- Email: [email protected]
V compound semiconductors
Sensors and Actuators 85 Ž2000. 324–329 www.elsevier.nlrlocatersna
Hetero-micromachining of epitaxial IIIrV compound semiconductors Erwin Peiner ) , Klaus Fricke, Ingo Behrens, Andrey Bakin, Andreas Schlachetzki Institut fur ¨ Halbleitertechnik, Technical UniÕersity Braunschweig, Hans-Sommer-Str. 66, 38106 Brunswick, Germany Accepted 10 November 1999
Abstract Hetero-micromachining, a novel technique for the fabrication of miniaturized sensors and actuators is described. It is based on IIIrV compound semiconductor layers epitaxially grown on Ž001. silicon. Cantilevers composed of single indium phosphide or gallium arsenide layers or a layer sequence of different IIIrV compound semiconductors were realized exploiting the etching selectivity of the layer against the silicon substrate in KOH solution. Both etching and fracture properties of InP cantilevers are dependent on the concentration of silicon impurities in the layer. For GaAs, a fracture limit in excess of 1.5 GPa was found. Thermally actuated micromirrors that were fabricated by hetero-micromachining of InP could be deflected to up to 0.078rmW of electrical input power under quasistatic excitation conditions. Typically, an increase of this efficiency by an order magnitude was observed at resonance that was in the kHz range. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Hetero-micromachining; IIIrV compound semiconductors; Silicon impurities; Fracture limit; Thermally actuated micromirror
1. Introduction Bulk and surface micromachining are the conventional techniques for the fabrication of miniaturized silicon sensors and actuators. Both techniques suffer from weaknesses, e.g. considerable consumption of chip area in the former case and rather restricted mechanical performance in the latter. As a tradeoff, epi-micromachining has been described where the mechanical structure is realized using an epitaxial layer w1x. Different crystalline materials with potentially large etching selectivity can be combined by heteroepitaxy. As an example indium phosphide on silicon ŽInPrSi. was investigated with respect to its etching behaviour in KOH solution w2,3x. InP microcantilevers could be fabricated using this hetero-micromachining process. A fracture limit of 0.9 GPa was measured indicating that stability requirements for micromechanical applications can be met. In this study, hetero-micromachining with InPrSi, GaAsrSi and an InPrIn 0.53 Ga 0.47 As double heterostructure on GaAsrSi is described. GaAs may be preferable to
) Corresponding author. Tel.: q49-531-391-3761; fax: q49-531-3915844; http:rrwww.tu-bs.derinstituteriht. E-mail address: [email protected] ŽE. Peiner..
InP due to potentially higher piezoelectric coefficient and fracture strength w4x. Heterostructures incorporating latticematched ternary compound layers are of interest since they are basic constituents of electronic and optoelectronic devices that may be integrated with the mechanical structure. As an example for a device, a thermal actuator is realized by hetero-micromachining with InPrSi.
2. Experimental The basic steps of the hetero-micromachining process are depicted in Fig. 1 using the example of a thermally actuated micromirror. N-type Ž001. silicon wafers of resistivity and thickness of 2–4 V cm and 420–540 mm, respectively, are utilized as substrates. First Ža., a heteroepitaxial indium phosphide layer is grown by metalorganic vapour-phase epitaxy ŽMOVPE. w5x. Prior to growth, the substrate is deoxidized for 15 min at 9508C under H 2 flow. For stabilization of the surface reconstruction, AsH 3 is added at 8508C during the cool-down phase. A thin buffer layer Ž40 nm. is deposited at 4008C with a ratio of the group V and III components ŽVrIII. in the reactor of 2700. The total pressure P is 100 mbar. After buffer-layer growth, the process is routinely interrupted and the graphite susceptor is thermally cleaned at 12008C in a high-vacuum chamber. Subsequently, the main layer is grown at 6408C,
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Table 1 Layer sequence of structures in Fig. 3 Žtop layer: In 0.53 Ga 0.47 As.
Fig. 1. Basic fabrication steps of a micromirror by hetero-micromachining.
VrIII s 140 and a total pressure of 20 mbar resulting in a growth rate of 1 mmrh. Subsequently, a AurCr Žor AurTi. bilayer is deposited and structured using lift-off. In the second step Žb., the layer is patterned by wet etching using HBr: CH 3 COOH:K 2 Cr2 O 7 Ž1:1:1.. The resulting step height that corresponds to the thickness h of the layer or the layer sequence is measured by mechanical surface tracing ŽDektak 3 ST .. By the following etching in KOH Ž30%. at 608C Žc., membranes suspended by four beams were obtained exploiting the etching selectivity of Si with respect to InP. In Fig. 2, a scanning electron microscopy ŽSEM. photograph of processed micromirrors is displayed. AurCr-coated InP cantilevers and micromirrors are thermally actuated exploiting the bimorph effect between the metal layers and the IIIrV semiconductor. Cantilever or micromirror action is measured using the laser beam deflection technique, i.e. via the position of a laser spot after reflection at the device surface. Gallium arsenide microcantilevers are fabricated from layers grown at 4008C, VrIII s 4, 100 mbar Žbuffer layer, 15 nm. and 7008C, VrIII s 80, 50 mbar Žmain layer, 2–3 mm. w5x. For structuring, HCl:HNO3 Ž1:2. is used. Multi-
Fig. 2. Micromirrors in InP realized by hetero-micromachining.
