1050 C. (a) APCVD poly SiGe and (b) RPCVD poly SiGe.

changed, the columnar structure is transformed to a large grain structure and the average grain size is increased to 400 nm. The above structural modifications are also reflected in the XRD patterns displayed in Fig. 4. For RPCVD samples, the pattern before and after annealing is basically unchanged. The only difference is a slight decrease in the line width associated with the random phase. This decrease corresponds to an increase from 50 to 70 nm in the average grain size. For APCVD, however, we observe that the intensity of the line is decreased by a factor of two and that the more line is present. These two factors point toward intense a decrease in the oriented phase and to an increase in the random one. This behavior is also reported for LPCVD poly Si deposited at 620 C and annealed at temperatures higher than 1100 C [8], which illustrates that recrystallization occurs at lower temperatures in APCVD poly SiGe than in poly Si. The FWHM of the lines corresponding to the random phase is strongly decreased, calculations show that the scattering domain is increased from 16 to 150 nm.

IV. STRESS

IN

POLY SIGE FILMS

The stress of the layers has been determined using an Eichorn & Hausmann MX 203 stressmeter, which derives the stress from the curvature of the wafer before and after deposition. We have assumed that the thin nucleation layer prepared at atmospheric pressure does not appreciably perturb the determination of stress. Also, the thicker nucleation layer prepared by LPCVD is not influencing measurements, as it has been deposited on both sides of the wafer. The stress has been measured on films prepared at atmospheric and reduced pressure. As-grown or annealed films have been used. Annealing has been performed for 30 min in a nitrogen atmosphere at temperatures up to 1050 C To avoid systematic errors, a silicon control wafer has been kept in the deposition system, at the deposition temperature, for a time equivalent to the one of the growth and then annealed at the highest temperature. No appreciable changes in the curvature of the wafer were detected in this case.

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(b) Fig. 4. X-ray diffraction pattern of 1-m-thick poly SiGe films annealed at 1050 C. (a) APCVD poly SiGe and (b) RPCVD poly SiGe.

The stress in thin films is the sum of thermal and intrinsic stress. Thermal stress is caused by a difference in the expansion coefficient of the substrate and of the thin film. Intrinsic stress has two components: the first originates from volume contraction associated with crystallization and is tensile. The second component is compressive and is due to the existence of a preferred growth orientation [6], disorder at the grain boundary [8], effects related to different deposition rates [9], or the incorporation of impurity atoms [10]. It has been verified that the dominant stress component is the intrinsic one [11], which is greatly affected by the film structure. The measured values of the stress for as-grown and annealed samples are reported in Fig. 5 as a function of growth or annealing temperature. We note that the stress is compressive ( 145 MPa) in as-grown APCVD poly SiGe and tensile ( 60 MPa) in as-grown RPCVD. The compressive stress of as-grown APCVD poly SiGe might be due to highly disordered grain boundaries which are typically associated texture [6]. This compressive stress with the dominant is almost a factor of three lower than that of typical LPCVD poly Si deposited at a pressure of 0.1 Torr and at a temperature of 620 C [12]. In Fig. 5, stress data for APCVD poly Si deposited at 850 C are also reported (diamonds). We note that stress values, similar to those of APCVD poly SiGe, are

Fig. 5. Dependence of stress on annealing temperature ( APCVD poly SiGe grown at 650 C, RPCVD poly SiGe grown at 625 C, and APCVD poly Si grown at 850 C):

obtained only at higher temperatures in APCVD poly Si. This shows that the presence of the germanium atoms lowers the temperature at which the same structural order is obtained in APCVD poly SiGe and poly Si. We also found that the stress in APCVD poly SiGe is nearly the same (within 5%) independently of the nucleation layer used. This shows that the nucleation layer does not play a significant role in determining the structure of the film. The magnitude of stress in as-grown RPCVD is lower than that of APCVD poly SiGe, and this might be due to the presence of both randomly oriented and columnar grains. The measured tensile stress indicates that the layer is deposited in the amorphous state and then crystallizes in the furnace. Crystallization results in contraction against the boundary constraints, inducing a tensile stress, which can be approximated as the radial traction, applied at the edge of the imaginary unconstrained crystallized film, required to restore it to its original amorphous diameter. As for the dependence on the annealing temperature, it is clear that the stress of RPCVD poly SiGe is nearly independent of this parameter. This is consistent with the fact that only slight changes are brought about to the structure of the film, by annealing, as determined from the XRD patterns and the TEM cross sections. Meanwhile, the stress of APCVD poly SiGe strongly depends on annealing. By increasing the annealing temperature, it starts to decrease due to the motion of dislocations in the direction of the stress gradient [12]. At temperatures close to 950 C, the stress becomes tensile; this is due to recrystallization, which, as we have already discussed, increases the tensile stress. As for APCVD poly Si, the decrease in stress, due to the motion of dislocations, is observed, but the stress does not become tensile as poly Si recrystallization occurs only above 1050 C [7]. V. POLY SIGE MICROMACHINED STRUCTURES In this section, we describe some micromachined structures realized by using poly SiGe. The purpose of this presentation is twofold: 1) analyzing their shape and their deformation to obtain further information on stress and stress gradient and 2) demonstrating that poly SiGe allows fabricating sus-


