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Masks as an Application-Specific Product

by Steve Carlson, Photronics, Inc.

Historically, the lithography community has treated masks as a commodity. An “off-the-shelf” reticle could be ordered from a mask shop on short notice without worrying about too many details. If one reticle design worked well, the pattern could be changed and sent to the mask shop without worrying about any changes in data or design. If a reticle worked well in one fab, it would surely work well in another.

With the advent of deep sub-wavelength (DSW) lithography,1 masks can no longer be treated as commodities. A mask pattern change brings many more variables that must be comprehended in order to ensure that the resulting wafer lithography yields acceptable results. A reticle that works well on a stepper or scanner from one manufacturer may not print good wafers when a different system or process is used.

In this article, some new phenomena seen in DSW lithography are discussed. Then there is an explanation as to why relying solely on the mask purchased as an “off-the-shelf” or stand-alone solution is becoming an increasingly time-consuming and expensive approach. Finally, some solutions are presented for expensive and complex lithography problems by looking at other elements of the integrated lithography system.

New phenomena in deep sub-wavelength lithography

In DSW lithography, there are some new phenomena that make the lithographer’s job more difficult:

• reticles that meet conventional specifications but still have killer defects

Lateral Translations One type of reticle enhancement for DSW lithography utilizes hard-phase shifters. This technique can produce a lateral image shift if the depth of the etch varies across the feature or reticle.

An aberration in the projection optics can also produce a small lateral image shift.2 Of course, these aberrations aren’t unique to DSW lithography. What’s new is that the error budgets are becoming so small that these small lateral image shifts can now account for a significant percentage of the error budget (Figure 1).

Mask Error Enhancement Factor Current leading-edge projection optics systems in steppers reduce the size of the features to one-quarter or one-fifth their size on the reticle. Historically, you could rely on the errors being reduced by the same reduction factor. This would be expressed as a mask

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Estimate of current production overlay

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• accelerated mask error enhancement factor (MEEF) • more severe proximity effects

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Figure 1. SIA Roadmap for wafer level control.

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error enhancement factor (MEEF) of one. In sub-wavelength lithography, these errors are “enhanced” and result in a greater error contribution. If the value of the MEEF is two, then a stepper would print an error on the wafer which is half the size of the defect on the reticle instead of one-quarter the size.

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at the 0.25 micron node, it would not be uncommon to see that write time double or even triple for an aggressive mask design at the 0.18 micron node.

Yield

This effect increases with decreasing line-width on a given system. In DSW lithography, various optical phenomena tend to magnify errors on the mask to an even greater extent.3 One stepper might have a MEEF of 1.5 and another might have a MEEF of 1.8, depending on the aberration function, illumination optics, and details of how the stepper is set up.

In modern leading-edge mask making, several yield bottlenecks exist. Aggressive specifications for CD uniformity, targeting, placement, and defectivity commonly apply simultaneously to the same mask. In this case, the impact on final shippable yield can be dramatic. For instance, if the yield of each parameter is 90 percent, a mask that requires the same level of complexity for all four areas could result in a final plate yield of 65 percent or less.

Proximity Effects

Project Complexity

In DSW lithography, enhanced interactions between different features are seen. Iso-dense bias is an example of a proximity-induced line-width variation that accelerates with line-width reduction. Within the same lithography system, 20 nm of iso-dense bias at the 180 nm node, and more than twice that amount when the system is extended to the 100 nm node, might also be seen.4

Complex manufacturing techniques drive up mask costs. For example, some advanced mask processes require dry rather than wet etch. A dry etcher is a more expensive machine and has a longer process cycle time.

The Traditional Solution The traditional way to solve these problems has been to focus on the mask. That approach still works today.

Making sure to etch the mask correctly, resolve phase conflicts, and minimize proximity-induced translations can reduce the impact. The MEEF can be rendered irrelevant by making a mask without any errors. Making reticles with aggressive OPC features can minimize proximity effects.

The problem with focusing exclusively on the mask is that it’s a time-consuming and expensive way to solve the problem. Here is what drives the costs of advanced photomasks.

Drivers of mask costs

The main factors that determine mask costs are write time, yield, and the complexity of the mask-making process.

Write Time The amount of data required for writing the pattern explodes if the problem of proximity effects is being solved by adding aggressive OPC and scattering bars.

If, for instance, it took six to ten hours to write a plate

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There is a strong feeling that if a 180-degree phase shift is executed by etching the mask in three 60-degree increments, the probability that certain types of defects will result in unacceptable wafer results is minimized.

This manufacturing technique obviously requires more lithography steps and etch iterations. It increases the cycle time and the opportunity for negative yield impact.

Masks as part of an integrated lithography system

In certain situations, it might be cost effective to invest in an expensive mask as the stand-alone solution. In memory chip production, for example, where one mask will print tens of thousands of wafers, the cost of the mask could be amortized over hundreds of thousands of chips. On the other hand, in a true ASIC environment where the mask will expose only a small number of wafers, the impact of the reticle complexity and costs could completely dominate the non-recurring engineering overhead for a given design and perhaps even limit the market for that design.