Material
Thickness
In 0.53 Ga 0.47 As InP In 0.53 Ga 0.47 AsrInP InP GaAs
0.6 mm 1.1 mm 20=Ž7r7 nm. 1.0 mm 1.5 mm
layer samples incorporating indium gallium arsenide lattice-matched to indium phosphide ŽIn 0.53 Ga 0.47 As. Žcf. Table 1. can be selectively removed using HCl:H 3 PO4 Ž1:4. for InP and C 6 H 8 O 7 :H 2 O Ž1:7. for In 0.53 Ga 0.47 As. Epitaxy of In 0.53 Ga 0.47 As is performed at 6408C, VrIII s 140 and 20 mbar w6x. For w100x Žor w010x.-oriented IIIrV compound structures, as in the case of Fig. 1, it is possible to completely remove the silicon underneath by frontside etching. For a complete release of w110x-oriented cantilevers and resonators, we start from wafers which are thinned to a residual thickness of 300 mm by etching in KOH Ž30%, 608C.. Furthermore, the wafer backside is thermally oxidized to a thickness of 1.6 mm prior to epitaxy. After structuring this oxide layer, double-sided etching can be performed to account for the etch-stop behaviour of the 1114 crystal planes of silicon in KOH. Bending stiffness Dx and fracture limit of the micromechanical elements are measured with test structures like cantilevers of different widths w using a load-deflection technique w2,3x. An external load F varied from 10 to 400 mN is applied to a cantilever and the resulting deflection d is measured in dependence on the loading position x along the cantilever axis. Fracture is observed when the stress in the cantilever exceeds a critical value sac .
3. Results and discussion 3.1. Etching selectiÕity Fig. 3 displays SEM photographs of resonators and cantilevers fabricated from a sequence of IIIrV-semiconductor layers using hetero-micromachining. The layers and their epitaxial thicknesses are given in Table 1. Freestanding structures of various dimensions could be fabricated confirming the high etching selectivity of the different IIIrV compounds with respect to silicon. Only at the bottom GaAs layer is an etch attack indicated ŽFig. 3b.. For precise determination of the thickness h of the fabricated structures load-deflection measurements were performed with cantilevers used as test structures. Fig. 4 shows load-deflection curves measured with a w100x-oriented GaAs cantilever of a nominal thickness of 2.7 mm as determined by step-height measurement in the course of
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Fig. 3. Top Ža. and bottom Žb,c. SEM photographs of microresonators and -cantilevers fabricated by hetero-micromachining.
the fabrication process Žcf. Fig. 1 and context.. The solid lines correspond to fits of:
d Ž x. s
x3 3 Dx
F
Ž 1.
to the experimental curves with Dx s Ewh3r12 as the fitting parameter. Using the Young’s modulus of E s 85 GPa and a cantilever width of w s 175 mm, we obtained h s 2.1 " 0.1 mm. This result was confirmed by SEM with the cantilever cross-section. We attribute the thickness
reduction to an etch attack at the backside of the latticemismatched epitaxial GaAs where the crystal structure of the epitaxial layer is not perfect. In fact, for a GaAsrSi layer affected by antiphase domains w7x, most of the structures were damaged during the hetero-micromachining process. Cantilevers composed of the layer sequence given in Table 1 were analysed assuming an average Young’s modulus of E s 110 and 70 GPa for the w110x and w100x orientation, respectively. In Table 2, the results obtained with different cantilevers are given. In all cases, h is less than the original thickness of the as-deposited layer sequence indicating an etch attack during the hetero-micromachining process. An effect which can cause an impaired etching resistance of GaAs and InP against KOH solution is the incorporation of silicon impurities into the layer. With InP intentionally doped to concentrations of 10 18 to 10 19 cmy3 , an etch attack was observed leading to a considerable increase of the surface roughness. During heteroepitaxy, silicon impurities can be incorporated into the growing layer by solid-state diffusion across the heterointerface between substrate and layer or by diffusion via the ambient gas. To prevent uptake of silicon into the gas phase, the susceptor is thermally cleaned prior to the main-layer epitaxy. Furthermore, backside and sidewalls of the substrate surface can be coated with a SiO 2 cap. Fig. 5 shows the silicon doping profile in InP layers grown on a bare substrate without process interruption prior to main-layer growth Žsingle-step process. and on a substrate with a 0.4-mm-thick thermal oxide cap. In the latter case, a thermally cleaned susceptor is employed Žtwo-step growth.. On the abscissa, the position within the layer is given starting at the heterointerface Ž z s 0. and proceeding towards the layer surface. The dashed curves fitted to the measured profile in the latter case describe the solid-state diffusion of silicon across the heterointerface via lattice sites Ž; erfc z . and diffusion channels Žln n ; z 6r5 . in the layer w8x. The horizontal line corresponds to a uniform background doping level of around 10 16 cmy3 . We find that using a SiO 2-capped substrate, the silicon concentration is considerably reduced across the entire layer by
Table 2 Features of cantilevers characterized by load-deflection measurements Orientation
w Žmm.
h Žmm.
sac ŽMPa.
w100x
100 180
w110x
80
3.6 4.2 4.2 4.0 4.0 3.8 3.8 4.3 3.8
315 372 174 664 734 515 ) 537 ) 417 ) 586
120 140 160 180 Fig. 4. Load-deflection curves of a GaAs microcantilever.
