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Characterization of KrF excimer laser annealed PECVD SixGe1−x for MEMS post-processing
Sensors and Actuators A 127 (2006) 316–323
Characterization of KrF excimer laser annealed PECVD SixGe1−x for MEMS post-processing Sherif Sedky a,b,c,∗ , Maria Gromova c , Tom Van der Donck d , Jean-Pierre Celis d , Ann Witvrouw c a Physics Department, The American University in Cairo, 11511 Cairo, Egypt Science and Technology Research Center, Physics Department, The American University in Cairo, Egypt c IMEC, Kapeldreef 75, 3001 Leuven, Belgium d Katholieke Universiteit Leuven, Department of MTM, Kasteelpark Arenberg 44, 3001 Leuven, Belgium
b
Received 27 February 2005; received in revised form 19 January 2006; accepted 19 January 2006 Available online 28 February 2006
Abstract This work studies the possibility to treat plasma enhanced chemical vapor deposited (PECVD) silicon germanium (Si1−x Gex ) thin films grown at 400 ◦ C or lower with a pulsed excimer laser for obtaining good MEMS structural layers. The main advantage of using PECVD is that a high growth rate (∼35 nm/min) can be achieved at low temperatures (≤370 ◦ C). It is demonstrated that optimizing the pulse fluence, number and rate yields high quality films characterized by a low defect density (∼102 defect/cm2 ), large grains (∼300 nm), a low mean stress and a low stress gradient. Furthermore, the electrical resistivity of the as grown material, deposited at 210 ◦ C, is reduced from 12 k cm to 1.3 m cm after laser annealing. © 2006 Elsevier B.V. All rights reserved. Keywords: Silicon germanium; Excimer laser; Grain size; Texture
1. Introduction Over the last two decades excimer laser annealing has been considered an efficient low thermal budget technique for locally modifying the physical properties of thin films without introducing any damage or alterations to the underlying layers. The early motivation for using pulsed laser annealing was to control the grain size and crystallinity of amorphous silicon [1,2], which was attractive for the fabrication of thin film transistors (TFTs) having a high field effect mobility on glass substrates [3,4]. Also, it was commonly used to tune the electrical properties of implanted semiconductors [5–7] especially for devices that require shallow doped regions. Furthermore, it has been demonstrated that pulsed laser annealing can noticeably improve the efficiency of solar cells as it enhances the minority carrier diffusion length [8,9]. The fact that pulsed laser annealing reduces the defect density due to the melting and re-crystallization mechanism widened the application of this technique to improve the
∗
Corresponding author. Tel.: +20 2 797 5902; fax: +20 2 795 7565. E-mail address: [email protected] (S. Sedky).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.01.035
electrical properties of metal induced crystallized amorphous silicon thin films [10]. Microsecond and nanosecond lasers have been recently used for low temperature deposition of a wide variety of materials such as bismuth telluride [11], aluminum nitride [12], vanadium oxide [13], and ferromagnetic materials [14]. A novel application for pulsed laser annealing is to control the stress and stress gradient in surface micromachined Micro Electro Mechanical Systems (MEMS) that are processed on top of standard pre-fabricated driving electronics [20]. This application is much more challenging as MEMS implies the use of rather thick layers and requires the optimization of the mechanical and electrical properties of these layers. Accordingly, the laser annealing conditions needed for MEMS applications are completely different from those conventionally implemented for TFTs or pulsed laser deposition. Due to the superior electrical, mechanical and thermal properties of silicon germanium, compared to silicon [15–19] it has been worthwhile to investigate the possibility of locally tuning the physical properties of this material by pulsed laser annealing. Preliminary research on low pressure chemical vapor deposited Six Ge1−x (25% ≤ x ≤ 60%) deposited between 400 and 425 ◦ C
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showed that pulsed laser annealing can completely eliminate a stress gradient by using a dual layer of Six Ge1−x . Moreover, the electrical conductivity can be as low as 0.5 m cm [19–22]. The main disadvantage of using LPCVD Six Ge1−x is its low deposition rate, which means that the sample will be exposed to the deposition temperature (>400 ◦ C) for a long time interval (upto 8 h or more depending on the layer thickness). In addition, because of the very low deposition rate, it is impossible to reduce the deposition temperature below 400 ◦ C, which might be too high for certain applications. This is why in this work the interaction between a pulsed laser and plasma enhanced chemical vapor deposited (PECVD) silicon germanium film is investigated. The use of PECVD allows the enhancement of the growth rate and gives the possibility to reduce the deposition temperature below 400 ◦ C [23–25]. The main motivation for this work is thus to investigate the possibility of reducing the SiGe deposition temperature below 400 ◦ C, increasing the growth rate, and tuning the physical properties of the MEMS active material, with a thermal treatment which has limited thermal penetration depth and accordingly does not affect the underlying layers. Therefore, SiGe has been deposited by PECVD at temperatures ≤370 ◦ C, with an average growth rate of 35 nm/min. Then pulsed excimer laser has been used to tailor the structural, mechanical and electrical properties of the film. The high hydrogen content in the PECVD films makes the interaction between the laser pulse and the film more challenging than in the case of LPCVD films. In general, PECVD films deposited at these low temperatures are amorphous as grown, and annealing in a conventional furnace at temperatures, slightly higher than the deposition temperature, leads to void formation due to hydrogen out-gassing. In this work, the laser annealing conditions have been tuned for optimal crystallization depth and layer quality, while at the same time avoiding thermal influence on the underlying layers. 2. Experimental setup Excimer laser pulses have been generated from a Lambda Physic Compex 205 system having krypton fluoride (KrF) as the lasing gas, resulting in a laser wavelength of 248 nm, a bandwidth of 300 pm and a pulse duration of 24 ns. The output pulse has a rectangular transversal cross section with a width of 0.6 cm and a height of 2.4 cm. The beam intensity has a Gaussian distribution in the vertical and horizontal directions. A beam guiding system has been used to reshape the pulse wave front into a square of 1.6 cm × 1.6 cm and to homogenize the intensity of the beam in the transverse direction. A schematic diagram of the beam guiding system is displayed in Fig. 1. It consists of a set of telescopic lenses composed of a horizontal cylindrical lens that shortens the pulse long axis and a vertical cylindrical lens to stretch the beam in the horizontal direction. The intensity of the beam is homogenized in the transverse direction using four arrays of cylindrical lenses. Two arrays, each of which is composed of 10 vertical cylindrical lenses, are used to homogenize the beam in the horizontal direction. The two other arrays consist of 10 horizontal cylindrical lenses in order to homogenize the beam in the vertical direction. Finally, the target is placed
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Fig. 1. Schematic diagram of the pulsed laser setup.
