Autumn00 proliferation of masks

Lithography S

P

E

C

I

A

L

F

O

C

U

S

A Proliferation of Masks Why We Need Them, How to Pay for Them

by Paul Chipman, DuPont Photomasks Inc.

Until recently, the manufacture of photomasks was fairly straightforward. Twenty years ago, we made 1X masters for Perkin-Elmer scanners. Ten years ago, we switched to 5X reticles. MEBES III’s, PBS, and KLA 200’s ran high yields with low costs. Now everything has changed; with the advent of the sub-wavelength era, mask complexity is growing almost exponentially. Different mask types are generating significant value for different types of semiconductor manufacturers. To help the chip industry stay on its accelerated road map, mask manufacturers must broaden product offerings, as shown in Figure 1, to include even more varieties of masks with multiple types of optical enhancements. Of course, we can afford to develop and deploy advanced photomask technology globally only if we can earn an attractive return for our shareholders.

Our customers can help keep the photomask industry healthy by developing meaningful specifications, paying reasonable prices and forming creative partnerships with us to help offset some of the significant investment and cost.

Multiple mask types

At DuPont Photomasks Inc. (DPI) it has become apparent customers with different products, processes, business models and lithography strategies need masks with different optical enhancement technologies.

For example, consider trying to build a polysilicon layer. Polysilicon layers are very sensitive to CD variations and require high resolution. For the 130-nm node, IC manufacturers can choose from several approaches: 1) alternating aperture phase shift masks (AAPSM, see Figure 2 for examples), 2) aggressive optical proximity correction (OPC), 3) embedded attenuated phase shift masks (EAPSM) with or without OPC, or finally 4) a 193-nm platform, which may or may not require optical enhancements at 130 nm, depending on the design.

The alternating aperture phase shift mask provides the most difficulty for the mask manufacturer, but it will also provide the most value for some types of customers. For example, a customer who is making microprocessors may favor AAPSMs because those masks potentially offer the best CD control, which has a significant impact on microprocessor speed. The customer pays

the incremental cost for an AAPSM because it will improve his binning yields and thereby increase his revenue and profitability. Similarly, a DRAM manufacturer might select an AAPSM because he can amortize the cost of the mask over tens of millions of identical devices. Certain chip designs are not as CD sensitive as others and therefore the design can drive which optical enhancement technique makes the most sense.

Additionally, ASIC-type customers process relatively few wafers per design. These customers might select OPC because the mask is less expensive with faster cycle times. The optimal type of OPC depends on the design as well as the design rules that are chosen. For example, the chip may not have enough real estate for scattering bars or aggressive OPC.

The Past

Sub-Wavelength Era

Binary Photomasks

Binary Photomasks Critical Gate Layers Alternating Aperture PSM

Attenuated, Embedded PSM for 248nm Aggressive Optical Proximity Correction

Attenuated, Embedded PSM for 157nm Attenuated, Embedded PSM for 193nm EUV Scalpel/EPL

Figure 1. The Sub-Wavelength Era brings broader product offerings.

Autumn 2000

Yield Management Solutions

27

S

P

E

C

I

A

F

L

O

C

U

S

For masks that appear to have similar design rules, we see a wide range of specifications between customers, sometimes even from different fabs owned by the same IC manufacturer. As many manufacturers buy their equipment from many of the same suppliers, we believe the difference in photomask specifications from fab to fab may have more to do with tradition and philosophy than science.

Cross Section

Multi-Phase Complimentary Figure 2. Alternating aperture phase shift masks.

The variety of phase shift masks will increase with the advent of the 130-nm and 100-nm nodes because the industry will move to shorter exposure wavelengths, which will require different blank materials and different pellicle materials. Each successive generation of steppers provides less of an advantage over the previous generation with respect to resolution. Many people believe 193-nm lithography will provide a significant benefit over 248-nm lithography only if we integrate optical enhancements into 193-nm masks from the very beginning. Mask manufacturers must prepare to provide 193-nm masks with strong shifters, EAPSM and OPC. We will have to provide a wide range of masks during the transition because some customers will be bringing up their 193-nm steppers, others will be pushing their 248-nm system and still other groups will be developing 157-nm systems.

While optical lithography has always surprised us with its incredible longevity, one day the semiconductor industry may indeed have to change to a non-optical technology. The leading candidates are EUV and EPL, which will require substantial investments in new photomask materials and manufacturing technology.

Meaningful specifications

In the past, mask manufacturers typically had the technical capability to exceed customers’ requirements, particularly with the advent of reduction lithography. A tighter specification wouldn’t require a new mask writer or a new inspection system. But now, each successive turn of the crank on specifications requires significant new investment and development.

