The Microwave Plasma Electron Flood

Marvin Farley, Vice President and CTO
Bo Vanderberg, Staff Scientist
Axcelis Technologies, Inc.

Wafer charge neutralization during the 1980’s was a hot topic in the field of ion implantation. It was a significant problem in high current implanters due to both charge induced implant non-uniformity and poor gate oxide reliability. The term “antenna ratio” became a common phrase in communications between customers and manufactures of ion implanters. The first generation of electron flood systems was based on secondary electron generation in the vicinity of the ion beam and wafer surface. Primary electrons having kinetic energy in the range of 300 eV were directed across a stationary ion beam to a metal or graphite target that would yield some secondary electrons in the vicinity of the beam. The energy distribution function EDF of the resulting electrons had two energy peaks, one at a few eV and another at the primary electron energy due to elastically reflected primaries. Some say this high energy peak was the major source of negative charging potential that forced users to balance positive charge control against negative charging or “over-flooding”.

Later during the middle 1990’s the secondary electron source was replaced with a Plasma Electron Flood system. The PEF systems were far more effective than their predecessors due to two main reasons; (1) low electron energy distribution function, and (2) efficient electron transport due to the positive charge of the plasma ions. In terms of prevention of wafer charging, the PEF is a very effective means of control. Most PEF systems in the field today are based on a simple arc discharge source, similar to a Bernas ion source, with a magnetic field aligned with the cathode to enhance ionization. A negative side effect of this type of source is cathode sputtering and evaporation, which generates contaminant atoms and ions in the emerging plasma. Although many steps have been taken to minimize transport of these unwanted contaminants, there is still a moderate probability that contamination coexists with the implant.

Modern applications, such as ion implantation of image sensing devices, have increased the requirement of low metals contamination, and a large ion implantation equipment manufacturer has recently announced an electrode-less PEF,   using an RF source to generate low energy plasma. This can certainly generate the plasma necessary to neutralize wafer charge without the risk of introducing refractory metals into the process, but it does so at the expense of high background gas pressure and may also create significant sputter contamination.

A better choice is a microwave PEF system, such as the one developed for our Purion M medium current implanter.  The microwave PEF based on electron cyclotron resonance ECR has the capability to produce high electron density at relatively low pressure through high efficiency ionization of the gas atoms within the microwave plasma chamber. The plasma chamber contains an array of permanent magnets that produce an alternating polarity of magnetic cusps covering the inside surface of the chamber with exception of the region containing the extraction slit. These cusps serve two purposes; 1) electron mirror or reflector to reduce electron loss area. 2) ECR surface defined as the surfaces having 875 gauss, the requisite resonance field for 2.45 GHz microwaves. The PEF is located in close proximity to the implant surface and adjacent to the scanned ion beam. The extraction slit is oriented parallel to the fast-scan direction and perpendicular to the axis of the ion beam providing for the maximum coupling (bridging) between the PEF and the ion beam.

The microwave components used to power the PEF consist of off-the-shelf WR284 waveguide elements. These are illustrated below beginning with the microwave launcher which is a magnetron tube connected to a ¼ wave launcher in the dimensions of the WR284 waveguide. The launcher is followed by a three port circulator (called isolator in the picture) that passes microwaves entering port 1 to 2, port 2 to 3, and essentially blocking port 2 to 1. This circulator is used to capture reflected microwaves in a water cooled dummy load attached to port 3 which prevents unnecessary overheating of the magnetron tube. Following the circulator is a three stub tuner which is used to match the impedance of the downstream elements and PEF chamber to that of the upstream microwave generation system. There are two windows in the downstream system, one for vacuum isolation and the other that is a ¼ wave matching element used to couple microwaves into the PEF chamber.
PEF Chamber

Performance of the microwave PEF has been encouraging. TEG wafers having antenna ratio of 100K : 1 and 1M : 1 have been implanted with 80 KeV As+ at 3 mA to a dose of 5E14 with and without the PEF operating. 100% of the 1M : 1 devices passed with the PEF turned on and 100% failed with it turned off. The operating efficiency of the microwave PEF is proving superior to the hot filament PEF in every way; low operating pressure, low floating voltage, output current, low contamination, and low input power. The maintenance period for the PEF is also on the order of years, without the need of quarterly filament or RF antenna replacement. Look for continuous improvement in design and performance of our microwave PEF systems in the future.

PurionM Elimination of Metals Contamination
The AEF eliminates all metal contaminants as measured via VPD-ICPMS. The microwave PEF design eliminates all traces of heavy metals common to PEF filaments (tungsten, moly).

Charge Monitor Results
These results demonstrate AS+ 80keV 5e14, 3mA run with PEF off and on. The charge monitor wafers show good correlation to PEF setting. Even the structures with the highest antenna ratios maintain high breakdown voltages with PEF on.