Steve McCoy

A comparative study of dopant activation in boron, BF/sub 2/, arsenic, and phosphorus implanted silicon

Ultra-low energy implants were used in combination with rapid thermal anneals in the temperature range 900/spl deg/C-1050/spl deg/C to study dopant activation in silicon. First, relatively long time anneals were performed in a conventional tungsten-based RTA to investigate the activation mechanisms. The activation was monitored using Hall measurement, where the rate of electrical activation was considered by measuring the time it takes to reach 50% activation. Using Arrhenius fits, an activation energy was extracted, and it was found that while boron has a mean activation energy for electrical activation of 4.7 eV in agreement with previous studies, arsenic and phosphorus have thermal activation energies of 3.6 eV and 4.1 eV, respectively. The 4.7 eV activation energy for boron is believed to be related to a point defect driven mechanism for electrical activation. Electrical activation of arsenic and phosphorus, however, seems to be related to dopant diffusion. In the second set of experiments, an arc lamp system was utilized to perform ultra-sharp spike anneals. For both dopants, it was found that for a given temperature, there is an optimum ramp-rate that produces the desired dopant activation and junction depth.

Low resistance, low-leakage ultrashallow p/sup +/-junction formation using millisecond flash anneals

Junction formation using solid phase epitaxial (SPE) regrowth has been gaining popularity due to its high activation and low thermal budget which results in lower diffusion. Recently, it was shown that by carrying out the SPE regrowth at 1050/spl deg/C using a single Flash from a millisecond annealing tool it is possible to obtain active concentrations as high as 6.5/spl times/10/sup 20//cm/sup 3/ (Flash SPE process)-much higher than low-temperature SPE-and near as implanted profiles. But the end-of-range (EOR) damage left beyond the amorphous-crystalline (a-c) interface results in poor leakage. We study the effect of a second Flash anneal at higher peak temperatures, and show that we can anneal out the EOR defects while causing minimal diffusion and deactivation of the B. This results in nearly two orders of magnitude reduction in leakage currents compared to a single Flash SPE.

Millisecond Annealing: Past, Present and Future

The challenge of achieving maximal dopant activation with minimal diffusion has re-awakened interest in millisecond-duration annealing processes, almost two decades after the initial research in this field. Millisecond annealing with pulsed flash-lamps or scanned energy beams can create very shallow and abrupt junctions with high concentrations of electrically active carriers, but solutions for volume manufacturing must also meet formidable process control requirements and economic metrics. The repeatability and uniformity of the temperature cycle is the key for viable manufacturing technology, and the lessons from the development of commercial rapid thermal processing (RTP) tools are especially relevant. Advances in the process capability require a fuller understanding of the trade-off between dopant activation, defect annealing. diffusion and deactivation phenomena. There is a strong need for a significant expansion of materials science research into the fundamental physical processes that occur at the short time scales and high temperatures provided by millisecond annealing.

Enhanced-Performance Germanium Nanowire Tunneling Field-Effect Transistors Using Flash-Assisted Rapid Thermal Process

We report the enhanced performance of Ge nanowire (NW) tunneling field-effect transistors (TFETs), realized using a millisecond flash-assisted rapid thermal process (fRTP) for dopant activation. The electrical characteristics of our fRTP-activated NW TFETs exhibit maximum drive currents up toImax ~ 28 μA/μm at Vdd = -3 V and improved subthreshold swings. By comparison, NW TFETs realized using conventional RTP for dopant activation show an order of magnitude lower current. We attribute these findings to a more abrupt doping profile at the tunnel junction, owing to reduced dopant diffusion and improved dopant activation.

Characterizing implant behavior during flash RTP by means of backside diagnostics

A promising new technique for achieving ultra-fast rapid thermal annealing of shallow implants is Flash-assist RTP/spl trade/ (fRTP/spl trade/). The Vortek fRTP tool produces a unique time-temperature profile on the wafer surface by first rapidly heating the bulk of the wafer to an intermediate temperature and then exposing the implanted surface of the wafer to an intense flash of radiation. The sudden increase and decrease of the wafer surface temperature results in a more gradual variation in the wafer backside temperature, which can be easily monitored with a radiometer. This paper describes the thermal physics involved in this annealing technique and shows how the backside measurement can be used to estimate the front-side temperature. The annealing behaviour of various boron and BF/sub 2/ implant conditions is presented. These data are presented graphically, in a manner that clarifies the advantages of fRTP over conventional spike annealing.