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(c) Fig. 6. SEM picture of cantilevers fabricated by using 1-m-thick poly SiGe. (a) As-grown APCVD poly SiGe. (b) As-grown APCVD poly Si. (c) As-grown RPCVD poly SiGe.

pended micromachined structures at a relatively low temperature (650 C). The first type of structures that we analyze is cantilevers. For their fabrication, poly SiGe has been deposited onto a thermal oxide layer and then patterned in the appropriate shape. To release the cantilevers, the thermal oxide has been etched in HF. To avoid stiction during the drying process, we used sublimation of cyclohexane at 5 C Sublimation is performed at atmospheric pressure on a cold plate under a continuous flow of nitrogen. The nitrogen flow enhances the sublimation process by removing the cyclohexane vapor. It also prevents water condensation. After the sublimation process is completed, the wafer is heated to room temperature. Fig. 6 shows scanning electron microscope (SEM) photographs of three cantilevers 10 m wide and 100 m long, fabricated using APCVD poly SiGe, APCVD poly Si and RPCVD poly SiGe respectively. For the analysis of these cantilevers, we recall that: 1) the initial slope of a cantilever is determined by the mean value of the stress, and is positive (negative) for tensile (compressive) stress and 2) the bending of the free end goes in the same direction of the stress

gradient [13]. By investigating the profile of APCVD poly SiGe and poly Si cantilevers, we notice that the initial slope is negative and in agreement with the measured compressive stress (cf. Fig. 5). The free end of APCVD poly SiGe bends toward the substrate, which means that the upper layers are more compressed than the lower ones. This stress gradient might originate from the variation in the grain size along the growth direction, whereas the initial slope of APCVD poly Si cantilever is in agreement with the measured compressive stress and the free end is bending upwards indicating that the lower layers are more compressed than the upper ones, which is typical for poly Si films [9]. The initial slope of the RPCVD poly SiGe cantilever is slightly upwards because of the low tensile stress. The cantilever is nearly flat, indicating that the stress is uniform along the direction of growth, which we relate to the uniformity of the grains and to the multiple growth orientations. The second structure that we have fabricated is a suspended membrane, connected to the substrate through thin long supports. This structure is used for surface-micromachined IR bolometers [1], where the long supports provide the required

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(c) Fig. 7. SEM picture of IR bolometers fabricated by using 1-m-thick poly SiGe. (a) As-grown RPCVD poly SiGe having 0.6-m-wide and 50-m-long supports. (b) As-grown APCVD poly SiGe having 0.8-m-wide and 50-m-long supports. (c) As-grown APCVD poly SiGe having 1-m-wide and 25-m-long supports.

thermal insulation of the detector. The performance of the device is optimized by minimizing the thickness of the supports and maximizing their length. Fig. 7 displays SEM pictures of IR bolometers realized by both APCVD and RPCVD poly SiGe. The comparison of Fig. 7(a) and (b) clarifies the limitations due to a large mean stress and stress gradient. It is clear that by using RPCVD poly SiGe, it is possible to realize suspended structures having a support width and length of 0.6 and 50 m, respectively [cf. Fig. 7(a)]. Meanwhile, for the same support geometry, using APCVD poly SiGe, the structures are touching the substrate, and this is mainly due to the mean compressive stress and to the stress gradient along the supports [refer to Fig. 7(b)]. To avoid this situation, the supports must be shortened and widened. We found that the optimal support dimensions, for an APCVD poly SiGe 50 m 50- m structure, are 25 m long and 1 m wide [refer to Fig. 7(c)]. This illustrates that the minimum thermal insulation achieved by RPCVD poly SiGe is a factor of four higher than that achieved by APCVD poly SiGe. A third structure, the rotating pointer, has been fabricated with the purpose of determining the strain in poly SiGe. It

has been designed following the guidelines of [14] and is displayed in Fig. 8(a). The pointer is deposited on top of thermal oxide and after it is released, the arms and and will either expand or contract depending on the stress being either compressive or tensile. Consequently, the pointer will move to the right or to the left for tensile or compressive stress, respectively. The strain is related to the pointer deflection by [14] (1) m, m, m, and m. where The prefactor 1.6 in (1) is a correction factor which takes into account the error introduced by the turning point width and by the arm length. It is clear from Fig. 8(b)–(d) that the sign of the stress deduced from the direction of the pointer deflection is in agreement with the one determined from the bow measurements. The average deflection of the pointer, for APCVD poly SiGe, is nearly 0.4 m [cf. Fig. 8(b)], which Meanwhile, corresponds to a strain of for RPCVD poly SiGe, the average pointer deflection is 0.2