in the focal plane of a projection lens that reduces the spot size down to 0.58 cm × 0.58 cm. The samples under consideration are divided into two sets as shown schematically in Fig. 2. The first set is composed of 6 in. Si-substrate having 250 nm of thermal oxide, on top of which there is 50 nm of evaporated Al, which has been coated with 0.5 m of silicon germanium (Si1−x Gex ) (Fig. 2a). Whereas, for the second set of Si1−x Gex is deposited directly on top of 1.6 m of thermal oxide (Fig. 2b). Si1−x Gex has been deposited in an Oxford Plasma Lab 100 system, which is a plasma enhanced chemical vapor deposition (PECVD) cold wall system. The silicon gas source is pure silane, whereas 10% germane in hydrogen has been used as the germanium gas source. One percent diborane in hydrogen has been used as the boron gas source. The deposition temperature has been varied from 210 to 400 ◦ C and the deposition pressure has been fixed at 2 Torr. For all depositions the RF power has been set to 15 W. 3. Effect of pulsed laser annealing on grain microstructure and texture The grain microstructure of the as grown and annealed films has been investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). It is clear from the TEM cross section displayed in Fig. 3a that as grown Si33 Ge67 deposited at 370 ◦ C is fully amorphous. Exposing this film to a single laser pulse having the minimum fluence (67 mJ/cm2 )
Fig. 2. Schematic cross section for the layers used to study the effect of pulsed laser annealing.
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Fig. 3. TEM cross section demonstrating the effect of pulse fluence on 0.5 m thick PECVD Si33 Ge67 deposited at 370 ◦ C. (a) As grown film, (b) film exposed to single pulse having a fluence of 67 mJ/cm2 , (c) film exposed to single pulse having a fluence of 158 mJ/cm2 , (d) film exposed to single pulse having a fluence of 300 mJ/cm2 , (e) film exposed to single pulse having a fluence of 517 mJ/cm2 and (f) film exposed to a single pulse having a fluence of 760 mJ/cm2 .
does not introduce noticeable change in the grain microstructure as demonstrated in Fig. 3b. Increasing the pulse fluence to 157 mJ/cm2 , results in the generation of fine grains extending over a depth of 200 nm (Fig. 3c). The grains are characterized by elongated morphology (as confirmed from the dark field images) and the grain size varies from 40 to 200 nm. The defect density, as estimated from the dark field images, is expected to be around 1010 defect/cm2 . This observation is similar to what has been reported for laser annealed amorphous silicon [26–28] and
accordingly, the presence of the fine grains might be due to the self-propagating silicon germanium liquid through the amorphous silicon germanium film. The absence of coarse grains at this energy density indicates that the molten depth is extremely shallow, which means that this fluence brings the SiGe into the partial melting regime [30] where the SiGe is in a supercooled state and crystallization might occur from unmolten SiGe seeds. At this point it is interesting to note that the crystallization depth at this energy density is much deeper than that reported for
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Fig. 4. Dependence of grain size on pulse fluence. Diamonds: blocky grains, squares: fine grains. Solid line fit according to Eq. (1), and dashed line fit according to Eq. (2).
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Fig. 5. Dependence of the crystallization depth on pulse fluence. Diamonds: blocky grains. Solid line fit according to Eq. (3), squares: fine grains. Dashed line fit according to Eq. (4).
empirical formula: amorphous silicon [1] and this is mainly due to the increased crystallization velocity originating from the high germanium content [29]. Fig. 3d shows that a further increase of the pulse fluence to 300 mJ/cm2 results in a deeper crystallization depth, characterized by two distinct regions: an upper low defect density region (∼102 defect/cm2 ) having blocky grains (grain size varies from 180 to 310 nm) and a bottom region having high defect density (∼1010 defect/cm2 ) fine grains. Thus, this fluence lies in the near complete melting regime [30]. Increasing the pulse fluence results in an increase in the maximum temperature and accordingly the melt depth is increased. This results in the generation of blocky coarse grains close to the surface and fine grains at the bottom. The depth of the blocky grain region significantly increases by increasing the pulse fluence, whereas the fine grain zone is diminishing as clear from the TEM cross sections in Fig. 3e and f. Hence, we are already in the complete melting regime [30]. A detailed inspection of Fig. 3d and f shows that as the pulse fluence reaches 500 mJ/cm2 , tiny pores are generated in the blocky grains, which are much more pronounced at the highest fluence (Fig. 3f). To have a better understanding of the origin of the observed pores, it should be noted that as the pulse fluence is increased, the film melt depth is increased noticeably [31]. The rapid increase in temperature associated with the pulse absorption causes hydrogen to evolve explosively resulting in damaging to film, which is pronounced in the pores observed in the TEM cross sections. Fig. 4 gives a quantitative idea about the effect of the pulse fluence on the maximum grain size of both blocky and fine grains. It is interesting to note that the blocky grain size, GSbg , has a logarithmic dependence on the pulse fluence, E, as clear from the solid line in Fig. 4, which can be expressed by the following empirical formula: GSbg = 298 ln(E) − 1502 nm
(1)
where E is the pulse fluence in mJ/cm2 . On the other hand, the dashed line in Fig. 4 indicates that the size of the fine grains, GSf , varies inversely to the pulse fluence according to the following
GSfg =
43967 (nm) E
(2)
At this point it is instructive to note that the crystallization depth of the blocky grains increases logarithmically with the pulse fluence, as clear from the solid line in Fig. 5, and it can be defined by the following formula: GDbg = 293 ln(E) − 1492 nm,
(3)
where GDbg is the blocky grain crystallization depth. This behavior is very similar to the dependence of the grain size on pulse fluence (Eq. (1)), which might indicate that the grains are expanding laterally and transversely at the same rate. On the other hand, the dashed line in Fig. 5 shows that the crystallization depth of the fine grain zone, GDfg , is decreasing linearly with the pulse fluence and is expressed as: GDfg = 269 − 0.297E nm,
(4)
An important feature, which has not yet been considered, is the localization depth of the laser annealing process. It is reported that Al recrystallizes at around 200 ◦ C [32], which in return results in textural changes. Accordingly, monitoring the textural changes in the bottom Al layer by X-ray diffraction, can give us an idea about the thermal penetration depth of the laser pulse. It should be noted that the thin top SiGe layer (∼0.5 m) allows the X-ray to be diffracted by the bottom Al layer. By inspecting the XRD patterns displayed in Fig. 6 we notice that for pulse fluences as high as 420 mJ/cm2 there is no change in the Al texture as compared to the as grown texture. This indicates that the temperature of Al did not exceed the SiGe deposition temperature, which is 370 ◦ C in our case. On the other hand, increasing the pulse fluence to 760 mJ/cm2 , results in a changed Al texture, which is clear from the generation of the {2 2 0} peak. This indicates that temperature of Al already exceeded the deposition temperature of Si1−x Gex and the annealing process is no longer depth-localized. Thus, it is recommended to limit the pulse fluence to 400 mJ/cm2 , or lower, to keep the laser treatment localized in depth.
