Si Atomic Layer Epitaxy Based on Si2H6 and Remote He Plasma Bombardment

Abstract Atomic layer epitaxy (ALE) of Si has been demonstrated by using remote He plasma low energy ion bombardment to desorb H from an H-passivated Si(100) surface at low temperaturea and subsequently chemisorbing Si 2 H 6 on the surface in a self-limiting fashion. Si substrates were prepared using an RCA clean followed by a dilute HF dip to provide a clean, dihydride-terminated (1 × 1) surface, and were loaded into a remote plasma chemical vapor deposition system in which the substrate is downstream from an r.f. noble gas (He or Ar) glow discharge in order to minimize plasma damage. An in situ remote H plasma clean at 250°C for 45 min was used to remove surface O and C and to provide an alternating monohydride and dihydride termination, as evidenced by a (3 × 1) reflection high energy electron diffraction (RHEED) pattern. It was found necessary to desorb the H from the Si surface to create adsorption sites for Si- bearing species such as Si 2 H 6 . Remote He plasma bombardment for 1–3 min was investigated over a range of temperature (250°C−410°C), pressures (50–400 mTorr) and r.f. powers (6–30 W) in order to desorb the H and to convert the (3 × 1) RHEED pattern to a (2 × 1) pattern which is characteristic of either a monohydride termination or a bare Si surface. It was found that as He pressures and r.f. powers are raised the plasma potential and mean free paths are reduced, leading to lower He bombardment energies but higher fluxes. Optimal He bombardment parameters were determined to be 30 W at 100 mTorr process pressure at 400°C for 1–3 min. He was found to be more effective than Ar bombardment because of the closer match of the He and H masses compared with that between Ar and H. Monte Carlo TRIM simulations of He and Ar bombardment of H-terminated Si surfaces were performed 3o validate this hypothesis and to predict that approximately 3 surface H atoms were displaced by the incident He atoms, with no bulk Si atom displacement for He energies in the range 15–60 eV. The He bombardment cycles were followed by Si 2 H 6 dosing over a range of partial pressures (from 10 −7 Torr to 1.67 mTorr), temperatures (250°C–400°C) and times (from 20s to 3 min) without plasma excitation, because it is believed that Si 2 H 6 can chemisorb in a self-limiting fashion on a bare Si surface as two silyl (SiH 3 ) species, presumably leading to a H-terminated surface once again. The Si 2 H 6 dosing pressures and times corresponded to saturation dosing (about 10 6 langmuirs). Alternate Si 2 H 6 dosing and He low energy ion bombardment cycles (about 100–200) were performed to confirm the ALE mode of growth. It was found that the growth per cycle saturates with long Si 2 H 6 dosing at a level which increases slightly with He bombardment time. At 400°C, for 2 min He bombardment at 100 mTorr and 30 W, the growth per cycle saturates at about 0.1 monolayers cycle −1 , while for 3 min He bombardment the Si growth saturates at about 0.15 monolayers cycle −1 . It was also confirmed that the growth is achieved only by using alternate He bombardment and Si 2 H 6 dosing. He bombardment alone for a comparable time (3 min × 100 cycles) causes a negligible change in the Si film thickness (less than 5 A). Similarly, thermal growth using Si 2 H 6 under these conditions for (3 min × 100 cycles) causes negligible deposition (less than 5 A).