How can phase-shifting photomasks overcome conventional light source and lens resolution limitations in chip production? Dr Franklin Kalk, executive vice-president and chief technology officer at Toppan Photomasks, describes how.
In the increasingly competitive semiconductor market, chip-makers are enlisting the help of equipment and material suppliers in their efforts to achieve higher productivity from existing processes.
Device 'shrinks' or the narrowing of the line widths in device designs have historically proved to be reliable methods of increasing chip-maker productivity. But now device line widths are so narrow that conventional light sources and lenses and/or binary photomasks cannot ensure that chip designs accurately print on the wafer.
Today's tight specifications make resolution a critical issue because narrowing line widths requires 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. The device's individual features are so small or so close together that they no longer resolve without some technological improvement, even using the most advanced lithography equipment available.
Chip designs are broken down into layers, and the chip's critical-dimension (CD) tolerances and overlay specifications are extremely tight (25nm and 80nm respectively). The industry's ability to reduce feature sizes has outpaced exposure wavelength reduction and numerical aperture increases.
Let's examine the factors that influence the printing of designs on wafers. Minimum feature size is equal to a process constant (which is affected by numerous elements, including resist contrast, etch quality and photomask enhancement – represented in the equation below 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's numerical aperture (which you want to be as large as possible and is constrained by the depth of focus):
|↓Minimum feature size =||
Note that the lambda and NA terms on the right-hand side of the equation are determined by the stepper (or scanner or step-and-scan).
So what can help a chip-maker – equipped with the latest deep-ultraviolet (DUV) step-and-scan system, and already using the most advanced etch process and the latest resists – produce devices with smaller geometries?
PHASE-SHIFTING PHOTOMASK SOLUTION
The solution is the photomask, and Toppan Photomasks' phase-shifting photomask in particular. This is a key enabling technology that allows semiconductor feature sizes to continue to shrink because phase-shift masks are capable of sharpening the effect of light 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.
Toppan Photomasks can offer several types of phase shifting photomask technique. These employ different materials, including substances such as molybdenum silicide (MoSiOxNy), a replacement for chrome, as well as traditional chrome with etched quartz regions.
Generally, these photomasks are categorised as embedded attenuated phase shift masks (EAPSMs) – soft shifters – and alternating aperture phase shift masks (AAPSMs) – hard shifters.
EAPSMs are similar to binary masks, in that they begin with a quartz substrate coated once with a material that the layer's design is then etched into. The most common material used in today's EAPSMs is molybdenum silicide.
Unlike chrome, molybdenum silicide allows a small percentage of light to pass through it. The amount that passes through is low and does not expose the resist on the wafer. However, because the light does pass through, it 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.
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 use selectively etched quartz areas. These etched areas cause the light to shift 180° out of phase with the light passing through the un-etched regions.
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 creating phase shift masks are contributing to a fundamental transition in semiconductor manufacturing. The company's advanced technology enables devices with line widths of 0.13mμ 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, Toppan Photomasks' phase shift masks are becoming a key driver in advancing technology.
A binary photomask is composed of transmissive (clear) and opaque elements, which form one layer of a circuit pattern. Light passes through the transmissive elements, exposing a pattern on the wafer.
Production of an advanced binary photomask with very tight critical dimensions has enormous potential to boost the performance of Toppan Photomasks' customers' products, as well as improving their wafer yields. Generally speaking, the tighter the critical dimensions of a device, the faster the speed of the device. Also, shrinking the line widths of devices allows more devices to be put on each wafer, leading to increased productivity.
OPTICAL PROXIMITY CORRECTION
Toppan Photomasks is proud of its ability to support the broad range of optical proximity correction (OPC) techniques required by the leading semiconductor manufacturers.
OPC embeds non-printing sub-resolution features onto a mask. While there are many different OPC techniques, they all use ultra-small features to compensate for the way light modifies the mask pattern when it reaches the wafer.
Using steppers to pattern wafers with sub-wavelength features causes what is known as proximity effects. These include closed contacts (holes), shortened or rounded lines, and topographic effects (underlying wafer layers).
These proximity effects can cause a rectangle pattern in a photomask to print as an oval on the wafer because light has a tendency to round off edges.
As feature sizes decrease, the error budgets and process latitude must be reduced correspondingly. Extending stepper lithography means that masks are not only used as a patterning tool. They actively overcome the challenges involved in shaping light and assist in the achievement of a design's critical dimensions.
Adding OPC features to high-quality masks virtually eliminates proximity effects and results in improved resolution and process latitude.