Pattern Size Considerations
The maximum area that the electron beam can write without having to perform a stage move is refered to as an exposure field. A pattern larger than this field will require one or more stage moves in order to write the entire pattern. This can have two effects on the pattern
- Increased write time. Each time the stage moves it has to settle and be declared stable before writing continues. This is more of an issue in the VB6 than the JEOL 9300.
- Introduction of errors due to field stitching. Simply put, field stitching is the ability of the EBL tool to line up features in field A with features in field B. If lines are written across the boundary of two fields gaps, notches and other anomalies can be observed. More information on field stitching is given below.
Field Stitching Considerations
Both of the EBL tools at CNF have field stitching performance specifications less than 40 nm. The JEOL 9300 can acheive field stitching accuracy less than 20 nm. For critical features, it is best to place them in the center of the field such that errors due to field stitching never become an issue. Also, it is unwise to place small features at the corners of a field. Placing a 20 nm square centered at the corner of a field will actually be written by 4 fields.
Generating a high resolution electron beam for patterm writing is fairly straight forward. However, moving this beam around the patterning field is not trivial. Aberations in the focusing lenses and distortions induced by the deflection system result in a degradation in the performance of the beam the further it is deflected. One of the reasons why EBL systems cost so much is because they employ complex schemes for minimizing these effects. That said, the best pattern writing will still be acheived in the center of the patterning field. For writing features at or below the resolution specification of the EBL system you are using it is always wise to place them as close to the center of the field as possible.
As an electron from the writing beam strikes the surface of a substrate it undergoes various scattering events losing energy and causing the generation of secondary electrons. The energy range of most secondary electrons falls between 1 and 50 eV. Secondary electrons that are close to the substrate/resist interface are actually responsible for the bulk of the actual resist exposure process. While their range in resist is only a few nanometers they create what is known as the proximity effect. Simply put, the proximity effect is the change in feature size of pattern features as a consequence of nonuniform exposure. While the dose from the primary beam may be uniform across an entire pattern, the contribution of secondary electrons from the substrate may differ depending on pattern geometry. Two adjacent features will contribute a background dose of secondary electrons to each other resulting in a higher effective dose. This causes a broadening of the exposed features. This is particularly apparent with dense features (e.g. gratings). Consequently, dense arrays of features may require significantly less dose from the primary beam to print correctly.
Pattern size can also be adjusted to compensate for this effect. For example, 100 nm lines 100 nm apart are typically drawn in CAD as 90 nm lines 110 nm apart to get them to print correctly. This strategy stops working at the edges and corners of patterns. This sometimes requires the the creation of dummy patterns or devices outside of the primary pattern region to get the main features of interest to print correctly. One common practice is to draw a box around the pattern to normalize the dose in the primary pattern region.
Performing a Meaningful Dose Test
Exposing a pattern correctly usually requires performing a preliminary test exposure referred to as a dose test. In this test, the pattern is repeated several times on a test substrate. Each repetition is performed at a different dose or set of doses creating a matrix of different exposure conditions. Once the pattern is developed and pattern transfer has been performed the correct dose can be obtained through inspection in a suitable inspection tool (scanning electron microscopy, atomic force microscope, optical microscope, etc). There are several issues which can impact the usefullness of a dose test. Here are some guidelines:
Use the same type of substrate.
If there are films present on the surface of the substrate us a substrate with the identical film stack.
For large arrays of features, shooting the entire array as a test is not an efficient use of time. However, reducing the size of the array to an unrealistically small extent can give incorrent results during the test due to differences in the proximity effect.
Expose your patterns so that they are easy to locate. For example, do not expose a test pattern consisting of a 500 micron x 500 micron array of 50 nm squares in the middle of a 150 mm wafer. You will probably never find them. Including some locating features (large lines or a box surounding the pattern) can help tremendously. If you are exposing an array of patterns use as small of a repeat vector as possible. This will make locating the entire array easier and minimize the chances of getting lost when travelling in between adjacent elements of the array.
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