Ion Implantation and Synthesis of Materials - Studium

172 12 Surface Erosion During Implantation: Sputteringon experimental data of PtSi sputtering with Ar ions. Setting Y = 4.5 and r = 2.0,(12.10) predicts the steady-state surface Pt concentration ratio to be N Pt /N Si =2(4.5 − 1) −1 = 0.57, or a Pt concentration of N Pt /(N Si + N Pt ) = 36%.To calculate the Pt dose required to reach this maximum concentration, theparameter R P is estimated to be 29 nm for 45 keV Pt in Si. To reach ~90% of thesteady-state concentration, a thickness of ~4R P or 116 nm, has to be sputtered.This thickness corresponds to 5.8 × 10 17 atoms cm −2 (based on an atomic densityof 5.0 × 10 22 cm −3 for Si), which requires a Pt dose of 1.3 × 10 17 cm −2 for asputtering yield of 4.5.12.6.2 Pt Implanted with 45 keV Si IonsFor Si implanted in Pt, the parameter r is now defined by J Pt /J Si = r N Pt /N Si andis approximately 1/2. With Y = 3.0 and r = 1/2, (12.10) predicts the maximumsurface Si concentration to be N Si /N Pt = 0.25, or a Si concentration of N Si /(N Pt + N Si ) = 20%.To achieve ~90% of the maximum Si concentration, a sputter removal of athickness of 32 nm is required. This thickness corresponds to 2.1 × 10 17atoms cm −2 (based on an atomic density of 6.6 × 10 22 cm −3 for Pt), which requiresa Si dose of 0.7 × 10 17 cm −2 for a sputtering yield of 3.The discussions about the two implantations (1) Pt implanted into Si and (2) (Siimplanted into Pt), are summarized in Table 12.1. The sputtering yield of Case (1)is higher than that of Case (2). However, because of the difference in r values,Case (1) can achieve a higher implanted concentration than Case (2). For the samereason, it also takes much more sputtering for Case (1) to reach its maximumconcentration.12.6.3 PtSi Implanted with SiImplantation of Si into PtSi would be expected to result in a Si-rich Pt–Si mixture.However, instead of increasing the Si concentration, the Si implantation may decreaseTable 12.1. Comparison of two ion implantations (45 keV)Pt into SiSi into Pisputtering yield Y 4.5 3.0preferential sputtering parameter r 2 1/2maximum implanted concentration 36 at. % 20 at. %implanted layer thickness W 29 nm 32 nmmaterial sputtered to reach ~90% 116 nm 32 nmof dose required for max. conc. 1.3 × 10 17 Ions cm −2 0.7 × 10 17 Ions cm −2