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Fig. 5. Silicon impurity concentration profile in heteroepitaxial InP on silicon.
more than an order of magnitude to values of 2 = 10 19 cmy3 at the heterointerface and less than 10 16 cmy3 at the layer surface. Cantilevers fabricated from these layers showed an only slightly reduced thickness corresponding to 95 " 3% of the value realized by epitaxy. For cantilevers of a high concentration of silicon impurities in the indium phosphide, however, h amounted to only 80 " 3% of the original thickness indicating an etch attack by the KOH solution.
Fig. 7. Thermally actuated micromirrors in InP realized by hetero-micromachining.
3.2. Fracture limit The fracture stress sac of multilayer cantilevers fabricated by hetero-micromachining is shown in Table 2. For w110x-oriented structures, considerably higher values of sac were found than for the w100x orientation. In the latter case, fracture did not occur immediately at the cantilever clamp but in the frame. Due to different thermal expansion coefficients of the constituent layers, additional stress must be expected in such multilayers as compared to homogeneous cantilevers w9x. An effect on the fracture properties of hetero-micromachined cantilevers may also be expected by the amount of
incorporated silicon impurities. In Fig. 6, the cumulative frequency W of fracture of InP cantilevers with a high silicon concentration at the heterointerface exceeding 10 20 cmy3 is displayed in a Weibull plot in comparison with the results obtained for cantilevers fabricated from InP on SiO 2-capped silicon grown in a two-step process. We find that the stress s 0 at a value of W s 63.2% amounts to 0.49 GPa which is almost a factor of 2 lower than for the reference case. The slope of the line is m s 3.8 corresponding to a smaller distribution width. A typical example for load-deflection measurements with GaAs cantilevers is shown in Fig. 4. For comparison, also GaAs cantilevers of low crystal quality, i.e. GaAs
Fig. 6. Failure distribution of InP microcantilevers caused by stress-induced fracture in a Weibull representation.
Fig. 8. Quasistatic characteristics of thermally actuated micromirrors Žcf. Fig. 7..
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Fig. 9. Frequency response of a thermally actuated micromirror.
affected by antiphase domains were investigated. In this case, a fracture limit of only 165 to 400 MPa was observed. A much higher fracture limit was observed with the cantilever of Fig. 4. Up to stress figures of 1.24 to 1.49 GPa applied to the cantilever clamp fracture did not occur. This is within the range of the fracture stress of 1 to 3 GPa observed for cantilevers that were fabricated from standard silicon wafer material w10x. 3.3. Micromirror As an example for an actuator device micromirrors were fabricated by hetero-micromachining of indium phosphide. Micromirrors can be used as light-steering elements in a variety of applications like printers, bar code readers and image displays, e.g. on-head displays for virtual-reality systems w11x. For actuation electrostatic w12x, piezoelectric w13x and magnetic forces w14x are commonly utilized. In the present study, thermal actuators were realized exploiting the bimorph effect of metallized InP resonators and cantilevers. Fig. 7 shows SEM photographs of micromirrors fabricated by hetero-micromachining of InP. The mirrors are suspended by four straight Ža., right-angle bent Žb. and two-times right-angle bent beams. For actuation, a double layer of AurCr which is resistant against the etch attack by KOH solution was deposited on top of the suspensions. Contact pads were positioned on the frame close to the clamp of the suspensions. For electrical connection, a gold wire of 25 mm in diameter was attached by ultrasonic bonding at 1208C. The connected suspension is heated by passing a current between adjacent pads. Strain due to the different thermal expansion coefficients of the metallization and the InP lead to a deflection of the mirror. In Fig. 8, the deflection angle of the micromirrors displayed in Fig. 7 is plotted vs. the electrical input power. The measurements were performed under quasistatic excitation. Straight lines fitted to the experimental points show a slope increasing with increasing length of the suspen-
sions corresponding to a decreasing bending stiffness. Measurements under variation of the excitation frequency revealed resonance frequencies of 2.9 kHz and 880 Hz in cases b and c, respectively. Fig. 9 shows the frequency response of a micromirror which is two-side clamped by orthogonally arranged suspensions. The corresponding low-frequency transfer characteristic displayed in the inset of the figure exhibits a deflection angle of 0.038rmW of electrical input power. Resonant modes defined by the mechanical structure were observed at 2.7 and 5.3 kHz. For cantilever mirrors of a length of 475–635 mm and a width of 50–70 mm we found an angle of deflection of 0.068rmW of electrical input power and a fundamental resonance mode in the range form 0.94 to 1.62 kHz. As shown in Fig. 9, the conversion of electrical power into mechanical deflection is by a factor of around 10 more efficient, if the device is operated at resonance. Degradation of device performance was not observed after up to 10 7 cycles of operation.
4. Conclusion Free-standing micromechanical elements were fabricated by hetero-micromachining with IIIrV compounds on silicon. Stress figures of several hundreds of MPa up to 1.5 GPa for GaAs could be applied to cantilever test structures without observing fracture. Both etching selectivity of InP to Si and fracture strength of InP cantilevers were found to be dependent on the concentration of Si impurities. By optimization of the epitaxy process, incorporation of Si could be reduced to 10 16 cmy3 at the layer surface. Using these layers, micromirror devices were fabricated by hetero-micromachining showing linear transfer characteristics with a slope efficiency ranging from 0.003 to 0.078rmW. The resonance frequencies of the thermally actuated devices were in the kHz range.
Acknowledgements We are grateful to Doris Rummler and Nicole Riedel ¨ for active technical support to the technological processes and characterization. This work is funded by the German Research Foundation ŽDFG..