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Fig. 6. XRD patterns of the bottom Al layer and the top PECVD Si33 Ge67 deposited at 370 ◦ C. (a) As grown, (b) after a single laser pulse at 420 mJ/cm2 and (c) after a single laser pulse at 760 mJ/cm2 .
At this point it should be noted that the grain microstructure is not only affected by the pulse fluence, but also the pulse number and rate might have a significant impact. To clarify this issue, it is instructive to refer to the XRD patterns in Fig. 7. It is clear from Fig. 7b that increasing the pulse number to 100 at a rate of 10 Hz results in the generation of a weak {2 2 0} Al peak, which is not pronounced if the film is only exposed to a single pulse (Fig. 7a). Furthermore, some splitting in the {3 1 1} SiGe peaks can be seen. For the same number of pulses, increasing the pulse rate to 50 Hz, gives rise to a prominent {2 2 0} Al peak and results in a more obvious splitting in the SiGe {3 1 1} peak. The splitting of the SiGe peaks might be due to the diffusion of the silicon atoms [33,34], which is activated by the increased amount of heat dissipated in the film associated with the higher pulse rate. This results in intermixing between the silicon and germanium atoms, which results in a Ge concentration gradient across the film thickness. Hence, the process cannot be considered any more depth localized due to the observed textural changes in the bottom Al layer. To guarantee that the heat generated by the laser pulse is localized in depth, and at the same time,
Fig. 7. XRD patterns demonstrating the effect of pulse rate at a fixed fluence of 300 mJ/cm2 on the bottom Al layer. (a) Single pulse, (b) 100 pulses at 10 Hz and (c) 100 pulses at 50 Hz.
Fig. 8. XRD patterns of 0.4 m thick PECVD Si25 Ge75 deposited at 370 ◦ C and exposed to: (a) 500 pulses at 50 Hz and 160 mJ/cm2 and (b) 100 pulses at 50 Hz and 300 mJ/cm2 .
to avoid any damage to the SiGe layer, it is recommended to use a large number of pulses with a low pulse fluence applied at the maximum rate. The exact values depend on the layer thickness. The XRD pattern displayed in Fig. 8a shows that for a 0.4 m thick Si25 Ge75 layer, the optimal laser annealing conditions would be 500 pulses having a fluence of 160 mJ/cm2 and applied at 50 Hz, as this does not affect the bottom Al layer nor introduce any splitting in the SiGe peaks. It is important to note that the effectiveness of increasing the pulse number and rate is more pronounced at lower pulse fluence. From the TEM cross section shown in Fig. 3b it is clear that the minimum fluence of 67 mJ/cm2 does not introduce any change in to the film. On the other hand, Fig. 9 shows that 500 pulses of 67 mJ/cm2 at 50 Hz results in a crystallization depth
Fig. 9. TEM cross section demonstrating the effect of 500 pulses at 67 mJ/cm2 and 50 Hz on the grain microstructure of PECVD Si33 Ge67 deposited at 370 ◦ C.
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of 50 nm and a grain size of around 25 nm. Further inspection revealed the presence of blocky and spherical grains. 4. Micromachined devices In general, the measured mean stress in as grown PECVD Six Ge1−x films deposited at temperatures between 300 and 400 ◦ C is compressive with the upper layers more compressed than the lower ones. This results in an out of plan deflection as clear from the profile of the surface micromachined cantilever and the diamond structure displayed in Fig. 10a and b. Exposing this film to 500 laser pulses at 158 mJ/cm2 significantly reduce the stress gradient and the mean stress as clear from Fig. 10c and d. By comparing the surface micromachined structure displayed in Fig. 10a and c it is clear that due to the compressive stress in the as grown material the film is buckling (Fig. 10a). After laser annealing the structure is flat and suspended which confirms low tensile stress. On the other hand, the top layers of the as grown film is much more compressed than the lower ones, as clear form the out of plane deflection of the surface micromachined cantilevers displayed in Fig. 10b. After laser annealing, the top layers are tensile due to recrystallization, and this results in a more uniform stress distribution across the film thickness as clear from the flat profile of the surface micromachined cantilever displayed in Fig. 10d. Also, it has been found that exposing 0.77 m thick PECVD Si31 Ge69 films deposited at 300 ◦ C to 500 pulses at 70 mJ/cm2 reduces their mean stress and sheet resistance, from 93 MPa compressive to 48 MPa compressive and from 450 k/sq. to 600 m/sq., respectively. Using the same pulse number and rate, while increasing the fluence to
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100 mJ/cm2 converts the mean stress of a similar film on top of an Al layer from 85 MPa compressive to 90 MPa tensile. This illustrates the possibility of fine-tuning the mechanical properties of SiGe layers by optimizing the laser annealing conditions. 5. Laser annealing of SiGe deposited at 210 ◦ C From the previous section it is clear that the optimal annealing conditions of a SiGe layer deposited at 400 ◦ C with regard to mechanical and structural properties are 500 pulses at 50 Hz and a fluence of ∼160 mJ/cm2 . As the latest fabrication technologies are accompanied by a reduction in the backend thermal budget, it is instructive to investigate the effect of pulsed laser annealing on Si1−x Gex films deposited at 210 ◦ C. Such temperature is compatible with a wide variety of substrates and driving electronics. The effect of pulse fluence and number on the electrical resistivity of 0.4 m thick PECVD Si71 Ge29 layer deposited at 210 ◦ C is demonstrated in Fig. 11. By investigating the squares in Fig. 11 it is clear that exposing the film to a single laser pulse at 500 mJ/cm2 reduces the resistivity to 1.3 m cm. The initial resistivity of this film before laser annealing was 12 k cm. Decreasing the pulse fluence is associated with increasing resistivity which is mainly due to the shallower penetration depth as previously explained in Section 3. It should be noted that 500 mJ/cm2 is the maximum pulse fluence these films can stand, without having voids or peeling off, is compared to a maximum pulse fluence of 760 mJ/cm2 for the films deposited at 370 ◦ C (Fig. 3f). This might be due to the higher hydrogen content associated with decreasing deposition temperature. On the
Fig. 10. Surface micromachined structures realized by 1.4 m thick Si31 Ge69 films. (a and b) as deposited at 400 ◦ C, (c and d) after pulsed laser annealing.
