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Product > Mask Technology > PSM
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| There are different kinds of photomasks produced: Binary mask, EAPSM and AAPSM.
The most popular one is binary mask. While binary masks are relatively easy to produce, it is increasingly difficult to form well-defined pattern images on the wafer surfaces if the design feature becomes smaller than a certain critical dimension.
PSM utilizes a special layer of material over ordinary binary photomask in order to introduce phase-shift of light passing through the layer. In doing so, we could achieve a superior resolution in design pattern on the surface of wafers, compared to binary masks. Phase shifting of light is achieved when the light passes through two different optical paths: one through a phase-shift layer and the other through the Quartz layer without phase shift layer. While PSM offers advantages over the binary masks in the area of transcribing fine patterns on the wafer surfaces, the manufacturing process of PSM often involves more complicated steps, resulting in a lower yield compared to binary mask yield.
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| Binary mask is consisted of Cr pattern formed over Quartz substrate. Light can go through the clear Quartz area which is not covered by Cr layer. However, light can not pass through the Quartz area which is under the Cr pattern. Due to the selective transmission of light over the photomask, specific pattern can be created on the PR layer covering the Silicon wafers. As the pattern geometry shrinks beyond a certain critical dimension, the interference of lights passing through the photomask Cr patterned area increase as well. As a result of the interference and diffraction, PR areas that are not supposed to be exposed to the incident light gets over-exposed, producing incorrect pattern on the wafer surface after the wafers are developed..
In other words, when the Cr pattern is below a certain critical dimension, it can no longer acts as a perfect barrier of light and this becomes the limiting factor in creating high resolution photomask. Due to the diffraction and interference of lights around the fine Cr pattern area, binary mask can be useful only up to a certain geometry size. In general, binary masks are used up to 0.18um technology node and photomask with 0.18um or smaller CD consists of both binary and PSM masks in order to circumvent any problems likely to be caused by light interference and diffraction phenomenon.
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| EAPSMconsists of MoSiON layer as the phase shifting material on the Quartz substrate. EAPSM with either 6% or 8% transmission is often used. Light going through the MoSiON area will have a phase difference of 180 degree compared with the light passing through the clear area of the mask.
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AAPSM is a form of PSM invented by Levenson (IBM) and utilizes selective etching of the Quartz substrate. Depending on the depth of etching on the Quartz, lights could be induced to have 180 degree phase shift. Since Cr layers are not involved in general with AAPSM method, light transmission is not hindered and there exists no difference in the transmission of light over the etched or un-etched area of the Quartz substrate. At the boundary of etched pattern area, there is a complete cancellation of lights due to 180 degree phase difference between the adjacent areas. While it is possible to make AAPSM without using Cr layer altogether, Cr layer is also used sometimes in order to create pattern on the mask.
Alternative PSM incorporates phase shift area on both sides of the pattern so that there are no interference or diffraction over the pattern area regardless of the feature size. The advantage of this type of PSM is that maximum pattern resolution on the wafer can be achieved regardless of the Cr pattern size on the mask.
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