Ni Removal from Si Wafers by Low-Pressure UV-Chlorine Cleaning
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UHV-compatible research tool via a cryo-pumped loadlock. Experiments on oxide-covered surfaces were performed on samples prepared by UV/air exposure of clean hydrogen-terminated
Si(100). The UV/C12 CVC chamber (Figure 1) is constructed from electropolished stainless steel. An electronic grade quartz liner and Suprasil disk are sealed to the chamber using Viton o-rings. Gases reach the sample via holes drilled in the quartz liner. The anodized Al sample stage is mounted on a magnetically coupled rotary-linear feedthrough. The sample is radiantly heated from the rear using a quartz halogen lamp. The reactor pressure is adjusted manually using a gate valve between the reactor and the turbopump (Edwards 381/160). An "ozone-producing" low-pressure Hg lamp (BHK Inc.) is used as the UV photon source. A multi-technique UHV surface analysis chamber (base pressure -1 x 10-10 Torr) described elsewhere is used for on-line AES and hot-filament Ni deposition [9]. Surface compositions were calculated using sensitivity factors found in the PHI AES Handbook. Suprasil Disk SiC-Coated Graphite
N2-Purged UV Lamp-__ý O-rings
eHolder
S~Quartz Liner Baratron Gauge
Cl2 Feed
lllll3001l/s Anodized At Sample Stage
Halogen Lamp
Turbomolecular Pump
Figure 1. UV/C1 2 CVC reactor. Nickel was deposited by hot-filament physical vapor deposition (PVD) from a Ni filament (Alfa AESAR 99.994%) inter-twined with two tungsten filaments. A constant-current power supply (Keithly 228) was used to deposit Ni in reproducible quantities. The current through the filament was fixed at 4 to 5 A which required a -3 V potential. The background pressure in the UHV chamber during PVD was typically 1 x 10-8 Torr. The Ni-contaminated samples were transferred to the CVC chamber in vacuo via the loadlock. Cl 2 (Air Products, electronic grade) was fed at 20 sccm, and the pressure measured by the Baratron gauge was adjusted to 300 mTorr. Each sample was UV/C12 treated for 2 min, and processing temperatures between 100 - 200'C were examined. Samples were subsequently cooled under N 2 without UV irradiation and transferred back to the UHV chamber for AES. Similar UV/C12 exposures were also performed on uncontaminated bare and oxide-covered Si surfaces. XP spectra were collected off-line using a PHI 3057 XPS system with a dual-anode x-ray source and PC interface. Al Ka x-rays (1486.6 eV) were used due to the overlap of the Ni AE peaks (LMM, 488, 479, and 545 eV) and the 0 Is photoelectron peak (531 eV) when using the Mg anode. The pass energy was set at 24 eV.
488
Atomic force microscopy (AFM; Digital Instruments 30001) was used to examine the surface morphology after UV/C12 CVC and to measure Si etch rates. In the etch rate experiments, a 1000 A thermal oxide was grown on a 100-mm wafer and patterned using conventional photolithographic techniques. Square areas of various sizes were etched in the oxide to expose bare Si. Samples were then UV/C12 processed at substrate temperatures between 150-250 0 C, the oxide mask was removed by wet etching, and the step form
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