References w1x P.J. French, P.T.J. Gennissen, P.M. Sarro, New silicon micromachining techniques for microsystems, Sens. Actuators, A 62 Ž1997. 652–662. w2x K. Fricke, E. Peiner, M. Chahoud, A. Schlachetzki, Fracture properties of epitaxial InP microcantilevers, Proc. Eurosensors XII, IOP, Bristol, 1998, pp. 723–726.
E. Peiner et al.r Sensors and Actuators 85 (2000) 324–329 w3x K. Fricke, E. Peiner, M. Chahoud, A. Schlachetzki, Fracture properties of InP microcantilevers, by hetero-micromachining, Sens. Actuators 76 Ž1999. 395–402. ˚ Schweitz, Gallium arsenide as a w4x K. Hjort, J. Soderkvist, J.-A. ¨ mechanical material, J. Micromech. Microeng. 4 Ž1994. 1–13. w5x E. Peiner, S. Mo, H. Iber, G.-P. Tang, A. Schlachetzki, Characterization on thin buffer layers for strongly mismatched heteroepitaxy, Thin Solid Films 283 Ž1996. 226–229. w6x E. Peiner, H.-H. Wehmann, H. Iber, S. Mo, G.-P. Tang, A. Bartels, A. Schlachetzki, A. Koch, K. Dettmer, M. Hollfelder, High-quality In 0.53 Ga 0.47 As on exactly Ž001.-oriented Si grown by metal-organic vapour-phase epitaxy, J. Cryst. Growth 172 Ž1997. 44–52. w7x S.N.G. Chu, S. Nakahara, S.J. Pearton, T. Boone, S.M. Vernon, Antiphase domains in GaAs grown by metalorganic chemical vapour deposition on silicon-on-insulator, J. Appl. Phys. 64 Ž1988. 2981– 2989. w8x A. Bartels, E. Peiner, R. Klockenbrink, A. Schlachetzki, The distribution of charge concentration in InPrSi, J. Appl. Phys. 78 Ž1995. 224–228. w9x R. Kuzmenko, A. Gansha, J. Schreiber, W. Kircher, S. Hildebrandt, S. Mo, E. Peiner, Stress-induced E 0 photoreflectance spectra on GaAsrSi and InPrSi heterostructures, Phys. Status Solidi A 152 Ž1995. 133–146. w10x C.J. Wilson, A. Omeggi, M. Narbutovskih, Fracture testing of silicon microcantilever beams, J. Appl. Phys. 79 Ž1996. 2386–2393. w11x T.A. Furness III, Where there is no vision . . . , Proc. Eurosensors XII, Southampton, 13–16 September, 1998, Plenary Paper 1.1.4. pp. 30–49. w12x W. Lang, H. Pavlicek, Th. Marx, H. Scheithauer, B. Schmitt, Electrostatically actuated micromirror devices in silicon technology, Sens. Actuators 74 Ž1999. 216–218. w13x A. Schroth, C. Lee, S. Matsumoto, R. Maeda, Application of sol–gel deposited thin PZT film for actuation of 1D and 2D scanners, Sens. Actuators 73 Ž1999. 144–152. w14x L.O.S. Ferreira, S. Moehlecke, A silicon micromechanical galvanometric scanner, Sens. Actuators 73 Ž1999. 252–260.
Biographies Erwin Peiner was born in Bad Munstereifel, Germany, in 1960. He ¨ received his Diplom-Physiker and PhD degrees, both in Applied Nuclear Physics, from the University of Bonn, Germany in 1985 and 1988, respectively. In 1989, he joined the Institut f ur ¨ Halbleitertechnik of the Technical University Braunschweig, where his main field of interest is the combination of IIIrV compound semiconductors and silicon for MEMS. Klaus Fricke was born in Soltau, Germany, in 1970. He received his Diplom-Physiker degree from the Technical University Braunschweig in 1996. Currently, he is pursuing his Dr.-Ing degree in the field of Si micromachining and its combination with MOS technology.
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Ingo Behrens was born in Bremen, Germany, in 1971. He received his Diplom-Physiker degree from the Technical University Braunschweig in 1999. His research activities are in the field of sensor and actuators by hetero-micromachining of IIIrV compound semiconductors. Andrey Bakin was born in St.-Petersburg Žformer Leningrad., Russia, in 1958. He received his Diploma of Engineer and PhD degrees, both from the St.-Petersburg Electrotechnical University Žformer LETI., Russia in 1981 and 1985, respectively, working there until 1988 on growth of IVrVI-compound semiconductors and IR-sensors fabrication. For one year in 1988–1989, and 3 months in 1993, he worked at the Institut f ur ¨ Halbleitertechnik of the Technical University Braunschweig, Germany, where his main field of interest was epitaxial growth of IIIrV-compound semiconductors. From 1989 to 1994, he worked as senior researcher and assistant professor at the St.-Petersburg Electrotechnical University, where he worked on the sublimation growth of SiC crystals. In 1994–1995, he worked at the Department of Physics and Measurement Technology of Linkoping University, Sweden on the investigation of silicon carbide ¨ CVD and sublimation growth. From 1995 to 1999, he was an associate professor at the St.-Petersburg Electrotechnical University, where he worked on the sublimation growth of SiC crystals. In 1999, he joined Institut f ur ¨ Halbleitertechnik of the Technical University Braunschweig, Germany, where his main field of interest is epitaxy of compound semiconductors using MOCVD for different micro- and opto-electronic devices. Andreas Schlachetzki was born in Breslau, Germany, in 1938. He received his Diplom-Physiker and PhD degrees, both from the University of Cologne, Cologne, Germany, in 1964 and 1969, respectively. While at the University of Cologne, he worked in ferromagnetism and ultrasonics. From 1970 to 1971, he investigated the magnetic properties of rareearth-hydroxide single crystals at He temperatures at Becton Center, Yale University, New Haven, CT. In 1971, he joined the Research Institute of the German Post Office, Darmstadt, Germany, where he was involved in the epitaxial growth of GaAs, and in basic aspects of fast digital circuits utilizing the Gunn effect in GaAs. In 1975, he worked for 6 months at the Electrical Communication Laboratory of Nippon Telegraph and Telephone, Tokyo. From 1976 to 1984, he was a professor at the Technical University Braunschweig, where he worked on the growth of InGaAsP and its use for integrated optical receivers. From 1984 to 1987, he was a professor at the Technical University Berlin and at the same time head of the Division Integrated Optics at the Heinrich-Hertz-Institut fur ¨ Nachrichtentechnik, Berlin. Since 1987, he has been the head of the Institut fur ¨ Halbleitertechnik of the Technical University Braunschweig where his main activities are in the field of monolithically integrated IIIrV components on Si substrates.