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m [refer to Fig. 8(c)], which yields an average strain of The estimated uncertainty on these values is about 10%. It should be noted that the strain in APCVD poly Si is similar to that of APCVD poly SiGe [cf. Fig. 8(b) and (d)]. VI. CONCLUSION

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In this paper, we have proposed poly SiGe as a material suitable for micromachining applications. The effect of the deposition conditions on the structural and mechanical properties of APCVD and RPCVD poly SiGe films has been investigated. It has been demonstrated that the presence of the germanium atoms reduces the deposition temperature required to obtain an ordered structure characterized by low stress. Thus, poly SiGe appears very promising for micromachining applications that have a limited thermal budget. It has been demonstrated that the stress of the as-grown material can be tuned to the desired value by changing the deposition temperature and pressure. The effect of the annealing temperature on the structure of both RPCVD and APCVD poly SiGe has been investigated. It has been shown that RPCVD poly SiGe has a stable structure, which is not affected by annealing temperatures up to 1050 C Meanwhile, the stress in APCVD poly SiGe changes from compressive to tensile at annealing temperatures higher than 900 C The effect of the mean stress and the stress gradient on the stability of surface-micromachined suspended structures has been investigated. It has been verified that RPCVD poly SiGe has a uniform stress distribution along the growth direction. This feature, when combined with the low-tensile stress, makes this material suitable for realizing suspended membranes having supports which are 0.6 m wide and 50 m long. REFERENCES

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(d) Fig. 8. (a) SEM picture of the rotating pointer used to determine the strain in poly SiGe. (b) As-grown APCVD poly SiGe. (c) As-grown RPCVD poly SiGe. (d) As-grown APCVD poly Si.

[1] S. Sedky, P. Fiorini, M. Caymax, A. Verbist, and C. Baert, “IR bolometers made of polycrystalline silicon germanium,” Sens. Actuators A, vol. 66, pp. 1–3 and p. 193, Apr. 1998. [2] J. Mizuno, K. Nottmeyer, T. Kobayashi, K. Minami, and M. Esashi, “Silicon bulk micromachined accelerometer with simultaneous linear and angular sensitivity,” in Int. Conf. Solid State Sensors and Actuators, vol. 2, 1997, p. 1197. [3] W. Geiger, B. Folkmer, U. Sobe, H. Sandmaier, and W. Lang, “New designs of micromachined vibrating rate gyroscopes with decoupled oscillation modes,” in Int. Conf. Solid State Sensors and Actuators, vol. 2, 1997, p. 1129. [4] T. J. King, J. P. McVitte, K. C. Saraswat, and J. R. Pfister, “Electrical properties of heavily doped polycrystalline silicon germanium film,” IEEE Trans. Electron Devices, vol. 41, no. 2, p. 228, 1994. [5] S. Sedky, P. Fiorini, M. Caymax, A. Verbist, and C. Baert, “Thermally insulated structures for IR bolometers, made of polycrystalline silicon germanium alloys,” in Int. Conf. Solid State Sensors and Actuators, vol. 1, 1997, p. 237. [6] L. S. Fan and R. S. Muller, “As-deposited low-strain LPCVD polysilicon,” in Solid State Sensors and Actuators Workshop, 1988, p. 55. [7] T. Kamins, Polycrystalline Silicon for Integrated Circuit Applications. Norwell, MA: Kluwer, 1988, ch. 2. [8] T. I. Kamins, “Design properties of polycrystalline silicon,” Sens. Actuators A, vols. 21–23, p. 817, 1990. [9] P. Krulevitch, R. T. Howe, G. C. Johnson, and J. Huang, “Stress in undoped polycrystalline silicon,” in Transducers ’91, 1991 Int. Conf. Solid State Sensors and Actuators, p. 949. [10] A. Benitez, J. Bausello, E. Cabruja, J. Esteve, and J. Samitier, “Stress in low pressure chemical vapor deposition poly crystalline silicon thin films deposited below 0.1 Torr,” Sens. Actuators A, vols. 37–38, pp. 723–726, 1993.

