Abstract

Continuous in-situ particle monitoring with a CLS 700T particle counter can provide an extremely simple and cost effective means of monitoring a BOE tank of a wet bench. Since the CLS-700T particle counter measures particle levels in the tank itself, there is no question of contamination from external sources such as wafer handling. The data is specific to a given wet bath so there is no confusion as to which tank is causing the problem. Since the particle counter is continuously monitoring, problems can be identified very shortly after they occur. Finally, the CLS-700T can identify many different situations when the tank is out of specification during normal processing of wafers, before a problem occurs.

Introduction

Buffered Oxide Etch (BOE) is a mixture of Hydrofluoric Acid and Ammonium Fluoride. It is used to strip oxides and photoresist from silicon wafers during manufacturing in semiconductor fabs. Inside the fab, the mixture is diluted with ultrapure water (UPW) in a process bath according to proprietary recipes. Wafers are dipped into the bath to perform a variety of etches. Dip times are varied to give the desired type of etch, from a light, clean etch to a complete strip. Afterwards, the wafers are placed into a rinse tank to remove the acid from the wafers and carriers.

Wet etch processes are by nature dirty. The design of a good wet etch system takes this into account. A well-designed wet bench re-circulates its chemical through filters and then returns it to the bath. For good performance, the bath must be turned over frequently. As the filter loads up, the efficiency of the pump will degrade, causing fewer bath turnovers. This will eventually allow the tank to become dirty and contaminate the wafers being etched.

Traditionally, the cleanliness of wet benches has been determined with un-patterned monitor wafers. Typically, one of two methods is employed. With the first method, two or more monitor wafers, which have been prepared and checked for particles with a surface particle counter, are loaded into an empty cassette. They are then run through the etch, rinse, and dry process and taken back to the surface particle counter. If fewer than a prescribed limit of particles are added to the wafers, the bath is within specifications. Otherwise, one of the baths in the sequence, presumably the etch tank, is beyond specification and must be serviced. In the second method, monitor wafers, which have been tested as described above, are placed at each end of a boat of process wafers. The wafers are then run through the normal etch, rinse, and dry sequence. Then the monitor wafers are removed and taken back to the surface particle counter. The same limit comparison is made to determine the tank's compliance.

There are a number of drawbacks to these methods. Both of these techniques are extremely sensitive to operator handling. The wafers are exposed to the environment for extended periods of time allowing additional opportunity for wafer contamination. The possibility that the excessive contamination came from a source other than the actual tested bath leads to a tendency to disregard the results until another test can be completed. During this time, the tank may continue to process production wafers. If the tank is indeed contaminated, the production wafers may be compromised. Another problem occurs between test intervals when an incorrectly prepared lot of wafers is processed in the tank by mistake. The tank is completely contaminated but remains in production until the next scheduled test, which may be hours away. Witness wafers take up production space, require special handling and preparation, and cost money.

The CLS-700T particle counter from Particle Measuring Systems is an In-Situ Particle Monitor (ISPM) for wet benches. It can take a sample of liquid from a bath, measure the level of particles and compare that level to prescribed limits. It operates continuously without operator intervention and, in combination with Facility Net software, can notify users of problems via automatic reports, computer messages, or even personal pagers.

Installation

Position the CLS-700T particle counter next to the bath to be monitored. Connect power and air to the system. Plumb the sample inlet tube directly into the bath to be measured. Plumb the sample outlet and exhaust to the weir or overflow side of the bath. Connect the CLS-700T to Facility Net for data collection and instrument control.

The CLS-700T particle counter can receive a signal from the tank or process too that determines when sampling is to occur. This prevents the CLS-700T from operating when the tank is empty. The signal input can either be a switch closure or 0-5 volt signal from the tank or process tool.

System Verification

The first step after installation is to verify correlation with witness wafers. The following are examples:

A tank was scheduled for this test and monitored until baseline was reached. Once at baseline, a series of witness wafers were run through the bath. The counts were checked and verified that the tank was within specification. The recirculating pump was taken offline, and the tank was allowed to sit for 15 minutes without any wafers being introduced to the tank. The counts in the CLS-700T particle counter increased measurably during this time.

Another series of witness wafers were run through the tank and checked. The tank was out of specification according to the wafers as well. The recirculating pump was turned on again, and within 10 minutes the CLS-700T reported the tank within operating limits. The witness wafer test was repeated and confirmed the tank was indeed within specification and ready to process wafers again. The conclusion from this test is that the CLS-700T particle counter can perform the qualification of the process tank without using witness wafers or additional manpower and it can do it more quickly and reliably than the witness wafer method.

Monitoring with the ISPM

Installing the CLS-700T particle counter on the BOE tank resulted in savings in other areas as well. Any disturbance of the etch system, including maintenance, can result in increased particle generation. The most common mistake made during routine maintenance is improper filter seating. When this occurs, the contaminated chemical bypasses the filter allowing the bath to increase suspended particles.

In one instance, a sudden increase in particle counts was caused when the witness wafers were processed. Although they tested within specifications, the background level in the tank had increased to a level which was unacceptable for production wafers. The paging system on the CLS-700T monitoring software alerted the equipment technicians in time to prevent production wafers from being introduced. After properly seating the filter, the counts dropped dramatically, indicating a return to acceptable background levels. The process operators were preparing to run production wafers when the equipment technicians arrived to correct the problem. In addition to saving time diagnosing the problem, the CLS-700T saved production wafers by immediately picking up the failure.

Another problem detected by the ISPM is the failure to pre-wet hydrophobic filters. When this happens, very little chemical is allowed to pass through the filter. This creates a stagnant bath, and every boat of wafers adds particles to it. Again, this contamination can affect production. Additionally, the pump is under an extreme load. If undetected for a long enough period, its life will be shortened appreciably. This leads to another possible problem; the pump could intermittently or abruptly stop. The results are the same as the other examples: contaminated wafers.

Conclusion

The CLS-700T particle counter is capable of qualifying the BOE etch tank more quickly and reliably than the previous method of using witness wafers. Additionally, other maintenance issues can be readily identified and corrected before production wafers are compromised.

This saves wafers, and reduces the need for expensive rework. Eliminating witness wafers can result in cost savings exceeding $50,000 per year.

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