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laser fluences as high as 420 mJ/cm2 did not induce any structural changes in the thin buried aluminum film, demonstrating the thermal localization of this technique. After laser annealing, the resistivity of PECVD Si71 Ge29 deposited at 210 ◦ C can be reduced to 1.3 m cm compared to 12 k cm for the as grown film. References
Fig. 11. Dependence of resistivity on pulse fluence for 0.4 m thick PECVD Si71 Ge29 deposited at 210 ◦ C. Squares: single pulse, diamonds: 100 pulses at 50 Hz.
Fig. 12. Dependence of resistivity on pulse rate for 0.4 m thick PECVD Si71 Ge29 deposited at 210 ◦ C. Pulse fluence has been fixed to 120 mJ/cm2 and pulse number is 100.
other hand, the diamonds in Fig. 11 show that decreasing the pulse fluence below 100 mJ/cm2 results in a noticeable increase in resistivity, even for 100 pulses. This indicates that the crystallization depth is extremely shallow at such low fluence. At this point it is interesting to investigate the effect of pulse rate at low pulse fluence. In Fig. 12 the dependence of resistivity on the pulse rate for 100 pulses at a fixed pulse fluence of 120 mJ/cm2 is shown. In Fig. 12 it can be seen that increasing the pulse rate to 50 Hz results in a noticeable decrease in resistivity, which is mainly due to atomic intermixing which changes the Ge content across the film thickness as previously discussed. Thus, there are layers with higher Ge content and accordingly the hole mobility will increase [35] and this explains the decreased electrical resistivity associated with increasing the pulse rate. 6. Conclusions The laser annealing conditions using KrF excimer laser has been optimized to yield high quality PECVD SiGe films after annealing. A large number of 160 mJ/cm2 pulses at a high repetition rate (∼50 Hz) leads to a reduction in the SiGe mean stress from highly compressive, as grown, to low tensile. Furthermore, the stress gradient is significantly lowered. Moreover,
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Biographies Sherif Sedky was born in Cairo (Egypt) in 1969. He received the BSc degree, with honors, in electronics engineering in 1992, and the MSc degree in engineering physics in 1995 both from Faculty of Engineering, Cairo University, and the PhD degree in microelectronics in 1998 from the Katholieke Universiteit Leuven, Belgium. In 1995 he joined the MEMS group of the
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Interuniversity Microelectronics Center (IMEC) in Leuven (Belgium). From 1999 to 2002 he was assistant professor at Cairo University. During the academic year 1999–2000, he was a postdoctoral fellow at the Katholieke Universiteit Leuven, and a visiting professor at the same university during Summer 2001, 2003–2005. In 2002 he was a visiting researcher at the University of California, Berkeley. At present, he is associate professor at The American University in Cairo. He is a member in the Institute of Electrical and Electronic Engineers (IEEE), and the materials research society. He holds four patents and authored and coauthored over 40 international publications and a book chapter in the field of design, fabrication and monolithic integration of MEMS with the driving electronics using polycrystalline silicon germanium as a structural material. He is the author of the book titled: “Post-processing techniques for integrated MEMS”. Currently, he is establishing a MEMS fabrication facility at The Science and Technology Research Center at The American University in Cairo. He is a recipient of the Egyptian prestigious national award in advanced technological sciences in 2002, and the graduate studies award from Cairo University in 1996. He served on committees of several international conferences. Maria Gromova received an MS degree in communication engineering from the Technical University of Sofia, Bulgaria, in 1999. She is currently pursuing a PhD degree in electronics engineering at IMEC, Leuven, with main research interests including post-CMOS integration of MEMS. In particular hydrogenated microcrystalline SiGe development for MEMS applications. Tom Van der Donck graduated in 2001 as electronical engineer and in 2003 as metallurgical engineer at the Katholieke Universiteit Leuven. He is currently working towards his PhD degree in engineering at the department of metallurgy and materials engineering (MTM) at the Katholieke Universiteit Leuven (Belgium). His research in the surface engineering and tribology group at the department of MTM is in the field of wear behaviour and adhesion of thin films and coatings. Jean-Pierre Celis graduated in 1972 as metallurgical engineer at the Katholieke Universiteit Leuven. He obtained his PhD degree in 1976 from the department of metallurgy and materials engineering (MTM) at the Katholieke Universiteit Leuven (Belgium). He is currently full professor in metallurgy and materials engineering at the Katholieke Universiteit Leuven, and he is responsible for the scientific and applied research on surface engineering and tribology at the department of MTM. His basic and applied research is focussed on the synthesis of functional coatings for tribological applications and for applications in microelectronics, and on the unraveling of the tribochemical behavior of materials in sliding contacts. In that respect, he developed test equipment for testing the fretting wear behavior of materials in ambient air of different relative humidity and at high temperature. He developed also testing facilities for corrosion-wear in which electrochemical transient techniques are implemented. The concepts of wear rate based on energy dissipation and of active wear track area in tribo-corrosion were developed in his research group. He is fellow of the Institute of Metal Finishing (GB), member of the ASTM G2 group (USA), and Belgian representative in the International Tribology Council. Recently he established with one of his coworkers, Dr.ir. Dirk Drees, a spin-off company on tribology, namely Falex Tribology N.V. located near Leuven (B). Ann Witvrouw received the MS degree in metallurgical engineering in 1986 from the Katholieke Universiteit, Leuven, Belgium, and both the MS degree in applied physics in 1987 and PhD degree in applied physics in 1992 from Harvard University, Cambridge, MA. In 1992, she joined the Interuniversity Microelectronics Center (IMEC), Leuven, where she worked on the reliability of metal interconnects until the end of 1998. During this time, her research was focused on the mechanical stress in films and lines, electromigration and stress induced voiding. In 1998 she switched to research in micro-electromechanical systems at IMEC, where she is now responsible for advanced MEMS process technologies including the integration of MEMS and CMOS. She has been the coordinator of the IST project SUMICAP and currently she is the coordinator of the IST project SiGeM.