Hetero-micromachining of epitaxial IIIrV compound semiconductors Erwin Peiner ) , Klaus Fricke, Ingo Behrens, Andrey Bakin, Andreas Schlachetzki Institut fur ¨ Halbleitertechnik, Technical UniÕersity Braunschweig, Hans-Sommer-Str. 66, 38106 Brunswick, Germany Accepted 10 November 1999
Abstract Hetero-micromachining, a novel technique for the fabrication of miniaturized sensors and actuators is described. It is based on IIIrV compound semiconductor layers epitaxially grown on Ž001. silicon. Cantilevers composed of single indium phosphide or gallium arsenide layers or a layer sequence of different IIIrV compound semiconductors were realized exploiting the etching selectivity of the layer against the silicon substrate in KOH solution. Both etching and fracture properties of InP cantilevers are dependent on the concentration of silicon impurities in the layer. For GaAs, a fracture limit in excess of 1.5 GPa was found. Thermally actuated micromirrors that were fabricated by hetero-micromachining of InP could be deflected to up to 0.078rmW of electrical input power under quasistatic excitation conditions. Typically, an increase of this efficiency by an order magnitude was observed at resonance that was in the kHz range. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Hetero-micromachining; IIIrV compound semiconductors; Silicon impurities; Fracture limit; Thermally actuated micromirror
1. Introduction Bulk and surface micromachining are the conventional techniques for the fabrication of miniaturized silicon sensors and actuators. Both techniques suffer from weaknesses, e.g. considerable consumption of chip area in the former case and rather restricted mechanical performance in the latter. As a tradeoff, epi-micromachining has been described where the mechanical structure is realized using an epitaxial layer w1x. Different crystalline materials with potentially large etching selectivity can be combined by heteroepitaxy. As an example indium phosphide on silicon ŽInPrSi. was investigated with respect to its etching behaviour in KOH solution w2,3x. InP microcantilevers could be fabricated using this hetero-micromachining process. A fracture limit of 0.9 GPa was measured indicating that stability requirements for micromechanical applications can be met. In this study, hetero-micromachining with InPrSi, GaAsrSi and an InPrIn 0.53 Ga 0.47 As double heterostructure on GaAsrSi is described. GaAs may be preferable to
) Corresponding author. Tel.: q49-531-391-3761; fax: q49-531-3915844; http:rrwww.tu-bs.derinstituteriht. E-mail address: [email protected] ŽE. Peiner..
InP due to potentially higher piezoelectric coefficient and fracture strength w4x. Heterostructures incorporating latticematched ternary compound layers are of interest since they are basic constituents of electronic and optoelectronic devices that may be integrated with the mechanical structure. As an example for a device, a thermal actuator is realized by hetero-micromachining with InPrSi.
2. Experimental The basic steps of the hetero-micromachining process are depicted in Fig. 1 using the example of a thermally actuated micromirror. N-type Ž001. silicon wafers of resistivity and thickness of 2–4 V cm and 420–540 mm, respectively, are utilized as substrates. First Ža., a heteroepitaxial indium phosphide layer is grown by metalorganic vapour-phase epitaxy ŽMOVPE. w5x. Prior to growth, the substrate is deoxidized for 15 min at 9508C under H 2 flow. For stabilization of the surface reconstruction, AsH 3 is added at 8508C during the cool-down phase. A thin buffer layer Ž40 nm. is deposited at 4008C with a ratio of the group V and III components ŽVrIII. in the reactor of 2700. The total pressure P is 100 mbar. After buffer-layer growth, the process is routinely interrupted and the graphite susceptor is thermally cleaned at 12008C in a high-vacuum chamber. Subsequently, the main layer is grown at 6408C,
0924-4247r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 0 0 . 0 0 3 1 8 - 6
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Table 1 Layer sequence of structures in Fig. 3 Žtop layer: In 0.53 Ga 0.47 As.
Fig. 1. Basic fabrication steps of a micromirror by hetero-micromachining.
