Photolithography for Device Fabrication

Indtroduction

Photolithography is a technique used in semiconductor device fabrication. A light sensitive layer is added to a semiconductor wafer. Light is applied to the parts of the light-sensitive layer to remove the layer where needed. After this is done, etching can be performed exclusively to the parts of the wafer without a layer of the photo-sensitive layer.

Photoresist

The light-sensitive layer added to the wafer is called the photoresist. To apply photoresist to a wafer, first clean the wafer. The photoresist should cover most of the wafer. This should be done while the wafer is sitting in the spinner. The spinner rotates the wafer so that the photoresist has an even coating. Then the spun wafer is placed on a heating plate. The RPM and temperature of the hot plate are important. The photoresist data sheet can indicate the required spin rate and temperature.

Photoresist Applied to Wafer

When the photoresist layer is uneven, an interference pattern can be seen on the wafer, as shown below. The interference pattern shown is less than ideal. It is normal however that there will be more photoresist build-up at the edges of the wafer. The excess photoresist at the edges of the wafer can be removed using acetone and a q-tips or swabs.

Wafer with Interference Pattern after applying photoresist

There is also the question of whether to cut the wafer before applying photoresist or afterwards. The advantage of cutting the wafer afterwards is that the built up layer of photoresist at the edges will not be used, since the cut wafer will be taken from the middle. A disadvantage of cutting the wafer afterwards is that cutting the wafer can cause damage to the photoresist layer. Also, the issue of built up photoresist at the edges of the cut wafer will remain. If the cut wafer is not round, there will be more build-up at corners of the cut wafer. If using a silicon wafer, cutting a clean square will be more difficult than when using a III-V semiconductor. Generally, one can create a square or rectangle by making a notch in the side of the wafer. The lattice of the material will cause a break to be a straight line.

Mask Aligner

The mask is what is used to select which parts of the photoresist layer will be removed and which will stay. Masks are ordered from a company such as Photronics with a .gds file for the die. The mask and the cut wafer are placed in a mask aligner. Here, a vacuum press is used. UV light is directed at the wafer from above the mask aligner.

Developing

The UV light from the mask aligner breaks the bond of the photoresist where it was applied. Now the broken-bond photoresist needs to be removed using a developer solution. The amount of time that the wafer is rinsed in the developer solution is critical. Too much time can cause the photoresist to be removed further than needed. This is especially important for small features, such as a waveguide.

Wafer soaked in CD-26 Developer solution

Next, we can view the wafer using a microscope. The combination of the thickness of the photoresist, how even the photoresist layer is, the type of photoresist, how fast it was spun, how hot it was baked on the hot plate, the mask, the developer solution, how long it was soaked in the developer solution and how much care was given to the wafer during the process, including the presence of dust will all contribute to the overall result of the wafer. Below, curved waveguides are shown on the microscope. These are layers of photoresist. For a higher magnification, an electron microscope can be used.

Curved Waveguides – Photolithography
Photolithography Playlist

Photonic Components: Multimode Interference Waveguides

Multimode Interference Waveguides, also termed MMI Couplers, are used to split light from one waveguide into two or more paths. MMI couplers are designed to match the power at each output port. The length, width and positioning of the output ports are critcal to the design of the MMI coupler. The MMI coupler is also difficult to build in device fabrication due to the sensitivity of the width of the multimode waveguide to the performance.

Below are two MMI couplers, designed in Rsoft. The 3dB Coupler has two output ports of half the input power. The approach for both couplers is to monitor the optical power at each output port in the simulation. Initially, we design the length of the multimode section to be longer than estimated. The length of the multimode waveguide section is reduced to the length at which the optical power in each of the output paths is equal.

3dB Coupler

Simulation Result: 3dB Coupler
Rsoft CAD Setup: 3dB Coupler

MMI Coupler

rsoft7.1
rsoft7.2