Characterization of KrF excimer laser annealed PECVD SixGe1−x for MEMS post-processing Sherif Sedky a,b,c,∗ , Maria Gromova c , Tom Van der Donck d , Jean-Pierre Celis d , Ann Witvrouw c a Physics Department, The American University in Cairo, 11511 Cairo, Egypt Science and Technology Research Center, Physics Department, The American University in Cairo, Egypt c IMEC, Kapeldreef 75, 3001 Leuven, Belgium d Katholieke Universiteit Leuven, Department of MTM, Kasteelpark Arenberg 44, 3001 Leuven, Belgium
b
Received 27 February 2005; received in revised form 19 January 2006; accepted 19 January 2006 Available online 28 February 2006
Abstract This work studies the possibility to treat plasma enhanced chemical vapor deposited (PECVD) silicon germanium (Si1−x Gex ) thin films grown at 400 ◦ C or lower with a pulsed excimer laser for obtaining good MEMS structural layers. The main advantage of using PECVD is that a high growth rate (∼35 nm/min) can be achieved at low temperatures (≤370 ◦ C). It is demonstrated that optimizing the pulse fluence, number and rate yields high quality films characterized by a low defect density (∼102 defect/cm2 ), large grains (∼300 nm), a low mean stress and a low stress gradient. Furthermore, the electrical resistivity of the as grown material, deposited at 210 ◦ C, is reduced from 12 k cm to 1.3 m cm after laser annealing. © 2006 Elsevier B.V. All rights reserved. Keywords: Silicon germanium; Excimer laser; Grain size; Texture
1. Introduction Over the last two decades excimer laser annealing has been considered an efficient low thermal budget technique for locally modifying the physical properties of thin films without introducing any damage or alterations to the underlying layers. The early motivation for using pulsed laser annealing was to control the grain size and crystallinity of amorphous silicon [1,2], which was attractive for the fabrication of thin film transistors (TFTs) having a high field effect mobility on glass substrates [3,4]. Also, it was commonly used to tune the electrical properties of implanted semiconductors [5–7] especially for devices that require shallow doped regions. Furthermore, it has been demonstrated that pulsed laser annealing can noticeably improve the efficiency of solar cells as it enhances the minority carrier diffusion length [8,9]. The fact that pulsed laser annealing reduces the defect density due to the melting and re-crystallization mechanism widened the application of this technique to improve the
∗
Corresponding author. Tel.: +20 2 797 5902; fax: +20 2 795 7565. E-mail address: [email protected] (S. Sedky).
0924-4247/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2006.01.035
electrical properties of metal induced crystallized amorphous silicon thin films [10]. Microsecond and nanosecond lasers have been recently used for low temperature deposition of a wide variety of materials such as bismuth telluride [11], aluminum nitride [12], vanadium oxide [13], and ferromagnetic materials [14]. A novel application for pulsed laser annealing is to control the stress and stress gradient in surface micromachined Micro Electro Mechanical Systems (MEMS) that are processed on top of standard pre-fabricated driving electronics [20]. This application is much more challenging as MEMS implies the use of rather thick layers and requires the optimization of the mechanical and electrical properties of these layers. Accordingly, the laser annealing conditions needed for MEMS applications are completely different from those conventionally implemented for TFTs or pulsed laser deposition. Due to the superior electrical, mechanical and thermal properties of silicon germanium, compared to silicon [15–19] it has been worthwhile to investigate the possibility of locally tuning the physical properties of this material by pulsed laser annealing. Preliminary research on low pressure chemical vapor deposited Six Ge1−x (25% ≤ x ≤ 60%) deposited between 400 and 425 ◦ C
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showed that pulsed laser annealing can completely eliminate a stress gradient by using a dual layer of Six Ge1−x . Moreover, the electrical conductivity can be as low as 0.5 m cm [19–22]. The main disadvantage of using LPCVD Six Ge1−x is its low deposition rate, which means that the sample will be exposed to the deposition temperature (>400 ◦ C) for a long time interval (upto 8 h or more depending on the layer thickness). In addition, because of the very low deposition rate, it is impossible to reduce the deposition temperature below 400 ◦ C, which might be too high for certain applications. This is why in this work the interaction between a pulsed laser and plasma enhanced chemical vapor deposited (PECVD) silicon germanium film is investigated. The use of PECVD allows the enhancement of the growth rate and gives the possibility to reduce the deposition temperature below 400 ◦ C [23–25]. The main motivation for this work is thus to investigate the possibility of reducing the SiGe deposition temperature below 400 ◦ C, increasing the growth rate, and tuning the physical properties of the MEMS active material, with a thermal treatment which has limited thermal penetration depth and accordingly does not affect the underlying layers. Therefore, SiGe has been deposited by PECVD at temperatures ≤370 ◦ C, with an average growth rate of 35 nm/min. Then pulsed excimer laser has been used to tailor the structural, mechanical and electrical properties of the film. The high hydrogen content in the PECVD films makes the interaction between the laser pulse and the film more challenging than in the case of LPCVD films. In general, PECVD films deposited at these low temperatures are amorphous as grown, and annealing in a conventional furnace at temperatures, slightly higher than the deposition temperature, leads to void formation due to hydrogen out-gassing. In this work, the laser annealing conditions have been tuned for optimal crystallization depth and layer quality, while at the same time avoiding thermal influence on the underlying layers. 2. Experimental setup Excimer laser pulses have been generated from a Lambda Physic Compex 205 system having krypton fluoride (KrF) as the lasing gas, resulting in a laser wavelength of 248 nm, a bandwidth of 300 pm and a pulse duration of 24 ns. The output pulse has a rectangular transversal cross section with a width of 0.6 cm and a height of 2.4 cm. The beam intensity has a Gaussian distribution in the vertical and horizontal directions. A beam guiding system has been used to reshape the pulse wave front into a square of 1.6 cm × 1.6 cm and to homogenize the intensity of the beam in the transverse direction. A schematic diagram of the beam guiding system is displayed in Fig. 1. It consists of a set of telescopic lenses composed of a horizontal cylindrical lens that shortens the pulse long axis and a vertical cylindrical lens to stretch the beam in the horizontal direction. The intensity of the beam is homogenized in the transverse direction using four arrays of cylindrical lenses. Two arrays, each of which is composed of 10 vertical cylindrical lenses, are used to homogenize the beam in the horizontal direction. The two other arrays consist of 10 horizontal cylindrical lenses in order to homogenize the beam in the vertical direction. Finally, the target is placed
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Fig. 1. Schematic diagram of the pulsed laser setup.
