Spie Press BookHow to Protect Reticles from Electrostatic Damage
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- 1 Introduction
- 2 Some Basic Elements of Electrostatics
- 2.1 Triboelectric charging
- 2.2 Conductive charging and equipotential bonding
- 2.3 Inductive charging
- 2.4 Air discharge
- 3 How Electric Field Interacts with a Reticle
- 3.1 Field induction in a reticle
- 3.2 Effect of equipotential bonding
- 4 How Electric Field Damages a Reticle
- 4.1 How feature spacing changes field induction
- 4.2 Reticle ESD
- 4.3 Field-induced migration
- 4.4 Haze
- 4.5 Temporal effects in field induction
- 5 Consequences of Field-Induced Reticle Damage
- 6 Field Penetration into Reticle Pods
- 7 Measurements of Field Penetration into Reticle Pods
- 8 Electrostatic Risks outside Reticle Pods
- 9 Discussion
- 10 Conclusions
I have spent a long time studying this subject, and I feel that the detailed understanding of it that I have developed through many years of study is important to the semiconductor industry. My involvement with reticles started when I joined ASM Lithography in 1997 as a product manager for Material Handling and Factory Integration. One of the priority issues that I was asked to address was the damage being caused to reticles by electrostatic discharges (ESD). While I was working at ASML, the research into reticle ESD damage at International Sematech was underway, and I started attending their public meetings.
I was responsible for helping to drive forward the development of static dissipative standard mechanical interface (SMIF) pods because at the time, ASML was the only company using single-reticle SMIF pods, and making them dissipative was considered by ESD experts to be the best way to reduce the problems that ASML's customers were facing. Unfortunately, at the Sematech meeting where the new static dissipative SMIF pod was "unveiled," Larry Levit presented data showing that the new pod was not quite as protective as it had been hoped it would be. On leaving ASML and moving to Microtome, I was able to spend more time investigating the concerns I had about the methods being adopted. After seventeen years, this has led to the understanding of the subject that I present in this Spotlight. Some of my research findings and the conclusions I have drawn from them have been controversial because they conflicted with the "established wisdom" at that time in the semiconductor and electrostatic control industries. That has certainly been true regarding the effect of equipotential bonding. This has been partly corrected with the publication of SEMI Standard E163, but during its writing there was strong opposition from the ESD Association to the guidance advising against using equipotential bonding with reticles because it had been effective in helping to protect sensitive electronic devices for many years and virtually every ESD expert actively promoted it.
Nevertheless, the evidence for the negative effect of equipotential bonding on reticles is incontrovertible, which demonstrates that fundamental mistakes were made when defining current reticle protection strategies. Reticles require a different approach to their electrostatic protection than electronic devices because they are uniquely sensitive to electric field and can be damaged in ways that do not affect electronic devices. This Spotlight is intended to explain this and convince those who are responsible for reticle management in semiconductor manufacturing facilities that a revision of their electrostatic protection strategy is required. I believe the semiconductor industry has unfortunately been taken down a "blind alley" over reticle electrostatic protection. This is not a criticism of the efforts of those who defined the electrostatic protection strategies in use today, for I was among them at the time. But, as knowledge improves, it is possible to identify weaknesses and errors that have been made and hopefully to correct them. To be able to make further progress, it is necessary for the semiconductor industry to reverse out of the blind alley it has gone down and head in a slightly different direction, but I do not believe that is as daunting a prospect as it perhaps sounds. SEMI Standard E163 points the way.
I believe that changing the focus from controlling electrical potential to appropriately managing electric field may also be advantageous elsewhere, such as in the flat-panel-display and MEMS industries, since their products may also be at risk from field-induced damage.
I sometimes meet people who are responsible for dealing with reticles within the semiconductor industry, both suppliers and end users alike, who tell me they simply do not see the problems that I describe. For example, after I had presented some of my initial findings about reticle electrostatic damage to a major semiconductor company in 2004, the audience and I were told in a very authoritative tone by the manager responsible for reticle handling "we do not have that problem here." Having a manager adopt such an attitude unfortunately stifles the kind of investigation and research that leads to discovery. To such people I would say that not seeing a problem is not evidence that it is not present. The physics are universal - no factory has special exemption from the laws of nature - so the problem has to be present to some extent. Whether it has "bitten" them yet is perhaps another matter, but I have certainly seen first-hand the effect of a fab being bitten by this problem and it was not pleasant. I was told that the fab had lost a major customer and the fab manager had lost his job because of it.
The advantage of taking a scientific approach to this problem rather than just working on what is being reported from fabs is that one can gather evidence that may show that something is going unnoticed. Then the observations and theories derived from them can be tested independently and either be proven or disproved. I am pleased to be able to say that whenever my statements and predictions about reticle electrostatic damage and protection have been tested independently in this way, they have been found to be correct.
This purpose of this Spotlight is not simply to condense all of the evidence and historical findings on the subject from the literature; it is to raise awareness that reticles continue to be at risk in most semiconductor manufacturing facilities as a result of the propagation of myths, misunderstandings, and mistakes about how best to protect them from electrostatic damage. Solutions that can prevent what may only seem to be a hypothetical problem for some fabs today from becoming a very painful reality tomorrow are available - they simply need to be adopted. I hope you find this informative and helpful. I also hope it prompts you to think again about this subject from a different perspective - just as I had to do myself almost 20 years ago - when I realized that what I had been taught and what all the ESD experts were saying at the time did not quite explain everything.
Gavin C. Rider