The Enabling Potential of Infusion Doping for Advanced Semiconductor Manufacturing

Recent developments at Epion Corporation have led to the availability of production capable infusion processing equipment for ultra shallow doping and surface engineering. Epion has developed the nFusion™ 300mm doping system, offering a solution that addresses these processing challenges and enables market demands to be met. Infusion processing is a unique doping and surface modification technology that utilizes a Gas Cluster Ion Beam (GCIB) source to produce a directed energetic chemical beam. Dopant-containing gas species can be introduced into this chemical beam and become incorporated into a substrate. Infusion is an ultra shallow, high rate process performed at room temperature. Infusion processing now offers a truly viable alternative to shallow plasma and ion beam processes and does not suffer from their limitations.

The Infusion Process
During infusion processing, surfaces are treated with a highly directional beam of high-energy (1–60 keV) gas clusters made up of large numbers (n>5000) of atoms or molecules and having unit or small electrical charge. The physical and chemical effects arising from the interaction of such large ionized clusters with a substrate are dramatically different from those observed in monomer ion processing (i.e. ion implant, plasma etch, and deposition processes) that use single atom or low molecular weight ions. In conventional processing the kinetic energy of each ion resides in a single or very small number of atoms. Clusters are delivered at high energy to a surface but this kinetic energy is shared with the thousands of atoms in the cluster. As a consequence, the average energy of an individual atom or molecule in GCIB processing is typically < 10eV. When a cluster strikes a surface, it produces a highly localized transient thermal and pressure spike (TTPS) at the surface lasting only a few picoseconds (Figure 1), while the cluster disintegrates. The TTPS then propagates in 3-dimensions and is quickly quenched. The process of the rapid dissipation of this energy into the near surface and its associated effects we call, ‘infusion'. It produces dramatically different physical and chemical characteristics than conventional processes. While process conditions for GCIB infusion are typically 1E-5 Torr and room temperature, the instantaneous surface conditions represent extraordinary pressures and temperatures relative to traditional plasma technologies. GCIB infusion performs in ways not possible with other techniques for precisely this reason. The unique characteristic of infusion doping is that atoms or molecules are infused into a substrate by bombardment from individual ionized clusters. The average number of atoms infused is determined primarily by the gas mix and energy and has been shown to be very stable and repeatable. Although a distribution of cluster sizes is generated, the doping effects are dependant primarily on the acceleration energy. This is because the doping mechanism is unique and fundamentally different from ion implantation. Unlike standard plasma and ion beam techniques, the doping depth is not affected by the atomic weight of the species. To illustrate this, diborane and germane gases are mixed and co-infused simultaneously (Figure 2). Even though the mass of B and Ge differ, both are infused to exactly the same depth. Also significant is the absence of end of range damage. Many important applications for surface engineering are possible with this new technique of thin film formation. The ability to create SiGe or pure Ge amorphous layers by Ge infusion is receiving interest as an alternative to SiGe epi for channel strain or Ge channels [2].

nFusion™ System Overview
Gas clusters are formed by the adiabatic expansion of gas through a nozzle. Clusters are ionized and accelerated by up to 60keV. Monomers and small clusters are filtered from the beam by a fixed magnet and a faraday is used to measure the beam current. Stable beams have been demonstrated for many hundreds of hours [3]. A mechanical scanning system is used to produce uniform doping across 200 or 300mm substrates. Fully automated processing tools incorporating cluster ion sources capable of up to 1000 micro Amperes (formed of both inert gases such as argon and reactive gases including CF4, NF3, B2H6, GeH4, O2, N2, and SiH4) are available.

The ability to transfer many atoms with each ion enables infusion processing to effectively produce extremely high equivalent beam currents as compared to ion implantation. Processes have been developed where up to 2000 boron atoms can become incorporated into silicon per cluster. The equivalent beam current is many times higher than existing high current implanters for low energy conditions. This gives infusion doping technology high throughputs for shallow doping profiles. Figure 3 shows the equivalent beam current as a function of energy. Doping profiles <10nm deep are possible with > 60 wafers per hour (wph) throughput for a 1E15 dose of boron. Important shallow doping applications which will benefit from this capability are for source drain extensions (SDE) and dual gate poly doping. These capabilities are possible because infusion processing is able to deliver high-energy clusters to the surface while minimizing space charge effects which tend to blow up ion beams and reduce beam current.