in the focal plane of a projection lens that reduces the spot size down to 0.58 cm × 0.58 cm. The samples under consideration are divided into two sets as shown schematically in Fig. 2. The first set is composed of 6 in. Si-substrate having 250 nm of thermal oxide, on top of which there is 50 nm of evaporated Al, which has been coated with 0.5 m of silicon germanium (Si1−x Gex ) (Fig. 2a). Whereas, for the second set of Si1−x Gex is deposited directly on top of 1.6 m of thermal oxide (Fig. 2b). Si1−x Gex has been deposited in an Oxford Plasma Lab 100 system, which is a plasma enhanced chemical vapor deposition (PECVD) cold wall system. The silicon gas source is pure silane, whereas 10% germane in hydrogen has been used as the germanium gas source. One percent diborane in hydrogen has been used as the boron gas source. The deposition temperature has been varied from 210 to 400 ◦ C and the deposition pressure has been fixed at 2 Torr. For all depositions the RF power has been set to 15 W. 3. Effect of pulsed laser annealing on grain microstructure and texture The grain microstructure of the as grown and annealed films has been investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). It is clear from the TEM cross section displayed in Fig. 3a that as grown Si33 Ge67 deposited at 370 ◦ C is fully amorphous. Exposing this film to a single laser pulse having the minimum fluence (67 mJ/cm2 )
Fig. 2. Schematic cross section for the layers used to study the effect of pulsed laser annealing.
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Fig. 3. TEM cross section demonstrating the effect of pulse fluence on 0.5 m thick PECVD Si33 Ge67 deposited at 370 ◦ C. (a) As grown film, (b) film exposed to single pulse having a fluence of 67 mJ/cm2 , (c) film exposed to single pulse having a fluence of 158 mJ/cm2 , (d) film exposed to single pulse having a fluence of 300 mJ/cm2 , (e) film exposed to single pulse having a fluence of 517 mJ/cm2 and (f) film exposed to a single pulse having a fluence of 760 mJ/cm2 .
does not introduce noticeable change in the grain microstructure as demonstrated in Fig. 3b. Increasing the pulse fluence to 157 mJ/cm2 , results in the generation of fine grains extending over a depth of 200 nm (Fig. 3c). The grains are characterized by elongated morphology (as confirmed from the dark field images) and the grain size varies from 40 to 200 nm. The defect density, as estimated from the dark field images, is expected to be around 1010 defect/cm2 . This observation is similar to what has been reported for laser annealed amorphous silicon [26–28] and
accordingly, the presence of the fine grains might be due to the self-propagating silicon germanium liquid through the amorphous silicon germanium film. The absence of coarse grains at this energy density indicates that the molten depth is extremely shallow, which means that this fluence brings the SiGe into the partial melting regime [30] where the SiGe is in a supercooled state and crystallization might occur from unmolten SiGe seeds. At this point it is interesting to note that the crystallization depth at this energy density is much deeper than that reported for
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Fig. 4. Dependence of grain size on pulse fluence. Diamonds: blocky grains, squares: fine grains. Solid line fit according to Eq. (1), and dashed line fit according to Eq. (2).
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Fig. 5. Dependence of the crystallization depth on pulse fluence. Diamonds: blocky grains. Solid line fit according to Eq. (3), squares: fine grains. Dashed line fit according to Eq. (4).
empirical formula: amorphous silicon [1] and this is mainly due to the increased crystallization velocity originating from the high germanium content [29]. Fig. 3d shows that a further increase of the pulse fluence to 300 mJ/cm2 results in a deeper crystallization depth, characterized by two distinct regions: an upper low defect density region (∼102 defect/cm2 ) having blocky grains (grain size varies from 180 to 310 nm) and a bottom region having high defect density (∼1010 defect/cm2 ) fine grains. Thus, this fluence lies in the near complete melting regime [30]. Increasing the pulse fluence results in an increase in the maximum temperature and accordingly the melt depth is increased. This results in the generation of blocky coarse grains close to the surface and fine grains at the bottom. The depth of the blocky grain region significantly increases by increasing the pulse fluence, whereas the fine grain zone is diminishing as clear from the TEM cross sections in Fig. 3e and f. Hence, we are already in the complete melting regime [30]. A detailed inspection of Fig. 3d and f shows that as the pulse fluence reaches 500 mJ/cm2 , tiny pores are generated in the blocky grains, which are much more pronounced at the highest fluence (Fig. 3f). To have a better understanding of the origin of the observed pores, it should be noted that as the pulse fluence is increased, the film melt depth is increased noticeably [31]. The rapid increase in temperature associated with the pulse absorption causes hydrogen to evolve explosively resulting in damaging to film, which is pronounced in the pores observed in the TEM cross sections. Fig. 4 gives a quantitative idea about the effect of the pulse fluence on the maximum grain size of both blocky and fine grains. It is interesting to note that the blocky grain size, GSbg , has a logarithmic dependence on the pulse fluence, E, as clear from the solid line in Fig. 4, which can be expressed by the following empirical formula: GSbg = 298 ln(E) − 1502 nm
(1)
where E is the pulse fluence in mJ/cm2 . On the other hand, the dashed line in Fig. 4 indicates that the size of the fine grains, GSf , varies inversely to the pulse fluence according to the following
GSfg =
43967 (nm) E
(2)
At this point it is instructive to note that the crystallization depth of the blocky grains increases logarithmically with the pulse fluence, as clear from the solid line in Fig. 5, and it can be defined by the following formula: GDbg = 293 ln(E) − 1492 nm,
(3)
where GDbg is the blocky grain crystallization depth. This behavior is very similar to the dependence of the grain size on pulse fluence (Eq. (1)), which might indicate that the grains are expanding laterally and transversely at the same rate. On the other hand, the dashed line in Fig. 5 shows that the crystallization depth of the fine grain zone, GDfg , is decreasing linearly with the pulse fluence and is expressed as: GDfg = 269 − 0.297E nm,
(4)
An important feature, which has not yet been considered, is the localization depth of the laser annealing process. It is reported that Al recrystallizes at around 200 ◦ C [32], which in return results in textural changes. Accordingly, monitoring the textural changes in the bottom Al layer by X-ray diffraction, can give us an idea about the thermal penetration depth of the laser pulse. It should be noted that the thin top SiGe layer (∼0.5 m) allows the X-ray to be diffracted by the bottom Al layer. By inspecting the XRD patterns displayed in Fig. 6 we notice that for pulse fluences as high as 420 mJ/cm2 there is no change in the Al texture as compared to the as grown texture. This indicates that the temperature of Al did not exceed the SiGe deposition temperature, which is 370 ◦ C in our case. On the other hand, increasing the pulse fluence to 760 mJ/cm2 , results in a changed Al texture, which is clear from the generation of the {2 2 0} peak. This indicates that temperature of Al already exceeded the deposition temperature of Si1−x Gex and the annealing process is no longer depth-localized. Thus, it is recommended to limit the pulse fluence to 400 mJ/cm2 , or lower, to keep the laser treatment localized in depth.
