Sputtering deposition of thin films at cryogenic temperatures

Abstract A sputtering system was adapted to deposit thin films onto substrates that were cryogenically cooled by a two-stage cryorefrigerator acting as a substrate holder. Aluminum films were sputter deposited onto NaCl crystals at various temperatures ranging from 293 down to 40 K. The purpose of this work is to observe composition and microstructural changes resulting from substrate cooling during deposition. Transmission electron microscopy techniques showed that aluminum films deposited using argon onto substrates at 80 K have an f.c.c. structure with a grain size of approximately 300 A as compared with similar films deposited onto substrates at ambient temperatures which showed grain sizes up to approximately 2000 A. All the aluminum films deposited at temperatures down to 40 K were found to be microcrystalline rather than amorphous. Sputter-deposited films of aluminum on substrates cooled to 40 K were obtained using neon as the sputter gas with marginal success because of the cryopump's limited capability for maintaining adequate neon flow for optimum sputtering. These films were found to be nodular in appearance with each nodule having a microcrystalline structure with an average grain size of approximately 2000 A. Low temperature sputter deposition using neon is being investigated with an oil diffusion pump replacing the cryogenic vacuum pump.