VrIII s 140 and a total pressure of 20 mbar resulting in a growth rate of 1 mmrh. Subsequently, a AurCr Žor AurTi. bilayer is deposited and structured using lift-off. In the second step Žb., the layer is patterned by wet etching using HBr: CH 3 COOH:K 2 Cr2 O 7 Ž1:1:1.. The resulting step height that corresponds to the thickness h of the layer or the layer sequence is measured by mechanical surface tracing ŽDektak 3 ST .. By the following etching in KOH Ž30%. at 608C Žc., membranes suspended by four beams were obtained exploiting the etching selectivity of Si with respect to InP. In Fig. 2, a scanning electron microscopy ŽSEM. photograph of processed micromirrors is displayed. AurCr-coated InP cantilevers and micromirrors are thermally actuated exploiting the bimorph effect between the metal layers and the IIIrV semiconductor. Cantilever or micromirror action is measured using the laser beam deflection technique, i.e. via the position of a laser spot after reflection at the device surface. Gallium arsenide microcantilevers are fabricated from layers grown at 4008C, VrIII s 4, 100 mbar Žbuffer layer, 15 nm. and 7008C, VrIII s 80, 50 mbar Žmain layer, 2–3 mm. w5x. For structuring, HCl:HNO3 Ž1:2. is used. Multi-
Fig. 2. Micromirrors in InP realized by hetero-micromachining.
Material
Thickness
In 0.53 Ga 0.47 As InP In 0.53 Ga 0.47 AsrInP InP GaAs
0.6 mm 1.1 mm 20=Ž7r7 nm. 1.0 mm 1.5 mm
layer samples incorporating indium gallium arsenide lattice-matched to indium phosphide ŽIn 0.53 Ga 0.47 As. Žcf. Table 1. can be selectively removed using HCl:H 3 PO4 Ž1:4. for InP and C 6 H 8 O 7 :H 2 O Ž1:7. for In 0.53 Ga 0.47 As. Epitaxy of In 0.53 Ga 0.47 As is performed at 6408C, VrIII s 140 and 20 mbar w6x. For w100x Žor w010x.-oriented IIIrV compound structures, as in the case of Fig. 1, it is possible to completely remove the silicon underneath by frontside etching. For a complete release of w110x-oriented cantilevers and resonators, we start from wafers which are thinned to a residual thickness of 300 mm by etching in KOH Ž30%, 608C.. Furthermore, the wafer backside is thermally oxidized to a thickness of 1.6 mm prior to epitaxy. After structuring this oxide layer, double-sided etching can be performed to account for the etch-stop behaviour of the 1114 crystal planes of silicon in KOH. Bending stiffness Dx and fracture limit of the micromechanical elements are measured with test structures like cantilevers of different widths w using a load-deflection technique w2,3x. An external load F varied from 10 to 400 mN is applied to a cantilever and the resulting deflection d is measured in dependence on the loading position x along the cantilever axis. Fracture is observed when the stress in the cantilever exceeds a critical value sac .
3. Results and discussion 3.1. Etching selectiÕity Fig. 3 displays SEM photographs of resonators and cantilevers fabricated from a sequence of IIIrV-semiconductor layers using hetero-micromachining. The layers and their epitaxial thicknesses are given in Table 1. Freestanding structures of various dimensions could be fabricated confirming the high etching selectivity of the different IIIrV compounds with respect to silicon. Only at the bottom GaAs layer is an etch attack indicated ŽFig. 3b.. For precise determination of the thickness h of the fabricated structures load-deflection measurements were performed with cantilevers used as test structures. Fig. 4 shows load-deflection curves measured with a w100x-oriented GaAs cantilever of a nominal thickness of 2.7 mm as determined by step-height measurement in the course of
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Fig. 3. Top Ža. and bottom Žb,c. SEM photographs of microresonators and -cantilevers fabricated by hetero-micromachining.
the fabrication process Žcf. Fig. 1 and context.. The solid lines correspond to fits of:
d Ž x. s
x3 3 Dx
F
Ž 1.
to the experimental curves with Dx s Ewh3r12 as the fitting parameter. Using the Young’s modulus of E s 85 GPa and a cantilever width of w s 175 mm, we obtained h s 2.1 " 0.1 mm. This result was confirmed by SEM with the cantilever cross-section. We attribute the thickness
reduction to an etch attack at the backside of the latticemismatched epitaxial GaAs where the crystal structure of the epitaxial layer is not perfect. In fact, for a GaAsrSi layer affected by antiphase domains w7x, most of the structures were damaged during the hetero-micromachining process. Cantilevers composed of the layer sequence given in Table 1 were analysed assuming an average Young’s modulus of E s 110 and 70 GPa for the w110x and w100x orientation, respectively. In Table 2, the results obtained with different cantilevers are given. In all cases, h is less than the original thickness of the as-deposited layer sequence indicating an etch attack during the hetero-micromachining process. An effect which can cause an impaired etching resistance of GaAs and InP against KOH solution is the incorporation of silicon impurities into the layer. With InP intentionally doped to concentrations of 10 18 to 10 19 cmy3 , an etch attack was observed leading to a considerable increase of the surface roughness. During heteroepitaxy, silicon impurities can be incorporated into the growing layer by solid-state diffusion across the heterointerface between substrate and layer or by diffusion via the ambient gas. To prevent uptake of silicon into the gas phase, the susceptor is thermally cleaned prior to the main-layer epitaxy. Furthermore, backside and sidewalls of the substrate surface can be coated with a SiO 2 cap. Fig. 5 shows the silicon doping profile in InP layers grown on a bare substrate without process interruption prior to main-layer growth Žsingle-step process. and on a substrate with a 0.4-mm-thick thermal oxide cap. In the latter case, a thermally cleaned susceptor is employed Žtwo-step growth.. On the abscissa, the position within the layer is given starting at the heterointerface Ž z s 0. and proceeding towards the layer surface. The dashed curves fitted to the measured profile in the latter case describe the solid-state diffusion of silicon across the heterointerface via lattice sites Ž; erfc z . and diffusion channels Žln n ; z 6r5 . in the layer w8x. The horizontal line corresponds to a uniform background doping level of around 10 16 cmy3 . We find that using a SiO 2-capped substrate, the silicon concentration is considerably reduced across the entire layer by
Table 2 Features of cantilevers characterized by load-deflection measurements Orientation
w Žmm.
