Photolithography is a process analogous to developing film in a darkroom.
A layer of photoresist (either positive or negative) is applied to the wafer surface, then exposed to untraviolet light through a mask.
After development, only selected areas of the wafer will be covered with photoresist, leaving the rest of the wafer bare.
This prepares the wafer for a subsequent etch step which requires certain regions of the wafer surface to be "masked off".
The photolithography process consists of the following steps:
Place the wafer in a tin plate.
Preheat the Dehydration Oven to between 200 - 250 degrees C. Place the tin plate containing the wafer into the Dehydration Oven for 30 minutes to "Pre-Bake" it.
The purpose of this step is to ensure that the wafer is completely dry.
Any moisture on the wafer surface would interfere with the photolithography process, causing it to yield poor results.
Using teflon tweezers, place the wafer on the wafer chuck in the center of the Photoresist Spinner.
After making sure the wafer is centered on the wafer chuck, press the button to lock on the vacuum line.
The vacuum will secure the wafer to the chuck.
Using the Nitrogen Gun, blow the wafer surface to remove any dust particles.
Using an eye dropper, flood the wafer surface with primer.
Wait ten seconds, letting the primer dwell on the wafer surface so that it will allow good adhesion between the photoresist and the wafer surface.
Then, press the spin button which will cause the wafer to spin at 5000 RPM.
Centrifical force will cause excess primer to move in waves away from the center of the wafer. Hold the spin button down until the waves disappear.
At this point, the wafer is primed and ready to accept photoresist.
Note: We cannot photograph the photoresist actual being applied becausewhite light causes it to be exposed.
It is worth noting that all lighting in the lab is covered by yellow filters expressly to prevent interference with the photolithography process.
In a manner similar to applying the primer, an eye dropper is also used to place photoresist on the wafer. Apply the resist so that about 2/3 of the wafer surface is covered.
It is not necessary to completely cover the surface because spinning will cause the photoresist to distribute over the entire wafer, and the primer will ensure that it adheres evenly.
Spin the wafer for 30 seconds.
Then, release the vacuum, and use telfon tweezers to carefully remove the wafer and place it back in the tin plate.
Pre-heat the Soft-Bake Oven to the temperature specified by the type of photoresist you are using. Typically this will be between 90-110 degrees C.
Place the tin plate containing the wafer into the Soft-Bake Oven for 30 minutes.
The purpose of the soft bake is to semi-harden the photoresist.
While the wafer is in soft bake, you may set up the mask aligner.
For each substrate layer requiring photolithography, there is a seperate mask.
A mask is a patterned glass plate.
A set of masks cost between $500-$1500, depending on the number of layers and the required precision.
Locate the appropriate mask for this process step.
Place the mask in the wafer holder so that the emulsion side of the mask will be facing downward toward the wafer.
Slide the wafer holder into the mask aligner, and tighten the two knobs to secure it in place.
Using teflon tweezers, carefully place the wafer on the wafer chuck of the Aligner.
Always orient the wafer so that the flat side of the wafer is facing the same direction (to the left, for example).
This is important because each mask layer after the first one will be precisely aligned with the pattern formed by previous mask layers.
Carefully slide the wafer chuck into the aligner so that the wafer is positioned under the mask.
Slowly raise the lever so that the wafer comes into contact with the mask.
A light will come on indicating that the wafer is in contact.
Then, press the button labeled "Seperation".
This will cause the wafer to move 0.5 microns away from the mask, far enough that it can be moved without scratching the wafer.
There are three knobs which allow you to move the wafer in the horizontal and vertical direction, and also to adjust the angle.
If this is the first mask layer, it is only important that the mask area completely cover the wafer so that devices are not lost.
For subsequent layers, the pattern on the wafer must be precisely aligned to the mask.
To make this posible, there are markings on each layer to guarantee proper alignment.
As a general strategy, first align the wafer so that rows of devices on the substrate are parallel to rows of devices on the mask.
To accomplish this, it will be necessary to iteratively move back and forth accross a row, adjusting the angular position and the vertical position until the angular component is correct.
Once the angular component is correct, it should be easier to align the horizontal and vertical markers.
This will cause the wafer to once again be in direct contact with the mask.
The exposure time should be set according to the particular type of photoresist and wattage of the bulb being used.
In our example, the exposure time is 10 seconds.
Gently but firmly press the expose button.
A very bright white light will come on inside the aligner, exposing the wafer through the mask for the specified amount of time.
For safety reasons, avert your eyes from the aligner during exposure.
It is worth noting that the aligner is sitting on a stabilizer table. Essentially, each leg of the table has its own nitrogen shock absorber, minimizing the chance that there will be any disturbing movement during exposure.
Once exposure is complete, the wafer should be released from the contact position.
Then, the chuck should be slid out and the wafer gently placed back in the tin plate and carried to the chemical sink, where it will be developed.
Fill the pyrex beaker about 2/3 full with developer solution. Fill another beaker about 2/3 full with deionized water.
Using teflon tweezers, carefully place the wafer into a chemically resistant carrier.
Be sure to lock the wafer in so that it is firmly secured.
Check your developer for the recommended development time. Typically, this will be around 30 seconds.
Set a timer, then immerse the wafer in the developer and agitate mildly until the time has expired.
Then, immediately plunge the wafer into the beaker filled with water to stop the development process.
Pull the wafer out of the deionized water beaker, and use the deionized water gun to completely rinse the wafer (front, back and also the carrier) for about 30 seconds.
Use the nitrogen gun to thoroughly dry the wafer, front and back.
Then, use teflon tweezers to carefully remove the wafer from the chemically resistant wafer carrier, and place it back in the tin plate.
The final step is to inspect the wafers under a microscope, to see if the patterns are clear and well defined.
Since this is something that takes some experience to know what you are looking for, our lab is equipped with a dual microscope so that both the instructor and the student can view the wafer at the same time.
If everything looks okay, the wafer is placed into the Hard Bake oven which should be preheated to between 120-130 degrees C.
The wafers should remain in the hard bake oven for 30 minutes. This prepares the wafer for the next processing step.
If the results were not good, the hard bake should not be performed.
The photoresist can be stripped off using 1165 stripper or an acetone solvent.
The photoresist process can then be repeated beginning with dehydration.