28

Autumn 2000

Yield Management Solutions

I encourage leading IC manufacturers to use scientific methods for determining specifications—simulations can be useful, but experiments are even better. Will tighter photomask specifications correspond to yield improvements in the fab? If not, don’t ask for them.

If so, making an expensive optically enhanced mask to satisfy a set of stringent specifications is of value when it has a direct relationship to chip performance.

The choice of specifications for defects, CD, registration, phase control and OPC features is a method of risk management. The previous strategy of a completely defect-free reticle probably made some sense when masks were a relatively inexpensive commodity. Now that masks are becoming an expensive and enabling technology for lithography, perhaps it’s not always the most cost effective approach. For example, ASIC companies trying to produce 100 die for a prototype out of five wafers with 1000 die/wafer may not need to worry about a defect which prints 0.1 percent of the time.

What’s the most cost effective approach to risk management in your situation? Would you like us to eliminate all your risk by making a perfect mask? As long as the tradeoffs in cost and cycle time are clearly understood, we are pleased to deliver perfect masks.

Fair prices

Pricing is an ongoing controversial subject. Fortunately, DPI has customers who come to say, “I’m willing to pay a fair price. Let’s figure out what that is and move on.” Fair prices adequately reflect the cost of building the product with a reasonable margin—a margin that allows us to develop the wide range of products that will enable our customers to stay on their roadmap.

Market pressure may tend to suppress margins, but you need a certain return on assets to continue to invest in this very capital-intensive business.

Fair pricing can also take into consideration the value of the mask to the customer. If a certain type of mask delivers a yield improvement, or if it extends the useful

S

lifetime of a stepper and thereby allows the customer to avoid capital investment, a higher price might be fair because the mask provides more value.

Partnerships

Historically, large captives such as AT&T, IBM and others invested in, developed, and shared advanced photomask technology. Over the past 15 years, most captives have divested their internal photomask operations to focus on their core competencies of designing and manufacturing semiconductors. The burden of research and development is falling on the merchant photomask producers, while the business model has not allowed for sufficient R&D expenditures. Joint ventures such as DPI’s Reticle Technology Center (RTC) are one solution. A joint venture isn’t the only possible vehicle.

Partnerships between suppliers and customers can take many other forms, such as sales contracts, commitments and development contracts.

P

E

C

I

A

F

L

O

C

U

S

Summary

Semiconductor manufacturers will need an ever-expanding variety of photomask technologies to stay on their roadmaps (Figure 3) and maximize their profits. To develop and deploy these technologies globally, we mask manufacturers will need our customers to provide meaningful specifications, pay reasonable prices and find some mutually satisfactory way to support our development so we can deliver whatever mask technology is optimal for your business at whatever time you need it.

10.0µm

Above Wavelength

5.0µm

Sub-Wavelength

3.0µm

Wavelength

2.0µm

Feature Size

1.0µm .05µm .03µm .02µm 0.1µm 1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

2002

Figure 3. Semiconductor lithography roadmap.

Over the past 20 years, mask costs have gone from five percent of semiconductor revenues to a low of about one percent in 1995. The “5X holiday”, pellicles, mask industry consolidation, and over-capacity have all held mask costs and prices low until recently. Today, photomask manufacturers must purchase $15M mask writers, $5M inspection tools, $2K photomask blanks, and significantly increase R&D spending. Phase shift masks require multiple separate writes and quartz etches. An optically enhanced photomask with phase shift and/or OPC features can write for 12-24 hours, or more. Photomask manufacturing is becoming more like semiconductor manufacturing with each generation. But, we don’t make hundreds of die per wafer; we can only make one at a time. Imagine where the semiconductor industry would be if they produced one die per wafer!

Photomask prices have begun to increase, driven solely by higher manufacturing costs, as the margins of photomask producers have not appreciably risen. If the photomask industry is to continue to invest to help enable our customers to remain on Moore’s Law, margins must improve, return on investment must improve, and we must earn the cost of capital to provide an acceptable return to our investors.

In the sub-wavelength era, photomasks provide value in numerous ways. Advanced photomasks have enabled our customers to accelerate shrinks which: * extend the life of their capital investment in facilities and equipment * enable more die per wafer * enable higher speed devices with lower power consumption As photomasks deliver increasing value to semiconductor producers, we anticipate the photomask industry will capture its fair share of this value to insure the global capability and capacity to meet the accelerated roadmaps in the sub-wavelength era. — by Ken Rygler, DuPont Photomasks Inc. Autumn 2000

Yield Management Solutions

29