Announcing Optolith - Advanced Capabilities For Simulating Optical Lithography

Silvaco recently released a powerful new optical lithography simulator called Optolith. This new simulator, which was developed in-house by Silvaco, provides unique capabilities for simulating optical lithography over non-planar topographies. Optolith is fully integrated with Elite and SSuprem4 and with all the MASTER Tools, including DeckBuild, TonyPlot, MaskViews, and the Virtual Wafer Fab.

A Completely New Simulator

Optolith was developed because customers wanted sophisticated non-planar simulation capabilities that were seamlessly integrated with Elite and SSuprem4. The SOLID simulator that was offered previously by Silvaco does not handle non-planar topographies, and its algorithms are not suitable for extension to non-planar topographies. It is also a relatively traditional code written in FORTRAN and is not well suited for inclusion within the MASTER Framework. After very careful consideration it was decided that the most effective way of meeting the present and future needs of customers was in-house development of a completely new optical lithography simulator. Optolith has therefore replaced SOLID in Silvaco's product line.

Optolith was completed in less than a year because it was possible to use all the capabilities provided by the MASTER Framework. Optolith was implemented using very modern, high quality software engineering that makes the code very maintainable and highly extensible.

 

Many Advanced Features

Optolith simulates 2-D aerial image formation and 2-D photoresist exposure and development. It is fully integrated with Elite and SSuprem4 so that lithography simulations over realistic topography can be studied. The Optolith imaging module has four submodules. These are concerned with the illuminating source, the mask, the projection system, and the image.

The shape of the illuminating source is arbitrary. Square or circular apertures can be defined and positioned at any location in the source region. Spatial filtering in the projection system is possible. Masks can have an arbitrary shape, and very general phase and transmission characteristics. MaskViews, Silvaco's general purpose layout editor, allows quick definition of phase shift, variable transmission, and binary masks.

The image calculation is done using an analytical Fourier Transform and then reconstructed using a Fourier Series summation. This allows great flexibility in the imaging simulation. The image calculation speed can be controlled by changing the size of the image window and the number of image points being calculated without a loss of accuracy.

The exposure calculation is based on the decomposition of a cross section of the aerial image into a spectrum of plane waves. The plane waves are propagated into the photoresist and underlying substrate. The interaction of these plane waves with the substrate topography is handled in a very general manner so that nonplanar topographies are accounted for. The intensity and photoactive compound concentration (PAC) are calculated using Dill's exposure model. High numerical aperture and defocus effects can be simulated. Optolith can simulate contrast enhancement layers with up to four different resist layers.

The post exposure bake module is based on 2-D diffusion of the photoactive compound. The development module uses a modified string algorithm. Several development models are available. There is built-in support for CD extraction and looping, so smile plots and swing curves are easily generated.

 

Examples

Figures 1, 2, 3, and 4 show an example of a structure that has undergone processing by SSuprem4, Elite and Optolith. The basic structure is constructed using SSuprem4 diffusion and oxidation, and Elite deposition machines. The structure consists of an aluminum metal line that is running over an isolation oxidation. Following photoresist deposition there is some thinning of the resist over the isolation oxide and a non-planar top surface results. An exposure was made using i-line illumination with a numerical aperture of 0.45. A positive defocus of 0.5 µm was used to compensate for depth of focus effects.

Figure 1shows the intensity calculated by Optolith for the structure following exposure to form two lines in the resist. A focusing effect at the edge of the oxide causes reflected light to create a point of high intensity in the area that should desirably be unexposed. In Figure 2, the photoactive compound concentration (PAC) is shown after the exposure of the photoresist. Again, the effect of the reflections near the edge of the isolation area can be clearly seen.

 

Figure 1. Intensity following exposure.

 

Figure 2. PAC following exposure.

 

Figure 3 shows the PAC after a post-exposure bake with a diffusion length of 0.08 µm. Figure 4 shows the resist after a development time of 40 seconds. This figure shows the effect of the reflected light on the final resist profile. It is clear from the shape of the resulting photoresist that such underlying topography must be taken into account in calculating final resist profiles.

 

Figure 3. PAC following post-exposure bake.

 

Figure 4. Resist following development.

 

Figures 5 and 6 show an NMOS structure including LOCOS isolation created using Elite and SSuprem4. Optolith was used to simulate exposure and development over aluminum for the etching of the contacts. The effects of multiple scattering off the highly reflective aluminum surface and underlying material layers can be seen at the contact position in Figure 5. The structure was exposed using a dose of 200 mJ, post-exposure bake was performed using a diffusion length of 0.1 µm, and the development was performed for 80 seconds using the Kim development model. Figure 6 shows the structure after development. The effects of reflection from the left side of the two contact bias result in exposure and subsequent thinning of the resist. This effect is highly sensitive to alignment and can only be predicted by taking into account the full underlying topography.

 

Figure 5. MOS structure PAC following
exposure and postbake.

 

Figure 6. MOS structure following development.

 

The overall results show the excellent capability of Optolith, when coupled with Elite and SSuprem4, to perform lithography simulation over completely general topographies.