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PLASMA PROCESS CONTROL

PLASMA PROCESS CONTROL 6.15

bias voltages greater than the plasma potential, the probe current saturates at the electron saturation
current.
Traditionally used by researchers, the Langmuir probe is a valuable instrument to understand
plasma physics. One example utilizing the Langmuir probe to study transformer-coupled plasma is
given in Ref. 61. Reliable-process endpoint has also been achieved using the Langmuir probe. 62
Surface contamination on the Langmuir probe adversely affecting its measurements has been
published. 63,64

6.3.4 Optical Emission Spectroscopy
Optical emission spectroscopy (OES) utilizes an optical sensor to detect and measure the spectral
emission from the plasma. Typically the sensors comprise a CCD image sensor and an optical filter.
It has become the most widely used method for detecting oxide-etch endpoint in semiconductor
65
manufacturing environments. Patent applications and grants that employ this technology for etch
endpoint and rate are numerous. Though they have proven to be a viable technology; limitations of
66
OES have been cited and justify alternative endpoint detection methods. The case is made that cur-
rent dual damascene via etch requires exposed areas in the ranges of 2 to .5 percent and with the
demands of shrinking critical dimensions, the exposed area may decrease to .2 percent. Presently the
limitations encountered with OES are approximately 1 percent of the exposed oxide area.

6.4 PROPERTIES OF DRY ETCH PROCESS

There are two types of methods of etching—chemical and physical. These combine to form four
basic plasma processes—sputtering or physical, chemical, ion-energy-driven, and ion-enhanced-
inhibitor etching. Chemical etch is performed by free radicals that bombard the target with a near-
uniform angular distribution. This process is characterized by high etch rates and occurs in
high-pressure conditions. Physical etch is performed by the sputtering of ions and is an anisotropic
process. Anisotropy is defined as the ratio of the vertical etch to the horizontal etch. With this
process only a marginal amount of sidewall removal occurs. This is the only etch process that can
remove nonvolatile material from the target. As compared to chemical etch, the selectivity for
physical etch is low. Selectivity is the ratio of the etch rate of the insulator to the etch rate of the
semiconductor. 17

6.5 FUTURE TRENDS AND CONCLUSIONS

To look to the future, we need to reflect on the past. In the 1980s the semiconductor industry relied
on single-frequency, capacitively coupled plasmas for manufacturing. The 1990s provided an array
of technology choices. Variants of ECR and inductively coupled plasma chambers were offered.
During this time all major etch companies were moving toward high-density plasma chambers.
Presently, the capacitively coupled chamber appears to be the mainstay. Today a large manufactur-
ing base of plasma chambers for semiconductor manufacturing comprise the AMAT P5000 PECVD
tool, LAM Rainbow and TCP series etch platforms and TEL Unity II DRM etch tool. Novellus
Concept 1 & 2 PECVD tools also share a significant part of the market.
There have been tremendous advances in etch capability of feature-size, selectivity, and critical-
dimension control that have been accomplished by continuing to refine the chamber and its inputs.
The industry will continue to be challenged with shrinking feature sizes. The present manufacturing
capability of height-to-depth ratios ranges from 20:1 to as much as 60:1. The industry trend today is
to increase the number and frequency of RF sources. Time will tell if this evolution of science will
yield better control of the plasma and achieve the expected manufacturing objectives.

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