h Žmm.
sac ŽMPa.
w100x
100 180
w110x
80
3.6 4.2 4.2 4.0 4.0 3.8 3.8 4.3 3.8
315 372 174 664 734 515 ) 537 ) 417 ) 586
120 140 160 180 Fig. 4. Load-deflection curves of a GaAs microcantilever.
E. Peiner et al.r Sensors and Actuators 85 (2000) 324–329
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Fig. 5. Silicon impurity concentration profile in heteroepitaxial InP on silicon.
more than an order of magnitude to values of 2 = 10 19 cmy3 at the heterointerface and less than 10 16 cmy3 at the layer surface. Cantilevers fabricated from these layers showed an only slightly reduced thickness corresponding to 95 " 3% of the value realized by epitaxy. For cantilevers of a high concentration of silicon impurities in the indium phosphide, however, h amounted to only 80 " 3% of the original thickness indicating an etch attack by the KOH solution.
Fig. 7. Thermally actuated micromirrors in InP realized by hetero-micromachining.
3.2. Fracture limit The fracture stress sac of multilayer cantilevers fabricated by hetero-micromachining is shown in Table 2. For w110x-oriented structures, considerably higher values of sac were found than for the w100x orientation. In the latter case, fracture did not occur immediately at the cantilever clamp but in the frame. Due to different thermal expansion coefficients of the constituent layers, additional stress must be expected in such multilayers as compared to homogeneous cantilevers w9x. An effect on the fracture properties of hetero-micromachined cantilevers may also be expected by the amount of
incorporated silicon impurities. In Fig. 6, the cumulative frequency W of fracture of InP cantilevers with a high silicon concentration at the heterointerface exceeding 10 20 cmy3 is displayed in a Weibull plot in comparison with the results obtained for cantilevers fabricated from InP on SiO 2-capped silicon grown in a two-step process. We find that the stress s 0 at a value of W s 63.2% amounts to 0.49 GPa which is almost a factor of 2 lower than for the reference case. The slope of the line is m s 3.8 corresponding to a smaller distribution width. A typical example for load-deflection measurements with GaAs cantilevers is shown in Fig. 4. For comparison, also GaAs cantilevers of low crystal quality, i.e. GaAs
Fig. 6. Failure distribution of InP microcantilevers caused by stress-induced fracture in a Weibull representation.
Fig. 8. Quasistatic characteristics of thermally actuated micromirrors Žcf. Fig. 7..
E. Peiner et al.r Sensors and Actuators 85 (2000) 324–329
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Fig. 9. Frequency response of a thermally actuated micromirror.
affected by antiphase domains were investigated. In this case, a fracture limit of only 165 to 400 MPa was observed. A much higher fracture limit was observed with the cantilever of Fig. 4. Up to stress figures of 1.24 to 1.49 GPa applied to the cantilever clamp fracture did not occur. This is within the range of the fracture stress of 1 to 3 GPa observed for cantilevers that were fabricated from standard silicon wafer material w10x. 3.3. Micromirror As an example for an actuator device micromirrors were fabricated by hetero-micromachining of indium phosphide. Micromirrors can be used as light-steering elements in a variety of applications like printers, bar code readers and image displays, e.g. on-head displays for virtual-reality systems w11x. For actuation electrostatic w12x, piezoelectric w13x and magnetic forces w14x are commonly utilized. In the present study, thermal actuators were realized exploiting the bimorph effect of metallized InP resonators and cantilevers. Fig. 7 shows SEM photographs of micromirrors fabricated by hetero-micromachining of InP. The mirrors are suspended by four straight Ža., right-angle bent Žb. and two-times right-angle bent beams. For actuation, a double layer of AurCr which is resistant against the etch attack by KOH solution was deposited on top of the suspensions. Contact pads were positioned on the frame close to the clamp of the suspensions. For electrical connection, a gold wire of 25 mm in diameter was attached by ultrasonic bonding at 1208C. The connected suspension is heated by passing a current between adjacent pads. Strain due to the different thermal expansion coefficients of the metallization and the InP lead to a deflection of the mirror. In Fig. 8, the deflection angle of the micromirrors displayed in Fig. 7 is plotted vs. the electrical input power. The measurements were performed under quasistatic excitation. Straight lines fitted to the experimental points show a slope increasing with increasing length of the suspen-
sions corresponding to a decreasing bending stiffness. Measurements under variation of the excitation frequency revealed resonance frequencies of 2.9 kHz and 880 Hz in cases b and c, respectively. Fig. 9 shows the frequency response of a micromirror which is two-side clamped by orthogonally arranged suspensions. The corresponding low-frequency transfer characteristic displayed in the inset of the figure exhibits a deflection angle of 0.038rmW of electrical input power. Resonant modes defined by the mechanical structure were observed at 2.7 and 5.3 kHz. For cantilever mirrors of a length of 475–635 mm and a width of 50–70 mm we found an angle of deflection of 0.068rmW of electrical input power and a fundamental resonance mode in the range form 0.94 to 1.62 kHz. As shown in Fig. 9, the conversion of electrical power into mechanical deflection is by a factor of around 10 more efficient, if the device is operated at resonance. Degradation of device performance was not observed after up to 10 7 cycles of operation.