Ultra Shallow Junction Formation
The generally accepted Ultra Shallow Junction requirements for the 45 nm process node include a doping depth of 10nm and <0.05% energy contamination [4]. Very low beam energies are required for standard ion implantation to be used. When an ion beam is decelerated to reduce the implantation energy, some of the ions may be neutralized and may continue to the wafer causing energy contamination. To avoid this problem, drift mode may be employed, but beam currents are greatly diminished and throughput suffers.

A number of solutions have been proposed to enable high throughput ultra shallow doping including plasma doping [5] or molecular species [4] such as decaborane (B10) or octa-decaborane (B18). Despite this, many issues remain. Important considerations for the manufacturability of any new process technique are the controllability and reproducibility in manufacturing. High throughputs must be maintained while producing very low contamination levels. The process must also be integrated into standard manufacturing flow. Infusion processing addresses all of these concerns. Figure 4 shows a comparison between traditional 0.3keV 11B monomer ion implantation doping and infusion doping by 5keV gas clusters made up of a mixture of B2H6 and Ar gases. Doping profiles are extremely abrupt and do not exhibit channeling. Significant channeling can be seen with the ion implantation process resulting in an Xj of 37 nm. Infusion doping is accomplished without the need for a pre-amorphizing implant step (PAI) due to the self–amorphizing properties of infusion.

Activation behavior of the boron for infusion doping is similar to traditional beam line implanted boron [Figure 5]. Devices have been fabricated by Renesas using 1E15 5keV B2H6 into the SDE region of pMOSFETS with 50nm gate lengths 3. A standard spike anneal at 1050 oC was performed and good device performance was realized. The measurements show the short channel effects (SCE) are improved by 20nm as compared to a 200eV implant operating in the drift mode.

Process repeatability is maintained with closed loop dose control. A marathon demonstration equivalent to 3000 wafers processed was performed during 200hrs of continuous operation using a 5KeV B2H6 process at a 1E15 dose. The boron doping depth at 1E18 atoms per cm2 was controlled to 12nm + 0.3nm as measured by SIMS [Figure 6]. After a spike anneal, the sheet resistance repeatability was <2% one sigma.

DRAM Poly doping
Advanced DRAM device dual gate poly structures require high doses of boron to compensate for n-type dopant in the poly. A number of factors impede implementation of doses higher than 1E16, < 70nm deep. High current implanters start to suffer significant degradation of throughput. Additional limitations are self-sputtering effects as well as out diffusion of the boron from subsequent anneals [4].

Infusion has a number of advantages for poly doping. No self-sputtering effects have been observed with infusion regardless of doping depth. Retained boron dose is linear with ion dose (Figure 7). No dose limit is reached and deposition will occur when the surface is saturated with dopant. This effect is unique to infusion and makes the process distinct from ion implantation or plasma doping.

Even for very high doses of boron, the surface of the poly still contains silicon indicating that dopant is continuously incorporated into the poly and does not "pile up" on the surface. XPS analysis shows that the B/Si ratio is 2.9 when a 5E16 dose of boron is infused into polysilicon, and no boron film is detected. After a spike anneal at 900 ºC, the surface is 50% silicon but the retained boron dose is identical to the pre-anneal dose demonstrating there is no out diffusion of the boron. The beam currents demonstrated in Figure 3 enable throughputs > 50wph at a 5E16 dose. Many more applications of high concentration shallow doping are envisioned.

References

1. M. Current, N. Cheung, S. Felch, B. Mizuno, C. Chan, K. Walter, "Plasma Immersion Ion Implantation: Applications for Semiconductor Materials and Coatings," a chapter in Ion Implantation Science and Technology, 8th ed., September, 2002, Fig. 3.2.
2. J. Borland, J. Hautala, M. Gwinn, T. Tetreault, and W. Skinner, Solid State Technology, May 2004, p.53.
3. J. Bachand, A. Freytsis, E. Harrington , M. Gwinn, N. Hofmeester, J. Hautala, M. Mack, K. Regan, IIT2002 Proceedings, IEEE, Piscataway, 2003 p 66
4. Borland et al, "Applying Equivalent Scaling" Semiconductor International, 1/1/2005
5. Tom Horsky, Octo-decaborane, IIT Conference, Taiwan 2004
6. Z.Fang et al, IWJT2005, Kyoto Japan
7. Kawasaki et al, IWJT2005, Kyoto Japan
8. Buh et al, 2005 USJ Conference, Daytona, USA