In that case, the mask could be treated as only one component in the integrated lithography system and the system could be adjusted to optimize for cost. “An integrated lithography system” refers to design data, the stepper’s illumination system, projection optics, the resist, the etcher, and the wafer itself (Figure 2).

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DESIGN

LITHOGRAPHY SYSTEM

WAFER

Design Source Illuminator RETICLE Projection Optics Resist Pattern Transfer Confirmation Figure 2. A new perspective on mask making is required.

In DSW lithography, a change in any of those elements can affect the final printed image on the wafer. The knowledge base of how these components of the system fit together are continuously being improved In one recent example, a customer requirement that had delivered good wafer results suddenly caused a different result at the wafer. Further investigation revealed that it was not a change in the reticle process or results which had caused the wafer change, but a change in the resist system. In this case, the metric was iso-dense bias. The new resist system had a different response under certain conditions for iso-dense bias.

That different response, coupled with a narrow process margin, can cause unacceptable results unless someone adjusts other reticle parameters to compensate.

The conventional approach would have been to investigate what went wrong with the reticle process and to spend additional time and money to make a new one.

In the integrated system approach, it may be more cost-effective or expedient to adjust another parameter, such as partial coherence, to improve the result.

The key point is that in DSW lithography the various elements of the lithography system are becoming increasingly interdependent. The lithography community should take collective advantage of this by learning to tune the various elements of the lithography system to make good wafers, rather than relying totally on the reticle as a stand-alone element as has been done in the past.

Here are some examples of how this might be accomplished. In its simplest form, it would require a more complete integration of the mask vendor and the customer in the design process. In that way, the more common errors can be avoided and opportunities for design for manufacturability leveraged.

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The next step in sophistication would be to characterize the projection optics in each stepper. For example, the projection optics might have a radial component to its variation. If a systematic iso-dense bias is a characteristic of a given lens design, that compensation could be built into the data for the reticle. The more comprehensive solution, however, may be to understand the drivers of this systematic error and build the solution into the stepper selection and specification strategy.

By characterizing the optics in each stepper, a new way of dealing with the mask error enhancement factor (MEEF) also is created. For example, a test reticle could be built to quantify MEEF for each stepper and then lenses selected to allow reasonable reticle specifications.

If the non-uniformity across the reticle has a systematic structure that corresponds to a particular lens aberration, that reticle could be restricted to steppers that lie below a given threshold for that particular aberration.

Here is another example. Recently a customer ordered a type of contact mask traditionally made with an e-beam mask writer. According to conventional wisdom, e-beam systems produce features with better fidelity under certain conditions. The customer agreed that a mask with better cycle time could be made using a laser-based system. The mask was written and the customer printed a contact layer with it. The features on the wafer were too small. The customer immediately assumed that the mask did not meet specification. Closer inspection revealed that while the features did meet the specification, the features on the mask did indeed look “different” (Figures 3 and 4).

The customer examined the mask and saw that the corner rounding was different from what was usually seen. The features on the wafer were indeed too small, but strictly speaking, the reason wasn’t the degree or nature of corner rounding on the reticle—rather, the

Figures 3 and 4. Slightly “different” mask features.

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measurement associated with the specification was insufficient to define the contact as acceptable. In this case, the area of the contact is more important than a given diameter measurement. After the area of the contacts was optimized, the laser-based reticle produced the same results on the wafer as the e-beam reticle, despite the popular myth that image fidelity is always the most important measure of reticle “quality”.

The conventional method of dealing with the problem may have been to reject the laser-based solution.

However, by looking more deeply at the system, the metrics could be modified and a correlation established that would allow a solution clearly more cost effective.

A different element in the lithography system was modified to get the desired result in the simplest and cheapest way.

Summary

In some cases, it might be more cost effective or efficient to focus entirely on making a “perfect” high-tech reticle

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that would work with a lithography system as presently constituted. But in other cases, costs can be cut and time saved by modeling the entire lithography system and improving or adjusting one or more elements to create a solution. Masks are no longer a commodity. The right mask for each situation depends on the intended application and what variables should be optimized.

Masks have truly become an application-specific product.

Reference: 1. DSW lithography is defined here as imaging features below one-half the wavelength of the exposure illumination 2. C. J. Progler, S. J. Bukofsky, D. C. Wheeler, “Method to Budget and Optimize Total Device Overlay”, SPIE Proceedings 3679, 1999 3. A. K. Wong, R. A. Ferguson, L. W. Liebmann, S. M. Mansfield, A. F. Molless, M. O. Neisser, “Lithographic Effects of Mask-Critical Dimension Error”, SPIE Proceedings 3334, 1998 4. B.W. Smith, R. Schlief, “ Understanding Lens Aberrations and Influences to Lithographic Imaging”, SPIE Proceedings 4000, 2000

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