Measurement of Intrinsic Stress in a-Si:H thin films Deposited in a Remote Hydrogen Plasma Reactor
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MEASUREMENT OF INTRINSIC STRESS IN a-Si:H THIN FILMS DEPOSITED IN A REMOTE HYDROGEN PLASMA REACTOR K. S. STEVENS AND N. M. JOHNSON Xerox Palo Alto Research Center, Palo Alto, CA 94304 ABSTRACT The effects of deposition conditions on the intrinsic stress in a-Si:H films deposited in a remote hydrogen plasma reactor are reported. The intrinsic stress is calculated from the change in substrate curvature induced by the a-Si:H film, as measured by the optical lever technique, and the results are compared with those for rf glow discharge deposited a-Si:H films. INTRODUCTION Motivated by the need to better understand the problem of adhesion of a-Si:H films and a desire to further explore the unique properties of a-Si: H films deposited in a remote hydrogen plasma (RHP) reactor, we have undertaken a systematic study of intrinsic stress in these films. Results are reported on the dependence of intrinsic stress on deposition temperature, silane (SiH 4 ) dilution in hydrogen (H 2 ), and phosphine (PH3 ) concentration. Additionally, intrinsic stress in rf glow discharge (GD) deposited a-Si:H films was examined as a function of deposition temperature for comparison with the RHP a-Si:H films. An optical lever technique was used to obtain substrate curvatures from which the intrinsic film stress was calculated. As is typical of electronic-grade a-Si:H films from GD deposition, the intrinsic stress in RHP films was found to be compressive and ranged from - 100 MPa to - 800 MPa. EXPERIMENT The a-Si:H films were deposited either in a remote hydrogen plasma reactor or in an rf glow discharge system. For each case, deposition conditions known to produce electronic grade a-Si:H films were chosen. The RHP system relies on monatomic hydrogen (H) from a hydrogen (H 2) plasma to dissociate silane (SiH 4 ) into reactive species which lead to a-Si:H film deposition on a heated substrate.1 The H2 plasma is sustained and confined by 2.45 GHz microwave radiation delivered to a microwave cavity which surrounds a portion of the alumina tube through which the H2 flows. The RHP deposition conditions were as follows: 200 sccm H2 flow, 10-40 sccm SiH 4 flow, 400 W plasma power, and 0.5 Torr chamber pressure. The GD system is a standard parallel plate reactor which operates at 13.56 MHz.2 The GD deposition conditions were as follows: 100% SiH 4 plasma with 100 sccm SiH 4 flow, 2W plasma power, and 0.1 Torr chamber pressure. The RHP films ranged in thickness from 0.5 to 0.75 pm, and the GD films were 0.4 pm thick. The RHP films were deposited on 1" X 0.75" quartz substrates and 0.5" square float zone silicon substrates. The GD films were deposited on 2.5" square quartz substrates. All substrates were -500 pm thick. Curvature measurements were performed with quartz substrates, and infrared absorption (IR) spectra were recorded with silicon substrates. The total stress in each a-Si:H film was calculated from the expression at = Ests2 / [6(1 -vs)
tfR] ,
(1)
where at = total film stress (by convention, a compressive film stress is negative ), Es = Young
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