4. Conclusion Free-standing micromechanical elements were fabricated by hetero-micromachining with IIIrV compounds on silicon. Stress figures of several hundreds of MPa up to 1.5 GPa for GaAs could be applied to cantilever test structures without observing fracture. Both etching selectivity of InP to Si and fracture strength of InP cantilevers were found to be dependent on the concentration of Si impurities. By optimization of the epitaxy process, incorporation of Si could be reduced to 10 16 cmy3 at the layer surface. Using these layers, micromirror devices were fabricated by hetero-micromachining showing linear transfer characteristics with a slope efficiency ranging from 0.003 to 0.078rmW. The resonance frequencies of the thermally actuated devices were in the kHz range.
Acknowledgements We are grateful to Doris Rummler and Nicole Riedel ¨ for active technical support to the technological processes and characterization. This work is funded by the German Research Foundation ŽDFG..
References w1x P.J. French, P.T.J. Gennissen, P.M. Sarro, New silicon micromachining techniques for microsystems, Sens. Actuators, A 62 Ž1997. 652–662. w2x K. Fricke, E. Peiner, M. Chahoud, A. Schlachetzki, Fracture properties of epitaxial InP microcantilevers, Proc. Eurosensors XII, IOP, Bristol, 1998, pp. 723–726.
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Biographies Erwin Peiner was born in Bad Munstereifel, Germany, in 1960. He ¨ received his Diplom-Physiker and PhD degrees, both in Applied Nuclear Physics, from the University of Bonn, Germany in 1985 and 1988, respectively. In 1989, he joined the Institut f ur ¨ Halbleitertechnik of the Technical University Braunschweig, where his main field of interest is the combination of IIIrV compound semiconductors and silicon for MEMS. Klaus Fricke was born in Soltau, Germany, in 1970. He received his Diplom-Physiker degree from the Technical University Braunschweig in 1996. Currently, he is pursuing his Dr.-Ing degree in the field of Si micromachining and its combination with MOS technology.
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Ingo Behrens was born in Bremen, Germany, in 1971. He received his Diplom-Physiker degree from the Technical University Braunschweig in 1999. His research activities are in the field of sensor and actuators by hetero-micromachining of IIIrV compound semiconductors. Andrey Bakin was born in St.-Petersburg Žformer Leningrad., Russia, in 1958. He received his Diploma of Engineer and PhD degrees, both from the St.-Petersburg Electrotechnical University Žformer LETI., Russia in 1981 and 1985, respectively, working there until 1988 on growth of IVrVI-compound semiconductors and IR-sensors fabrication. For one year in 1988–1989, and 3 months in 1993, he worked at the Institut f ur ¨ Halbleitertechnik of the Technical University Braunschweig, Germany, where his main field of interest was epitaxial growth of IIIrV-compound semiconductors. From 1989 to 1994, he worked as senior researcher and assistant professor at the St.-Petersburg Electrotechnical University, where he worked on the sublimation growth of SiC crystals. In 1994–1995, he worked at the Department of Physics and Measurement Technology of Linkoping University, Sweden on the investigation of silicon carbide ¨ CVD and sublimation growth. From 1995 to 1999, he was an associate professor at the St.-Petersburg Electrotechnical University, where he worked on the sublimation growth of SiC crystals. In 1999, he joined Institut f ur ¨ Halbleitertechnik of the Technical University Braunschweig, Germany, where his main field of interest is epitaxy of compound semiconductors using MOCVD for different micro- and opto-electronic devices. Andreas Schlachetzki was born in Breslau, Germany, in 1938. He received his Diplom-Physiker and PhD degrees, both from the University of Cologne, Cologne, Germany, in 1964 and 1969, respectively. While at the University of Cologne, he worked in ferromagnetism and ultrasonics. From 1970 to 1971, he investigated the magnetic properties of rareearth-hydroxide single crystals at He temperatures at Becton Center, Yale University, New Haven, CT. In 1971, he joined the Research Institute of the German Post Office, Darmstadt, Germany, where he was involved in the epitaxial growth of GaAs, and in basic aspects of fast digital circuits utilizing the Gunn effect in GaAs. In 1975, he worked for 6 months at the Electrical Communication Laboratory of Nippon Telegraph and Telephone, Tokyo. From 1976 to 1984, he was a professor at the Technical University Braunschweig, where he worked on the growth of InGaAsP and its use for integrated optical receivers. From 1984 to 1987, he was a professor at the Technical University Berlin and at the same time head of the Division Integrated Optics at the Heinrich-Hertz-Institut fur ¨ Nachrichtentechnik, Berlin. Since 1987, he has been the head of the Institut fur ¨ Halbleitertechnik of the Technical University Braunschweig where his main activities are in the field of monolithically integrated IIIrV components on Si substrates.