Introduction

Molecular contamination (AMC) is gaining an ever-increasing focus as semiconductor manufacturers, hard disk drive makers, aerospace equipment firms, and other leading edge technology producers face process drift and yield losses attributed to this gas phase form of contamination. Yield improvements through the reduction of airborne molecular contamination is now a significant challenge as facilities stabilize other sources of contamination. As more and more sites gain control over their particle contamination issues and circuit geometries continue to shrink, emphasis is shifting to airborne molecular contamination as the next target for improving productivity.

Molecular contamination takes many forms, such as condensable organics, dopants, and acidic and basic gas species, and can originate from a variety of sources such as makeup air, process gases, chemicals, materials, and even humans.

Acidic gas contamination is particularly insidious because of its reactivity with a number of other chemical species. This reactivity often occurs on surfaces that are very sensitive to contamination, such as semiconductor wafer surfaces, optical components within photolithography steppers and scanners, read/ write heads and magnetic coated disks on hard disk drives. A main driver of acidic gas contamination reduction has been photolithography process and equipment engineers. Not only can this contamination cause drifting of critical process parameters due to lens contamination and optical hazing, but also terrific costs may eventually be incurred in order to replace contaminated optics in steppers and scanners. Of particular interest are those acids, particularly organic acids such as sulfonic acids, which may be generated during the exposure process. Because they are produced in close proximity to the reduction lens' final optical element, these species can play a significant role in the formation of films that can potentially adhere to and contaminate optical surfaces.2 Reactivity of acidic gas contamination is typically a non-reversible process, and this often results in permanent changes to product surface properties. This can be especially catastrophic to exposed metallic product surfaces, such as aluminum, copper, and other metallic coatings. In addition to reacting on product surfaces, acidic gas contamination can have detrimental effects on cleanroom materials which may indirectly affect product surfaces, such as filters, sealants, epoxies, plastics used in tools and components, etc. For example, small amounts of acidic gases in the air can cause HEPA filters to accelerate the release of boron.3

Real-Time Monitoring of Molecular Contamination

Particle Measuring Systems' AiM® is designed to measure Surface Molecular Contamination (SMC). SMC is the result of gas phase molecules (AMC) interacting with product surfaces to form very thin chemical films, often altering the physical, electrical, chemical, and optical properties of the surface. Monitoring SMC with coated Surface Acoustic Wave (SAW) sensors is particularly useful as a tool to mimic actual semiconductor wafers, optical components, or disk drive media conditions. The data output can then be used to correlate end of line yield loss to conditions attributed to SMC. These conditions can stem from facility, process, operator, or equipment events such as tool maintenance, chemical refills or spills, material outgassing, loadlocks opening and closing, and chemical filter degradation or breakthrough. Major advantages offered by SAW technology are high-sensitivity and real-time monitoring of molecular contamination. Mass accumulation and/or chemical reactions occur on the surface of an oscillating SAW sensor. The frequency of oscillation changes in response to this mass interaction, which allows SAW sensors to offer 0.02 ng/cm2/Hz sensitivity, with 1- minute sample intervals. Other technologies that measure SMC such as witness wafers and Quartz Crystal Microbalances (QCM) are significantly less sensitive, and may not have real-time data output capability.

Acidic Gas Monitoring and Sensitivity

AiM monitors acidic gas contamination on copper and silver coated SAW sensor surfaces. Copper sensors react strongly with inorganic halogen gases (HCl, HF, HBr) while silver sensors react more strongly with sulfur oxide and nitrogen oxide gas species. An experiment was setup to show the reaction effect of a silver sensor coating to various concentrations of sulfur oxide (SOx) contamination. SO2 was provided via a permeation tube using CDA as a carrier agent that was then mixed with humidified CDA to achieve 40% RH conditions at 32°C. The gas was then distributed to an environmental chamber in which an AiM was enclosed within. Byproducts from the SO2 and humidified air may include sulfuric acid, as well as organic and inorganic sulfate compounds depending upon the CDA quality.

Figure 1 (Download this paper for all tables and figures.) (426.3 KB) demonstrates the results from two different concentrations of SO2 contamination on a silver coated sensor surface. SO2 concentrations of 1.0 ppb and 0.4 ppb are represented. The mass accumulation changes from 39.16 ng/cm2/day at an SO2 concentration of 1.0 ppb to 12.04 ng/cm2/day for an SO2 concentration of 0.4 ppb once equilibrium between the contamination and sensor surface has been established. This represents a contamination rate reduction of approximately 69%, compared to a 60% reduction in SO2 concentration (from 1.0ppb to 0.4ppb). Detection of low-ppt levels of contamination is possible with SAW technology.

Acidic Gas Contamination Rate Guidelines

Acidic gas contamination rates on copper and silver SAW sensors will vary from those contamination rates seen with SiO2 coatings. Cleanliness levels have been established for acidic gas contamination rates from data collected at a number of different semiconductor and HDD manufacturers. Table 2 describes the daily contamination rate guidelines for metallic coated sensors monitoring acidic gas contamination.

