Photoresist Detection In 300mm PVD Degas Chambers
Current state-of-the art methods for 200mm photoresist detection cannot be used successfully on 300mm tools due to the increase in pressure in the degas chambers and changes in tool operation.
Causes and methods of detection
Photoresist contamination can cause fabs big headaches. Even small amounts of residual photoresist can reduce product yields, cause 300mm PVD tool downtime, and cause elevated levels of particles in the process chamber over time, resulting in costly premature kit changes. Major photoresist hits can be especially costly - upwards of $600,000 if the target becomes contaminated. Photoresist can mistakenly be introduced into process chambers in a variety of ways. An example is when hot lots are rushed through, resulting in missed process steps, such as resist strip, causing immediate contamination throughout the PVD process tool. Another more common example is the incomplete removal of photoresist, which can be caused by a poorly controlled endpoint in the asher, such as from first-wafer, cold chamber effects. Such events are often associated with faults on integrated etch/strip tools.
Simple determinations of photoresist contamination (selecting an outgassing peak observed in a sample contamination event) frequently are not selective enough to properly warn or alarm the presence of photoresist. Photoresist formulations are proprietary, and therefore engineers will sometimes try to identify a single mass spec peak as an indicator of the photoresist formulation that they are currently using. However, the chemical structure of the photoresist is complex, and the outgassing signature changes dramatically after the bake, etch, strip and other process steps. Therefore, the selected mass spec peak may not be a reliable indicator of the photoresist formulation.
In many cases, such calculations of photoresist from selected peaks may only be sensitive to heavily coated photoresist wafers and therefore wafers with smaller but detrimental amounts of photoresist could go undetected. Conversely, looking for a combination of "signature" peaks often reduces effective photoresist detection. In situations where there are several different device process flows through a fab or foundry, engineers can spend a lot of time repeating this photoresist peak selection process and testing for each different device recipe.
Current 200mm methods of photoresist detection are not suitable for use on 300mm PVD tools. The pressure in 300mm degas chambers is typically 4-10Torr, which is significantly higher than the pressures in 200mm degas chambers. The mass spec-based sensors cannot withstand those pressures, and the associated algorithms do not account for changes in process recipes and tool operation. Therefore, current 200mm photoresist detection capabilities do not meet the demands of 300mm processing.
300mm resist detection
The 300mm Resist-Torr Module from MKS Spectra Products is a precise combination of high sensitivity process monitoring hardware and an advanced proprietary algorithm to quantitatively determine the amount of photoresist left on a wafer. By combining the factors affecting the 300mm degas step with mass spectrometry inputs, a quantitative, meaningful measure of photoresist contamination is derived - this is called the "PR Index".
During the development of the PR Index algorithm, several thousand wafers at many different fabs were run to refine and to prove out the PR Index algorithm. To ensure that the PR Index would be independent of photoresist formulation while providing accurate, low-level photoresist detection, several peaks were identified, together with methods of comparison that correspond to the active components of photoresist.
Tests were conducted at a major 300mm PVD tool OEM to prove that the PR Index is accurate and quantitative measure of photoresist contamination. A set of 5 test wafers consisting of a dummy wafer for baseline, 100% stripped of photoresist and 50% stripped photoresist were tested to determine if the 300mm Resist Detection Module could distinguish among them. Indeed these wafer conditions could be detected (Figures 2 and 3).
Figure 3 represents the resulting PR Index levels, which show how the measure accurately detects the difference in amounts of photoresist on wafers. An independent test was performed to determine the ability of the PR Index method to indicate degrees of residual photoresist proportionally from none through moderate to gross levels (Figure 4).
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