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Fig. 6. XRD patterns of the bottom Al layer and the top PECVD Si33 Ge67 deposited at 370 ◦ C. (a) As grown, (b) after a single laser pulse at 420 mJ/cm2 and (c) after a single laser pulse at 760 mJ/cm2 .
At this point it should be noted that the grain microstructure is not only affected by the pulse fluence, but also the pulse number and rate might have a significant impact. To clarify this issue, it is instructive to refer to the XRD patterns in Fig. 7. It is clear from Fig. 7b that increasing the pulse number to 100 at a rate of 10 Hz results in the generation of a weak {2 2 0} Al peak, which is not pronounced if the film is only exposed to a single pulse (Fig. 7a). Furthermore, some splitting in the {3 1 1} SiGe peaks can be seen. For the same number of pulses, increasing the pulse rate to 50 Hz, gives rise to a prominent {2 2 0} Al peak and results in a more obvious splitting in the SiGe {3 1 1} peak. The splitting of the SiGe peaks might be due to the diffusion of the silicon atoms [33,34], which is activated by the increased amount of heat dissipated in the film associated with the higher pulse rate. This results in intermixing between the silicon and germanium atoms, which results in a Ge concentration gradient across the film thickness. Hence, the process cannot be considered any more depth localized due to the observed textural changes in the bottom Al layer. To guarantee that the heat generated by the laser pulse is localized in depth, and at the same time,
Fig. 7. XRD patterns demonstrating the effect of pulse rate at a fixed fluence of 300 mJ/cm2 on the bottom Al layer. (a) Single pulse, (b) 100 pulses at 10 Hz and (c) 100 pulses at 50 Hz.
Fig. 8. XRD patterns of 0.4 m thick PECVD Si25 Ge75 deposited at 370 ◦ C and exposed to: (a) 500 pulses at 50 Hz and 160 mJ/cm2 and (b) 100 pulses at 50 Hz and 300 mJ/cm2 .
to avoid any damage to the SiGe layer, it is recommended to use a large number of pulses with a low pulse fluence applied at the maximum rate. The exact values depend on the layer thickness. The XRD pattern displayed in Fig. 8a shows that for a 0.4 m thick Si25 Ge75 layer, the optimal laser annealing conditions would be 500 pulses having a fluence of 160 mJ/cm2 and applied at 50 Hz, as this does not affect the bottom Al layer nor introduce any splitting in the SiGe peaks. It is important to note that the effectiveness of increasing the pulse number and rate is more pronounced at lower pulse fluence. From the TEM cross section shown in Fig. 3b it is clear that the minimum fluence of 67 mJ/cm2 does not introduce any change in to the film. On the other hand, Fig. 9 shows that 500 pulses of 67 mJ/cm2 at 50 Hz results in a crystallization depth
Fig. 9. TEM cross section demonstrating the effect of 500 pulses at 67 mJ/cm2 and 50 Hz on the grain microstructure of PECVD Si33 Ge67 deposited at 370 ◦ C.
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of 50 nm and a grain size of around 25 nm. Further inspection revealed the presence of blocky and spherical grains. 4. Micromachined devices In general, the measured mean stress in as grown PECVD Six Ge1−x films deposited at temperatures between 300 and 400 ◦ C is compressive with the upper layers more compressed than the lower ones. This results in an out of plan deflection as clear from the profile of the surface micromachined cantilever and the diamond structure displayed in Fig. 10a and b. Exposing this film to 500 laser pulses at 158 mJ/cm2 significantly reduce the stress gradient and the mean stress as clear from Fig. 10c and d. By comparing the surface micromachined structure displayed in Fig. 10a and c it is clear that due to the compressive stress in the as grown material the film is buckling (Fig. 10a). After laser annealing the structure is flat and suspended which confirms low tensile stress. On the other hand, the top layers of the as grown film is much more compressed than the lower ones, as clear form the out of plane deflection of the surface micromachined cantilevers displayed in Fig. 10b. After laser annealing, the top layers are tensile due to recrystallization, and this results in a more uniform stress distribution across the film thickness as clear from the flat profile of the surface micromachined cantilever displayed in Fig. 10d. Also, it has been found that exposing 0.77 m thick PECVD Si31 Ge69 films deposited at 300 ◦ C to 500 pulses at 70 mJ/cm2 reduces their mean stress and sheet resistance, from 93 MPa compressive to 48 MPa compressive and from 450 k/sq. to 600 m/sq., respectively. Using the same pulse number and rate, while increasing the fluence to
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100 mJ/cm2 converts the mean stress of a similar film on top of an Al layer from 85 MPa compressive to 90 MPa tensile. This illustrates the possibility of fine-tuning the mechanical properties of SiGe layers by optimizing the laser annealing conditions. 5. Laser annealing of SiGe deposited at 210 ◦ C From the previous section it is clear that the optimal annealing conditions of a SiGe layer deposited at 400 ◦ C with regard to mechanical and structural properties are 500 pulses at 50 Hz and a fluence of ∼160 mJ/cm2 . As the latest fabrication technologies are accompanied by a reduction in the backend thermal budget, it is instructive to investigate the effect of pulsed laser annealing on Si1−x Gex films deposited at 210 ◦ C. Such temperature is compatible with a wide variety of substrates and driving electronics. The effect of pulse fluence and number on the electrical resistivity of 0.4 m thick PECVD Si71 Ge29 layer deposited at 210 ◦ C is demonstrated in Fig. 11. By investigating the squares in Fig. 11 it is clear that exposing the film to a single laser pulse at 500 mJ/cm2 reduces the resistivity to 1.3 m cm. The initial resistivity of this film before laser annealing was 12 k cm. Decreasing the pulse fluence is associated with increasing resistivity which is mainly due to the shallower penetration depth as previously explained in Section 3. It should be noted that 500 mJ/cm2 is the maximum pulse fluence these films can stand, without having voids or peeling off, is compared to a maximum pulse fluence of 760 mJ/cm2 for the films deposited at 370 ◦ C (Fig. 3f). This might be due to the higher hydrogen content associated with decreasing deposition temperature. On the
Fig. 10. Surface micromachined structures realized by 1.4 m thick Si31 Ge69 films. (a and b) as deposited at 400 ◦ C, (c and d) after pulsed laser annealing.
