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Silicon On Insulator (SOI)

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Sputtered Silicon

(by Dr. Richard Hurley)

Sputtered silicon has potential as a bonding surface for transfer of pre-processed silicon layers to various insulating substrates. A possible wide range of applications (see for example [1,2]), exist including MEMS, 3-D mm wave components, stacked layer devices, flexible silicon layers on plastic or metal, transfer of pre-processed silicon layers to non-silicon substrates with large thermal expansion coefficient mismatches, silicon to sapphire and solar cell technology. We have found that the material appears quite suitable for low temperature processing and by choosing appropriate sputtering conditions a very smooth layer suitable for direct bonding can be obtained. Sputtering is a low temperature process with insignificant substrate heating during deposition. However, when annealing of bonded layers for thermal splitting or bond strengthening is required, gas trapped in the pores of the sputtered material is released at temperatures above 350 ⁰C and further increases of temperature lead to destruction of the bonded interface Fig 1 (a). The gas can be effectively driven out by thermal annealing at a temperature of 1000⁰C, [3]. For device applications when requirements demand that processing temperatures before bonding must be kept below about 300⁰C, this technique cannot be used. A collaborative study with the University of Surrey has shown that excimer laser-annealing of the sputtered silicon surface will remove trapped gases without significant heating of the underlying substrate Fig 1(b). Short pulse UV light is particularly suited to raising the temperature of a thin surface film in a controlled manner without significant substrate heating [4,5]. With silicon films, it is generally used to produce melting and re-crystallisation but has also been used with lower power densities for cleaning (Larciprete et al. [6]) and desorption of carbon and oxygen (Schlemm et al. [7]).  Hence the technique is suitable for thermal splitting applications where hydrogen and/or helium is implanted in a substrate whose temperature rise must be strictly limited.  Our experiments have shown that with correct choice of parameters it is possible using overlapping beam scanning to achieve optimum conditions for trapped gas removal and yet maintain a low roughness surface suitable for subsequent wafer bonding.

(a)  Without laser-annealing of sputtered surface before bonding. Bond-anneal at 400 ⁰C for 2 hours. Gas trapped in sputtered silicon layer builds up in bonded interface during heating and destroys bond.

(b) With laser-annealing. Bond-anneal at 1000⁰C for 2 hours. Only particulate voids are seen and bond remains good.

Figure 1.  Scanning acoustic microscope (SAM)  photos of bonded wafer pairs with sputtered silicon on one surface. Black areas are voided.

References

1. Bergmann R B, Werner J H. Thin Solid Films, 2002;403-404:162.

2. Werner J H, Dassow R, Rinke T J, Köhler J R, Bergmann R B. Thin Solid Films, 2001;383:95.

3. Hurley R E, Gamble H S. Vacuum, 2003;70:131

4. Coscia U, Ambrosone G, Minarini C, Parisi V, Schutzmann S, Tebano A, Restello S, Rigato V. Thin Solid Films, 2004;453-454:7.

5. Dutto C, Fogarassy E, Mathiot D. Applied Surface Science, 2001;184:362.

6. Larciprete R, Borsella E. Journal of Electron Spectroscopy and Related Phenomena,1995;76:607.

7. Schlemm H, Buchmann F, Geiler H –D. Applied Surface Science,1992;54:298.