Specifying masks

We design masks using our library of standard cells or we implement custom cells. After we agree on the design, a mask house builds the masks for your specific requirements and equipment. The order needs to specify a few parameters:

Size

The size of the masks depends on your mask aligner. In most cases, not a rule, the mask holder is one inch over-sized with respect to the wafers, meaning that, for example, a mask aligner for 150mm wafers needs 7" masks.

Thickness

This is the thickness of the glass. Many masks aligners are indifferent to the thickness of the glass mask but be aware that it is not a universal rule and some mask clamps require the specified thickness. Thicker masks are stronger and more rigid but cost more and absorb proportionally more light, therefore will tend to heat more.

Material

The transparency of the mask material is important in the UV region that is used to expose the photosensitive material. Transparency determines the percentage of energy reaching the photoresist and the percentage of energy absorbed in the material. The absorbed light is converted to heat which adds to the heat transferred by conduction from the light house and the combined heat affects the dimensional stability of the mask. Soda lime masks are the most economical; low-expansion (LE) borosilicate masks are the most stable and offer a favorable alternative to fused quartz masks (see the insert showing the CTE of the different materials). Fused quartz masks are required to work at shorter UV wavelengths but are unecessary in packaging work. Because of the coarse geometries and longer exposure wavelength, the masks material used for packaging have much less stringent requirements than for state-of-the-art ICs.

Flatness

The primary flatness of the mask comes from the fabrication of the blank. However, deformation will occur in thinner masks with very large span as in 300mm wafer work, which adds to the nominal fabrication flatness. Looser flatness tolerances can impact the layer-to-layer registration.

Polarity: Dark Field or Clear Field ?

The field defines where the design database requires chrome to appear on the mask. If the filled polygons in the design database represent chrome on the mask, the mask is "clear field" (sometimes referred to as "bright field"). In other words, the digitized data is dark. When the filled polygons of the database represent the clear areas of the mask, the mask is "dark field", i.e. the digitized data is clear. Note that this has nothing to do with the photosensitive polarity of the materials or with the process being additive or substractive. It simply defines where the chrome will remain on the mask in relation to the layout database. The layers created in the layout are reversed accordingly by the mask maker in an XOR operation.

Reading: Right or Wrong Reading Chrome Down

The process determines how the design should appear. If you look at the mask on the chrome side (not through the glass) this is called "chrome up". If you look at the mask to the chrome through the glass, it is called "chrome down". These appellations are used to specify whether the design is to be mirror imaged, i.e. "wrong reading" or look as it appears in the database, i.e. "right reading".

What polarity should a mask have?

The following paragraphs describe a potentially confusing aspect of mask design and ordering from a mask house.

A process can use negative or positive photosensitized materials, be it photoresist or polyimide. The polarity of the photosensitive material is only one factor determining if it is necessary to invert the image of a layer to achieve the desired result.

Keep in mind that light exposes the photosensitive material wherever chrome is etched on a mask. The key is to decide where you need the light to go through the mask. As example, we will use the case of etching a metal layer deposited on a wafer to define conductive lines. This is a substractive process. The conductor lines were drawn on the database as filled lines and polygons. Using a clear field mask, and a positive resist, results in having metal where the chrome was placed on the mask. If the photoresist is negative, the mask should be ordered as dark field.