Phase Shift Masks [PSM]
A Primer on Phase Shift Masks
The Productivity Paradox
In the increasingly competitive semiconductor market, chip makers are enlisting equipment and material suppliers in their efforts to achieve higher rates of productivity from existing processes. Device shrinks, or narrowing the linewidths of the device designs, have historically proved a reliable method to increase the chipmaker's productivity.
Now device linewidths are so narrow that conventional light sources and lenses, and/or binary photomasks cannot ensure the designs accurately print on the wafer.
Today's tight specifications make resolution a critical issue because narrowing linewidths require an increasingly high level of resolution. Although we can build the designs on ordinary binary masks, the lines blur together when reduced onto the wafer. Even utilizing the most advanced lithography equipment available, the device's individual feature sizes are so small or so close together that they no longer resolve without some technological improvement.
Chip designs are broken down by layers with a chip's critical-dimension (CD) tolerances and overlay specifications being extremely tight (25 nm and 80 nm, respectively). As the industry's ability to reduce feature sizes has outpaced exposure wavelength reduction and numerical aperture increases, let's examine all the factors that influence printing designs on wafers.
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| Minimum feature sizes are influenced by a stepper's wavelength and lens |
The Resolution Formula
Minimum feature size is equal to a process constant (affected by numerous elements, including resist contrast; etch quality; and photomask enhancements--represented by k1) multiplied by the stepper wavelength (which you also want as small as possible and is fixed--represented by the lambda symbol), the product of which is divided by the lens' numerical aperture (which you want as large as possible and is constrained by the depth of focus).
As you will note, the lambda and NA terms on the right hand side of the equation are determined by the stepper (or scanner or step-and-scan). Equipped with the latest deep-ultraviolet (DUV) step-and-scan system, what else will help a chipmaker using the most advanced etch process and the latest resists, produce devices with smaller geometries? The photomask.
In particular, Toppan Photomasks' phase shifting photomasks are a key enabling technology allowing semiconductor feature sizes to continue to shrink because phase shift masks are capable of sharpening the light's effects on photoresist for sub-quarter micron designs far better than ordinary binary masks. In relation to our equation, phase shift masks help bring down k1--the smaller the k1, the smaller the feature size--thus directly achieving the overall goal of shrinking the device at high yields.
There are several types of phase shifting photomask techniques produced by Toppan Photomasks, employing different types of materials, including substances such as molybdenum silicide (MoSiOxNy) a replacement for chrome, as well as traditional chrome with etched quartz regions. Generally, these types of photomasks are categorized as embedded attenuated phase shift masks (EAPSM), "soft" shifters, and alternating aperture phase shift masks (AAPSM), "hard" shifters.
Embedded Attenuated Phase Shift Masks
EAPSMs are similar to binary masks, in that they begin with a quartz substrate coated once with a material which the layer's design is then etched into. The most common material used in today's EAPSMs is molybdenum silicide.
| Embedded Attenuated Phase Shift Mask |
| also called Chrome Oxide/Molybdenum Silicide or Half Tone or Weak-Shifter |
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| FIGURE 1 |
Unlike chrome, molybdenum silicide allows a small percentage of the light to pass through; however, the amount that passes through is "weak" and does not expose the resist on the wafer. Because it does pass through, the light is 180° out of phase compared with the light passing through the quartz alone. Therefore, where the material and the quartz meet, light interferes in such a way as to sharpen the edges of the design, producing a faithful replica in the resist.
Figure 1. illustrates when destructive interference occurs any time there is a change from the coating material to the glass. That is, at every edge the sharpening begins by essentially dragging down the light from the bright zone into the dark zone, enabling device shrinkage using currently available tools and technology.
Alternating Aperture Phase Shift Masks
| Alternating Aperture Phase Shift Mask |
| also called Etched Quartz or Levenson-Type or Hard or Strong Shifter |
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| FIGURE 2 |
Alternating aperture is another method Toppan Photomasks uses to produce masks that engineer DUV destructive interference in order to print lines smaller than the wavelength of light. Going beyond the traditional chrome-on-glass approach, AAPSMs utilize selectively etched quartz areas.
This etched area causes the light to become 180° out of phase with the light passing through the unetched regions. As illustrated by the points of destructive interference in Figure 2., the transition between etched and unetched regions produces a dark or opaque area, thus patterning the photoresist in an extremely precise manner.
In conclusion
Phase shift masks enhance contrast to expose the photoresist and print features at resolutions that binary masks are unable to achieve with today's light sources and lenses. Toppan Photomasks' achievements in architecting phase shift masks contribute to a fundamental transition in semiconductor manufacturing. Toppan Photomasks' advanced technology enables devices with linewidths of 0.13 micron and below to be built using conventional DUV tool sets by reproducing patterns within the precise specifications demanded by high-performance devices.
Beyond adding value to the manufacturing process, our phase shift masks are becoming a key driver in advancing technology.