As our customers' technologies have advanced, phase shift masks (PSM) have evolved as a powerful tool to extend the capabilities of lithography equipment. In the Toppan PSM product family, we offer two distinct PSM technologies: Embedded Attenuated Phase Shift Masks (EAPSM) and Alternating Aperture Phase Shift Masks (AAPSM). These products have distinct technical qualities and design applications.
As design pitch gets smaller, diffractive effects reduce optical resolution. Standard binary photomasks make use of only light intensity to form an image: "masked" areas appear dark and open areas appear bright. But by using both the intensity and phase properties of light, diffractive effects on small pitch structures can be abated.
The key principle used in Phase Shift Masks is that light passing through any media will undergo a phase shift proportional to the thickness of the material. For example, light passing through a given thickness of quartz will emerge with a different phase angle than light passing through a thinner thickness of quartz. By engineering the thickness difference on adjacent structures, a 180⁰ phase shift can be achieved. Light from both 180⁰ (shifted) and 0⁰ (non-shifted) areas on the mask will interfere destructively and create a point of reduced light intensity on the wafer. Through careful design and layout, this characteristic can be harnessed to improve imaging resolution and DOF.
Alternating Aperture Phase Shift Masks
By etching into the quartz substrate to specified depth, a 180⁰ phase change can be created relative to non-etched spaces. Repeating the etched space on alternate open areas will create a periodic phase shifting structure that improves the imaging of the features defined by the opaque absorbers. Because the amplitude of the light through both spaces is nearly equal, this type of PSM is often referred to as a "strong shifter."
Phase angle uniformity is determined by the consistency of the etch depth across the photomask. After careful selection of the quartz etch chemistry, establishing tightly controlled plasmas, and eliminating loading effects, we have established a process that achieves world-class phase uniformity.
AAPSMs function best in highly regular structures, such as in memory and hard-disk drive applications. Non-regular patterns found in many logic designs present a special challenge. One such problem is when 0⁰ and 180⁰ etch quartz areas meet. At such a phase intersection, a dark interference line will be imaged in the resist. Toppan has solved this problem by utilizing stepped phase boundaries which we call "multiphase." Making use of our precision quartz etch process, we modulate the trench depth by 60⁰ steps to ensure that the phase boundary does not print. The multiphase technique has been heavily used in high-volume production wafer fabs for over a decade and is a technique that can be applied more generally to modulate the impact of phase defects on AAPSM masks.
Another technical issue occurring with AAPSMs is intensity balancing. The amplitude of the light passing through a quartz etched region is slightly less than that of an adjacent non-etched area. For sufficiently small line widths, this imbalance creates CD uniformity issues across the printed image. Toppan has solved this problem by performing a wet etch subsequent to the main quartz dry etch. This second step widens the space and increases the resulting image intensity. Because a small portion of the absorber is left hanging over the edge of the trench, we often refer to this as an "undercut" technique. As with our multiphase solution, undercut AAPSMs have been used extensively by the industry in mainstream applications.
We can manufacture a wide variety of quartz etch products using techniques like multiphase and undercutting. From dual-phase focus monitors to chromeless phase lithography, we have over a decade of experience in AAPSMs. No matter the wavelength you use or your final product application, we can customize a perfect solution for you.
Embedded Attenuated Phase Shift Masks
An alternative approach to create a phase shift is by replacing the chrome absorber with a semi-transparent material that changes the phase. A 6% transmission MoSi film is standard across the industry, though the film can be tuned to allow as much as 18% transmission to optimize lithographic performance.
Light from the MoSi absorber interferes with light from the clear space to cancel out at the feature's boundary and sharpen the image's edge. However, as you move away from the edge diffracted light from the nearby clear spaces can be constructively combined to unintentionally expose the resist. These areas are called "side lobes".
Side lobe printing is especially a concern for higher transmission films and tightly packed repetitive geometry. We have developed techniques to solve this issue. Selectively leaving chrome on top of the MoSi film wherever side lobes occur reduces the exposure intensity and typically eliminates the stray images. These chrome structures require precision overlay capability. By redesigning our overlay processes and working very closely with our suppliers, we have perfected these "tri-tone" EAPSMs.
EAPSMs offer several advantages. They are much simpler to manufacture than AAPSMs, and they are far easier to implement in wafer fabs. In fact, for dark-field layers like contact holes, EAPSMs are the only viable phase shifting technique.
Our Phase Shift Masks support a broad range of OPC techniques that compensate for a variety of proximity effects including closed contacts or holes, shortened or rounded lines, local pattern density non-uniformities, topographic effects and many other limiting phenomena. Adding OPC features to our high-quality masks virtually eliminates proximity effects and results in improved resolution and process latitude. Because OPC is applied to the pattern data before it is printed onto the mask, we will work closely with your teams to develop the perfect product for your needs.