Principles of Ion Implant

Transcription

1 Principles of Ion Implant Generation of ions dopant gas containing desired species BF 3, B 2 H 6, PH 3, AsH 3, AsF 5 plasma provides positive ions (B 11 ) +, BF 2+, (P 31 ) +, (P 31 ) ++ Ion Extraction Ions are extracted from the source due to a high electric field Ion Selection Magnetic field mass analyzer selects the appropriate ion (mass & charge) Ion Acceleration Further accelerate ions giving the ions their final kinetic energy. Beam Scan / Disk Scan Provides a uniform dose of ions over the wafer surface. In-situ Dose Monitoring K.D. Hirschman Silicon Processes: Ion Implantation 1 Implant Mechanics K.D. Hirschman Silicon Processes: Ion Implantation 2

2 Plasma source and ion extraction Gas feed Nielsen-type gaseous source Plasma Chamber mt level pressure + ions To pump V ext variable extraction voltage (typically ~30KV ) K.D. Hirschman Silicon Processes: Ion Implantation 3 Ion selection ( Analyzing Magnet) B v v X R-H-R B F Magnetic Field V : ion velocity KE = qvext = ½M v 2 R F = q v xb = M v 2 / R Centripetal Force radius of curvature Resolving Aperture selected ions Heavier ions The ions are extracted from the source and analyzed in a magnetic field. The Lorentz force makes the ions take a curved path with a radius of curvature that depends on the mass of each ionic species. By adjusting the magnetic field strength, only the selected ions will enter the accelerating column. Mass to charge ratio of the selected ions: M /q = R 2 B 2 / (2 Vext) K.D. Hirschman Silicon Processes: Ion Implantation 4

3 BF 3 Gas Spectrum K.D. Hirschman Silicon Processes: Ion Implantation 5 PH 3 Gas Spectrum K.D. Hirschman Silicon Processes: Ion Implantation 6

4 µτ level pressure Ion Acceleration Resolving Aperture E field Final Kinetic Energy of the Ion = q ( V ext +V acc ) Example: V ext = 30 KV V acc = 70 KV Vacc Column length = 3 Ground Energy of the Ion = 100 KeV K.D. Hirschman Silicon Processes: Ion Implantation 7 Projected Range (Rp) Rp of Boron, Phosphorous, Arsenic and Antimony in Silicon as a function of the ion energy Rp (µm) Rp depends on incident and target atomic masses (complex) Ion Energy (KeV) K.D. Hirschman Silicon Processes: Ion Implantation 8

5 Beam Scanning Electostatic scanning (low/medium beam current implanters (I < 1mA) Scan Patterns V y Vertical Scan V x Si wafer Horizontal Scan Y X This type of implanter is suitable for low dose implants. The beam current is adjusted to result in t > 10 sec/wafer. With scan frequencies in the 100 Hz range, good implant uniformity is achieved with reasonable throughput. K.D. Hirschman Silicon Processes: Ion Implantation 9 In-situ Dose Control End Station secondary electrons wafer ion beam V suppression ( V ) V Faraday cup ( V ) I A back plate Ι dt = Q=A q φ DOSE MONITORING Ground K.D. Hirschman Silicon Processes: Ion Implantation 10

6 Mechanical Scanning Stationary Ion Beam Beam Area ~ 20 cm 2 (diameter ~ 5cm) Si wafer Rotating Disc ( 1200 rpm ) Excellent wafer cooling needed. Substantial load/unload time wafers /disc. Excellent throughput for high dose implants. High current (~20mA) Scan speed adjustment to insure uniform dose K.D. Hirschman Silicon Processes: Ion Implantation 11