An Overview of ms Annealing for Deep Sub-Micron Activation

Millisecond annealing (MSA) has been developed over the last several years as a viable approach to achieve the high electrical activation, limited diffusion and high abruptness needed for junctions in the sub-65nm regime. This paper will provide an overview of the technology including the motivation, technology and some process results. Both main approaches for MSA, sub-melt laser and flash lamp annealing will be discussed as well as the potential challenges to bring these technologies into mainstream manufacturing.

Ultra-shallow junction formation using flash annealing and advanced doping techniques

As the demand for ever shallower, highly active and abrupt junctions continues, it is important to look at both the doping and activation portions of junction formation as a unit process. Advanced doping is useless without annealing methods that limit diffusion and provide high levels of electrical activation and new annealing techniques cannot make the junctions shallower than the as-doped profiles. This work has looked at optimizing several types of advanced doping (Plasma Doping and beamline ion implantation of molecular dopants) and a flash lamp-based ms annealing approach. With this combination, very shallow, abrupt and low resistivity junctions can be formed. Careful characterization was used to ensure the accuracy of the sheet resistance and junction depth measurements.

Crystal Damage Removal by Spike and Flash Annealing

The formation of extended defects resulting from the precipitation of the large amounts of interstitials and vacancies generated during the dopant and pre-amorphisation implantation is the major issue related to the formation of highly doped p+/n junctions. Interactions between defects and implanted dopants produce diffusion and activation anomalies that are among the major obstacles to the realisation of ultra- shallow junctions satisfying the ITRS requirements. The ideal thermal treatment should remove all the damage and get a high dopant activation with minimal diffusion. Modifying first the depth of the end-of-range damage by varying the pre- amorphisation implantation energy and the thermal budget by using second (spike) and millisecond (fRTP) annealing, the optimal values for implantation energy and thermal budget can be extracted for a complete defect annihilation. Transmission electron microscopy is used to determine the crystal quality.

Flash thermal processing through the melting point of silicon

The recent capability to achieve sub-millisecond surface temperature increases of greater than 600/spl deg/C over the entire surface of 300 mm wafers using flash-assist rapid thermal processing, fRTP/sup TM/ has allowed the exploration of silicons response to rapid thermal processes over the entire surface of the wafer. Previous results from high-speed processing of silicon through its equilibrium melting point have only been attainable by localized, small area, application of high-powered lasers. We report data obtained from a narrow band radiometer, operating at 1450/spl plusmn/30 nm with a 25 kHz sampling rate, that indicates anomalous emissions during melting. Interesting re-crystallization behavior of the bare silicon surface that is cooling 500.000/spl deg/C/s through its equilibrium melting point after a flash anneal from an intermediate temperature of 900/spl deg/C is also reported. Wafer survivability after exposure to greater than 600/spl deg/C surface temperature jumps from intermediate temperatures of 700/spl deg/C to 900/spl deg/C also demonstrates the inherent robustness of the wafer to withstand large thermal gradient at elevated bulk temperatures imposed during fRTP.

Experimental and theoretical results of dopant activation by a combination of spike and flash annealing

The diffusion length of the flash anneal is lowest for all different implant conditions. For the arsenic implant similar diffusion length is seen for all the processes that include a spike anneal due to the fact that the overall thermal budget is mainly determined by the spike anneal. Boron implants into crystalline as well as pre-amorphized silicon show similarly low sheet resistance independent of whether they are annealed with spike + flash, flash or flash + spike. For the arsenic implant by far the lowest sheet resistance is seen with a combination of spike + flash anneal. For the boron and arsenic implants the defect density after the flash anneal and the spike + flash anneal is below the weak beam dark field detection limit of the transmission electron microscope therefore suggesting low leakage due to defects present. From the simulations of the arsenic and the boron concentration profiles it can be learned that the spike profile determines the position of the chemical profile and the activation is increased by the subsequent diffusion-less flash anneal. Thus junction depth can be easily adjusted by the spike anneal condition in a spike + flash scheme, while still maintaining a high degree of dopant activation. This offers great flexibility to next generation junction formation. Although in this study the spike and flash annealing were performed in different tools, a combination of spike + flash can be easily run in the Mattson fRTPtrade system. The combination of spike + flash anneals also has been shown to improve the transistor drive current significantly without undesirable shifts in the other transistor characteristics.