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laser fluences as high as 420 mJ/cm2 did not induce any structural changes in the thin buried aluminum film, demonstrating the thermal localization of this technique. After laser annealing, the resistivity of PECVD Si71 Ge29 deposited at 210 ◦ C can be reduced to 1.3 m cm compared to 12 k cm for the as grown film. References
Fig. 11. Dependence of resistivity on pulse fluence for 0.4 m thick PECVD Si71 Ge29 deposited at 210 ◦ C. Squares: single pulse, diamonds: 100 pulses at 50 Hz.
Fig. 12. Dependence of resistivity on pulse rate for 0.4 m thick PECVD Si71 Ge29 deposited at 210 ◦ C. Pulse fluence has been fixed to 120 mJ/cm2 and pulse number is 100.
other hand, the diamonds in Fig. 11 show that decreasing the pulse fluence below 100 mJ/cm2 results in a noticeable increase in resistivity, even for 100 pulses. This indicates that the crystallization depth is extremely shallow at such low fluence. At this point it is interesting to investigate the effect of pulse rate at low pulse fluence. In Fig. 12 the dependence of resistivity on the pulse rate for 100 pulses at a fixed pulse fluence of 120 mJ/cm2 is shown. In Fig. 12 it can be seen that increasing the pulse rate to 50 Hz results in a noticeable decrease in resistivity, which is mainly due to atomic intermixing which changes the Ge content across the film thickness as previously discussed. Thus, there are layers with higher Ge content and accordingly the hole mobility will increase [35] and this explains the decreased electrical resistivity associated with increasing the pulse rate. 6. Conclusions The laser annealing conditions using KrF excimer laser has been optimized to yield high quality PECVD SiGe films after annealing. A large number of 160 mJ/cm2 pulses at a high repetition rate (∼50 Hz) leads to a reduction in the SiGe mean stress from highly compressive, as grown, to low tensile. Furthermore, the stress gradient is significantly lowered. Moreover,
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Biographies Sherif Sedky was born in Cairo (Egypt) in 1969. He received the BSc degree, with honors, in electronics engineering in 1992, and the MSc degree in engineering physics in 1995 both from Faculty of Engineering, Cairo University, and the PhD degree in microelectronics in 1998 from the Katholieke Universiteit Leuven, Belgium. In 1995 he joined the MEMS group of the
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Interuniversity Microelectronics Center (IMEC) in Leuven (Belgium). From 1999 to 2002 he was assistant professor at Cairo University. During the academic year 1999–2000, he was a postdoctoral fellow at the Katholieke Universiteit Leuven, and a visiting professor at the same university during Summer 2001, 2003–2005. In 2002 he was a visiting researcher at the University of California, Berkeley. At present, he is associate professor at The American University in Cairo. He is a member in the Institute of Electrical and Electronic Engineers (IEEE), and the materials research society. He holds four patents and authored and coauthored over 40 international publications and a book chapter in the field of design, fabrication and monolithic integration of MEMS with the driving electronics using polycrystalline silicon germanium as a structural material. He is the author of the book titled: “Post-processing techniques for integrated MEMS”. Currently, he is establishing a MEMS fabrication facility at The Science and Technology Research Center at The American University in Cairo. He is a recipient of the Egyptian prestigious national award in advanced technological sciences in 2002, and the graduate studies award from Cairo University in 1996. He served on committees of several international conferences. Maria Gromova received an MS degree in communication engineering from the Technical University of Sofia, Bulgaria, in 1999. She is currently pursuing a PhD degree in electronics engineering at IMEC, Leuven, with main research interests including post-CMOS integration of MEMS. In particular hydrogenated microcrystalline SiGe development for MEMS applications. Tom Van der Donck graduated in 2001 as electronical engineer and in 2003 as metallurgical engineer at the Katholieke Universiteit Leuven. He is currently working towards his PhD degree in engineering at the department of metallurgy and materials engineering (MTM) at the Katholieke Universiteit Leuven (Belgium). His research in the surface engineering and tribology group at the department of MTM is in the field of wear behaviour and adhesion of thin films and coatings. Jean-Pierre Celis graduated in 1972 as metallurgical engineer at the Katholieke Universiteit Leuven. He obtained his PhD degree in 1976 from the department of metallurgy and materials engineering (MTM) at the Katholieke Universiteit Leuven (Belgium). He is currently full professor in metallurgy and materials engineering at the Katholieke Universiteit Leuven, and he is responsible for the scientific and applied research on surface engineering and tribology at the department of MTM. His basic and applied research is focussed on the synthesis of functional coatings for tribological applications and for applications in microelectronics, and on the unraveling of the tribochemical behavior of materials in sliding contacts. In that respect, he developed test equipment for testing the fretting wear behavior of materials in ambient air of different relative humidity and at high temperature. He developed also testing facilities for corrosion-wear in which electrochemical transient techniques are implemented. The concepts of wear rate based on energy dissipation and of active wear track area in tribo-corrosion were developed in his research group. He is fellow of the Institute of Metal Finishing (GB), member of the ASTM G2 group (USA), and Belgian representative in the International Tribology Council. Recently he established with one of his coworkers, Dr.ir. Dirk Drees, a spin-off company on tribology, namely Falex Tribology N.V. located near Leuven (B). Ann Witvrouw received the MS degree in metallurgical engineering in 1986 from the Katholieke Universiteit, Leuven, Belgium, and both the MS degree in applied physics in 1987 and PhD degree in applied physics in 1992 from Harvard University, Cambridge, MA. In 1992, she joined the Interuniversity Microelectronics Center (IMEC), Leuven, where she worked on the reliability of metal interconnects until the end of 1998. During this time, her research was focused on the mechanical stress in films and lines, electromigration and stress induced voiding. In 1998 she switched to research in micro-electromechanical systems at IMEC, where she is now responsible for advanced MEMS process technologies including the integration of MEMS and CMOS. She has been the coordinator of the IST project SUMICAP and currently she is the coordinator of the IST project SiGeM.