Figure 1 (Download this paper for all tables and figures.) (426.3 KB)

A very clean monitoring environment accumulates less than 2.5 ng/cm2/day. Figure 2 shows accumulation within the minienvironment of a copper-plating tool. Contamination was measured on a copper sensor and the environments' average daily contamination rate was very clean at 0.94 ng/cm2/day.

Only looking at one variable (mass accumulation) can be deceiving at times. Multiple outputs from the AiM such as temperature, humidity, mass, frequency, and rate allow the user to better understand the contamination events within a facility or process. Figure 3 shows a contaminated monitoring location using a copper SAW sensor. The average daily contamination accumulation rate is about 6.3 ng/cm2/day. Viewing only the mass accumulation, the data may look somewhat stable. Monitoring the contamination rate as well identifies one large contamination event where the contamination rate is significantly higher than the average contamination rate. Another event shows a decrease in contamination rate for a sustained period of time. It can be important to understand the reason for these rate changes, in order to minimize the impact that a large contamination event can have on product, as well as try to reproduce and sustain events that cause a contamination rate decrease.

Acidic Gas Monitoring of Cleanroom Environments

As mentioned previously, acidic gas contamination can originate from a number of different sources. Makeup air is notorious for allowing sulfur and nitrogen oxides into the facility if the outside air is of poor quality. Within a facility, chemical systems, chemical spills, and process material can generate acid gas species that are detrimental to product surfaces. Since the majority of air is recirculated, it can take an extremely long time for these contaminants to be removed from the facility.

Figure 4 (Download this paper for all tables and figures.) (426.3 KB) shows the effects of acidic gas contamination in multiple locations within one facility. All data has been normalized to a contamination rate that would be expected in a clean facility (therefore, 0% increase is equivalent to a daily contamination rate of 2.5 ng/cm2/ day), thus showing the percentage increase in acidic gas contamination for each location relative to a clean facility. In this case, the copper sensor indicates that all locations are clean to relatively clean and the contamination is stable across the entire facility. Location 1, 4, and 5 are very clean as indicated by a contamination rate < 2.5 ng/cm2/day. As mentioned above, a clean facility would have a daily contamination rate on copper or silver sensor of < 2.5 ng/cm2/day, and a moderately clean facility would be 2.5 - 5.0 ng/cm2/day. The data from the silver sensor shows that there is significant contamination that is affecting the silver sensor surface (probably sulfur oxides) and fluctuates widely between locations. With the exception of locations 5 and 8, the silver sensor locations are considered to have extreme levels of acidic gas contamination, with locations 3 and 7 being excessively contaminated. Trying to manufacture or process critical materials in these locations could impact yield or product quality, especially if there is exposed metal on the critical product surface. If there is equipment or processes that are sensitive to acidic gas contamination within these areas, product must be handled carefully in these locations. In contaminated areas, queue times can be minimized, the ratio of makeup to recirculated air may be adjusted and optimized, and chemical filtration may be considered.

Acidic Gas Monitoring of Process Tools

The ability to identify, monitor, and resolve yield issues are benefits of monitoring process tools for acidic gas contamination using an AiM monitor. Process tools supporting wet etch, diffusion, CMP, Cu plating, and photolithography, among others, are susceptible to the impacts of acidic gas molecular contamination. Corrosion can occur on product surfaces, and reaction with basic species (amines, ammonia) can produce salt formations on optics that can cause hazing of optical components.

Figure 5 (Download this paper for all tables and figures.) (426.3 KB) displays the results of monitoring a chemical filter unit on an ASML 248 nm stepper. The daily contamination rate is 6.3 ng/cm2/day. Many of the contamination rate spikes were attributed through process logs to the introduction of a new wafer pod into the tool. In looking at a 4 hour rolling average of the contamination rate, a longer-term contamination event was seen with a significant increase from the background contamination rate beginning at 4:00 pm on August 8th. Contamination accumulation and rate information is a useful tool in determining chemical filter lifetimes and efficiencies. Process and equipment owners can lower the cost of ownership of chemical filters by understanding when a filter is not performing efficiently or when it has reached its lifetime, instead of using some empirical means to change filters, such as total time in service or number of wafers processed.

Summary

The AiM surface molecular contamination monitor is an extremely sensitive, real-time monitor. Surface molecular contamination is the result of AMC interacting with product surfaces to form very thin chemical films that can change the properties of that product surface.

Acidic gas contamination is one form of AMC that is particularly dangerous, as it has the ability to chemically and irreversibly react with product surfaces as well as materials that may indirectly affect product. Sub ppbv sensitivity has been verified for SOx contamination on a silver SAW sensor and guidelines have been established for surface molecular contamination accumulation rates on metallic sensors. Cleanroom monitoring applications and monitoring chemical filter data on process equipment have also been demonstrated.

(Download this paper for all tables and figures.) (426.3 KB)

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References

1. Middlebrooks, M.C., Airborne Molecular Contamination Control and the Effect of Filter Media Technical Parameters. Cleanrooms East (2000)
2. Grayfer, A., Kishkovich, D., Protecting 248nm and 193nm Lithography from Airborne Molecular Contamination during Semiconductor Fabrication. Proceedings of the SPIE (2001)
3. Airborne Molecular Contamination in Cleanrooms, Cleanrooms, 12 (1) January 1998, pp. 1-5