Shu Qin

Measurement and analysis of deposition-etch characteristics of BF3 plasma immersion ion implantation

The deposition-etch characteristics of BF3 plasmas were quantitatively measured and analyzed using a deposition monitor, and were correlated with plasma parameters. It was found that by controlling pressure and rf power, the source could be operated in regimes which were either deposition or etch dominant. This data was then applied to a −2 kV plasma immersion ion implantation BF3 process to explain retained dose characteristics. A good qualitative agreement between the deposition-etch data and implanted retained dose data was obtained.

Measurements of secondary electron emission and plasma density enhancement for plasma exposed surfaces using an optically isolated Faraday cup

We present secondary electron yield and plasma enhancement factor data for silicon surfaces exposed to Ar, He, N2, O2, H2, and BF3 plasmas, for incident ion energies from 0.5–10 keV. A fiber-optic isolated Faraday cup was used to directly measure the ion current Jion, allowing a direct measurement of the secondary electron yield. This method automatically accounted for the effect of pulse-induced plasma density enhancement due to the ionization of neutral gas by accelerated secondary electrons, which we observed and measured quantitatively. The values of the secondary electron yields measured by this method were higher than published values measured by the conventional (ultraclean surface and ultrahigh vacuum) methods but lower than published values measured by previous plasma immersion ion implantation methods.

Faraday dosimetry characteristics of PIII doping processes

A Faraday cup dosimetry system was developed and characterized to address the issues of plasma immersion ion implantation (PIII) dose measurements. Pure ion current was measured by using an electrostatic suppression mechanism combined with high-bandwidth fiber-optic electronics to isolate high-voltage pulses and eliminate the primary and secondary electron and displacement currents. The ion-current waveform measured by the Faraday cup was verified by an XPDP1 particle-in-cell simulation. All of the positively charged ions striking the target surface were counted for implant dose by the Faraday cup so that both high-energy implant dose during the pulse and low-energy implant dose between pulses can be separately determined. The dose of the high-energy implant during pulses, which is more influential on the junction depth, can be measured with a fairly good accuracy, although the low-energy implant dose cannot be accurately measured due to more complicated surface effects. Compared with other dosimetry methods for PIII doping processes, the Faraday dosimetry technique offers better repeatability and controllability for PIII processes due to its direct, in-situ manner.

Ion depletion effects in sheath dynamics during plasma immersion ion implantation-models and data

In plasma immersion ion implantation, the wafer is negatively pulsed while immersed in a dc ambient plasma. During this high voltage pulse, the sheath expands, and plasma ions are accelerated to the wafer. The essential character of this plasma sheath expansion can be described by a simple mathematical model, first proposed by Lieberman. In this article, we build on Lieberman’s model, extending it to describe the ion current before and after the pulse. We find that a dip in ion current is predicted immediately after the pulse, due to the depletion of ions within the sheath. This simple model is tested using Faraday cup data, and is also compared to a particle-in-cell simulation.

Active charge control in PIII—enlarging the process space

Abstract Charge control during the source-drain implants in the CMOS process is critical to avoid damage to the thin gate dielectric. PIII has always been touted as having good charge control, since the charge deposited on the gate during the implant pulse is neutralized by the plasma electrons between pulses, thus having a ‘built in plasma flood’. However, unless the time between pulses T bet is long, the gate can float to a positive potential in excess of 15 V. We show that T bet can be arbitrarily reduced without inducing any gate voltage stress by applying a positive bias during this time. This requires a special ‘Charge Balance Modulator’, as well as an additional electron source. The balance of charge is measured by an in situ charge monitor, which simulates the charge on a CMOS gate. Data from this charge monitor, as well as SPIDER wafers are compared to theory. The PIII process can be considered to be a blend of ion implantation as well as plasma processing. Depending on process conditions, combinations of ion implantation, as well as plasma deposition and etch combine to yield the final dopant profile. Some of these components may be advantageous or disadvantageous, depending on the process. An important component in controlling the relative amounts of these components is the pulse width and frequency. The Charge Balance Modulator allows control of these parameters independent of charging issues.