Low Temperature Dopant Activation Using Variable Frequency Microwave Annealing
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Low Temperature Dopant Activation Using Variable Frequency Microwave Annealing T. L. Alford, K. Sivaramakrishnan, and A. Indluru School of Mechanical, Chemical, and Materials, Arizona State University, Tempe, AZ 85287 School of Electrical Engineering, Arizona State University, Tempe, AZ 85287 Iftikhar Ahmad and R. Hubbard Lambda Technologies, Morrisville, NC 27560 N.D. Theodore Freescale Semiconductor Inc., 2100 East Elliot Rd., Tempe, AZ 85284
ABSTRACT Variable frequency microwaves (VFM) and rapid thermal annealing (RTA) were used to activate ion implanted dopants and re-grow implant-damaged silicon. Four-point-probe measurements were used to determine the extent of dopant activation and revealed comparable resistivities for 30 seconds of RTA annealing at 900 °C and 6-9 minutes of VFM annealing at 540 °C. Ion channeling analysis spectra revealed that microwave heating removes the Si damage that results from arsenic ion implantation to an extent comparable to RTA. Cross-section transmission electron microscopy demonstrates that the silicon lattice regains nearly all of its crystallinity after microwave processing of arsenic implanted silicon. Secondary ion mass spectroscopy reveals limited diffusion of dopants in VFM processed samples when compared to rapid thermal annealing. Our results establish that VFM is an effective means of low-temperature dopant activation in ion-implanted Si.
INTRODUCTION Goals established in the current International Technology Roadmap for Semiconductors (ITRS) anticipate physical gate lengths of silicon-based devices at 13 nm by 2013 [1]. These smaller feature sizes are instrumental in achieving faster devices. One major undesirable effect of reduced junction depths is increased contact resistance. To counteract this effect, shallow implants are done to elevate the concentrations (1020 cm-3) of electrically active dopants; thus, reducing the contact resistance and compensating for the effects of the reduction of device feature sizes [2]. Another adverse effect of high dose, low energy ion implantation is the increased amount of nuclear energy loss deposited into the near surface region of the silicon substrate [2]. This large amount of lattice damage results in increased sheet resistance. To repair the damage and activate the dopants, high temperature annealing is used. During a high temperature anneal, thermally activated processes (e.g., diffusion) can take place [2]. To lessen the extent of dopant diffusion during heating, rapid thermal annealing (RTA) is done with the use of lasers and/or lamps [3]. A shortcoming of both of these techniques is that they create uneven heating due to emissivity differences in near-surface device materials and because the photons used in lamp and laser heating cannot penetrate past the near surface region of silicon [4].
Earlier work shows that single frequency microwave annealing can activate dopants and remove radiation induced damage in boron implanted Si [5]. Alford et al. have shown that microwave annealing can repair lattice damage
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