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Analysis of ESD-Induced Reticle Defects by Jim Reynolds, Reynolds Consulting
In semiconductor manufacturing, the effects of static charge are particularly critical in the photolithography area. The photo process involves step-and-repeat operations using reticles. A damaged reticle can result in thousands of defective products. Due to the small dimensions and non-conductive substrates on the reticle, electrostatic discharge (ESD) can occur, causing significant yield losses. The present study was made as the result of just such an occurrence. The observed production yield loss was eventually traced to multiple ESD events, which caused pattern damage on the reticles. Some recommendations to the problem of reticle ESD damage are proposed. Introduction
Reticle deterioration over time has long been a concern of the wafer lithographer. Since the advent of the wafer stepper in the early 1980’s, events such as pellicle breakage, crystal growth under the pellicle, and electrostatic discharge (ESD) have created a low incidence of extremely expensive problems. The consequences of an undesired change on the image surface of a reticle can cause problems ranging from downgraded products to 100 percent yield loss. Isolating these losses to a damaged reticle can be very difficult and time consuming. Many semiconductor lithographers prefer periodic reinspection of reticles to the disastrous possibility, however remote, of undetected reticle damage. In 1997, a large European semiconductor manufacturer observed the yield on an established product drop to zero percent over a short period of time. This article is a discussion of that event and of ways to minimize the potential damage that can be caused by ESD. Reticles and ESD
Static charge is most commonly generated by triboelectric charging. Whenever two dissimilar materials are in contact and separated, a charge exchange occurs between the two surfaces, resulting in two oppositely 16
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Yield Management Solutions
charged objects. Once an object becomes charged, it may transfer its charge directly to another object. Induction charging can also occur when an isolated conductive object is brought near another charged object, without actually touching it. At any time when two objects, with a potential difference large enough to break down the insolating path that separates them, come close together, current will flow causing an ESD event. As a practical matter with reticles, the sources of charge are ubiquitous and the gaps over which discharge can occur are small now and getting smaller. Charges can build up on garments, work surfaces, packaging materials and air streams, just to name a few possibilities. Fab environments are normally dry with relative humidity at 40 percent or below and rapidly moving air. The quartz substrates used on most photomasks have a very low surface conductivity, allowing puddles of charge to build up on different areas of the reticle. Voltage differences of 5-8,000V are not uncommon on the surface of a reticle. The chrome which defines the pattern can conduct a high voltage to a region on the mask where the electrostatic potentials are lower. If chrome, run at a high potential, comes into close proximity of another at a lower potential, the resulting high electric field gradient will cause ESD. When this happens, extremely high temperatures are generated which melt both chrome and, in some cases, quartz, causing an undesired modification of the pattern. This reticle damage can occur over time (with small ESD events) and in varying degrees of severity.
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Figure 3.
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Figures 1-3. Progressive examples of ESD damage on advanced reticles.
Case study
Production yield losses at a major semiconductor manufacturer prompted the current investigation. The cause was arduously traced back to one of the reticles used in the photolithography process. We inspected the two-die reticle using a KLA-Tencor STARlight system and examined both the individual defects and the pattern of defects. Several levels of damage were discovered on this reticle. Figure 1 shows slight damage of the antireflective layer which lies on top of the chrome. This precursor to ESD was visible on the STARlight, even though this section of the reticle would probably go undetected on a pattern defect inspection system and would print normally. This level of damage can exist on a reticle that does not have additional damage, providing a means of early identification of a reticle that is prone to more catastrophic ESD.
When the potential of the distant region approaches or exceeds the breakdown voltage, the ESD shown in figures 1-3 occurs. Living with ESD
As long as photomasks are made of conductive chrome on non-conductive quartz substrates, the potential for ESD exists. It is up to the reticle maker and user to minimize exposure to this problem and the damage it can cause. The most direct method is to add conductivity to the air surrounding the reticle. This can be done with electronic air ionizers in front of the HEPA filters supplying air to the workstations where reticles are handled or used. Increasing the relative humidity as far as possible (50-55 percent) provides additional protection. Even with these measures, the only way to prevent yield loss due to ESD on reticles is to reinspect the reticles periodically using a STARlight or similar system. circle RS#033
Figures 2 and 3 show regions of the same reticle with increasing levels of ESD. Both show evidence of rapid current flow, coupled with deformation of the chrome image. This current has produced melting of both the chrome and quartz substrate. In figure 2, a small tail emerges from each side of the gap with a transmission of around 72 percent, as measured on the STARlight. In figure 3, a bridge with 53 percent transmission is found. Both of these cases would have produced bridging when printed on a wafer. Figure 4 is an atomic force microscope (AFM) rendering of the defect in figure 3, clearly showing the physical topology of the region. All three of these cases met the classic ESD requirements. Long conductors came from a distant part of the plate, bearing the potential of a charged region. A small gap separates them from a relatively short conductor which bears the potential of the local region.
Figure 4. AFM rendering of the bridge defect shown in figure 3.
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