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[11] V. M. Koleshko, V. F. Belitsky, and I. V. Kiryshin, “Stress in thin polycrystalline silicon films,” Thin Solid Films, vol. 162, p. 365, 1988. [12] D. Maier-Schneider, J. Maibach, E. Obermeier, and D. Schneider, “Variation in Young’s modulus and intrinsic stress of LPCVD-polysilicon due to high temperature annealing,” J. Micromech. Microeng., vol. 5, p. 131, 1995. [13] W. Fang and J. A. Wickert, “Determining mean and gradient residual stress in thin films using micromachined cantilevers,” J. Micromech. Microeng., vol. 6, p. 301, 1996. [14] B. P. Van Drie¨enhuizen, Integrated Electrostatic RMS-to-DC Converter Fabricated in a BIFET-Compatible Surface-Micromachining Process. The Netherlands: Delft Univ. Press, 1996, ch. 6.

Sherif Sedky was born in Cairo, Egypt, in 1969. He received the Master’s degree in engineering physics in 1995 from Cairo University, Cairo, and the Ph.D. degree in microelectronics and material science from the Catholic University of Leuven, Leuven, Belgium, in 1998. In 1996, he joined the Microsystems Group of the Interuniversity Microelectronics Center (IMEC), Leuven. He is also an Assistant Professor at the Department of Engineering Physics, Faculty of Engineering, Cairo University.

Paolo Fiorini was born in Rome, Italy, in 1953. He received the Ph.D. degree in solid-state physics from the University of Rome, Rome, in 1977. His thesis was on exitons in silicon. He has been active in the field of electrical and optical properties of semiconductors for many years, working at the University of Rome, IBM Research Center, Yorktown Heights, NY, and the Interuniversity Microelectronics Center (IMEC), Leuven, Belguim. He is currently a Professor at the Physics Department of the Third University of Rome, Rome.

Matty Caymax received the Ph.D. degree in 1984 from the Catholic University of Leuven, Leuven, Belgium. He joined the Electrotechnical Department, University of Leuven. In 1985, he was a Scientific Staff Member at the Interuniversity Microelectronics Center (IMEC), Leuven, in the field of Si and SiGe epitaxial growth by CVD.

Stefano Loreti was born in Rome, Italy, in 1962. He received the chemistry degree from Rome University LA SPIENZA in 1987 based on an investigation of micellar aggregates by EXAFS spectroscopy. He is currently a Researcher in the Italian National Agency for New Technology, Energy and Environment (ENEA). His recent activities are in structural and morphological characterization of semiconductor materials by X-ray diffraction and electron microscopy.

Kris Baert was born in Leuven, Belgium, on October 16, 1960. He received the Master’s degree in electrical engineering and the Ph.D. degree in microelectronics and materials science, both from the Catholic University of Leuven, Leuven, in 1984 and 1990, respectively. His Ph.D. dissertation was on low-temperature plasma-enhanced CVD of Si materials and their applications in solar cells and TFT’s. In 1988, he was a Visiting Scientist at the Konagai-Takahashi Laboratory, Tokyo Institute of Technology, Tokyo, Japan. From 1990 to 1992, he was with the Materials and Electronic Devices Laboratory, Mitsubishi Electric, Amagasaki, Japan, where he worked on poly-Si TFT’s for active-matrix liquid crystal displays. He is currently with the Interuniversity Microelectronics Center (IMEC), Leuven, where he is responsible for the activities on microsystem technologies.

Lou Hermans was born in Hasselt, Belgium, in 1956. He received the Ph.D. degree in physics from the Catholic University of Leuven, Leuven, Belgium, in 1983. In 1986, he was with the Interuniversity Microelectronics Center (IMEC), Leuven, as a Member of the IMEC spin-off cell involved in patents, technology transfer, and project acquisition. In 1992, he became Group Leader of the Microsystems Group involved in CMOS image sensor R&D and launching IMEC involvement in MEMS R&D. Since 1998, he has been Director of the Microsystems Department covering R&D on CMOS image sensors, microsystems fabrication technology, biochemical sensors, high-density interconnection technology, and semiconductor-based detectors.

Robert Mertens (F’95) received the Ph.D. degree from the Catholic University of Leuven, Belgium, in 1972. He was a Visiting Scientist at the University of Florida in 1973. After his return to Belgium in 1974, he became a Senior Research Associate of the National Foundation for Scientific Research of Belgium. In 1984, he joined the Interuniversity Microelectronics Center (IMEC), Leuven, as Vice President, where he was responsible for research on materials and packaging. These activities include also research on microsystems, photovoltaics, and solid-state sensors. Since 1984, he has been a Professor at the Catholic University of Leuven, where he is teaching courses on devices and technology of electronic systems. He has researched heavily doped semiconductors, bipolar transistors, and silicon solar cells.