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Revision 2 - 16 October 2000, David Sigmon
Revision 3 - 24 January 2003, Joel Pikarsky, Peter Sahlstrom

Karl Suss MJB 3 Mask Aligners

Trainers

Mask aligners are used in photo lithographic (from Greek, photo -light, lithos-stone, graph-writing, "writing on stone with light"!) processes to transfer a pattern from a photomask to a photoresist coating on a substrate.

During the lithography process, a photoresist-coated substrate is exposed to ultra-violet (UV) light through a photomask. UV light will pass through clear areas and be blocked by dark areas on the photomask. For a positive photoresist the areas that are exposed to light will be removed when the photoresist is developed. For a negative photoresist the areas NOT exposed will be removed when the photoresist is developed. In this way, the pattern on the glass photomask is transferred onto a photoresist layer and a photoresist mask is created.

Different photoresists are sensitive to light at different wavelengths. It is important to select an aligner with an output wavelength optimized for your resist.

Things you should know before you use any mask aligner:

What kind of photoresist you are using.
What wavelength of light your photoresist is sensitive to.
The thickness of your photoresist.
The thickness of your substrate (wafer).
The wavelength of light the mask aligner emits.
The size of the features you want to make.

For more detailed information on using specific mask aligner features please see the Karl Suss operating instructions (PDF format, 1,054KB) .

Features

Left MJB 3

vacuum contact exposure capability for smallest possible gap between mask and substrate
configured for 240 - 320 nm exposure wavelength
supports substrates up to 3" in diameter
supports masks up to 4"

Right MJB 3

equipped with IR back-side alignment system
configured for 320 - 365 nm exposure wavelength
supports substrates up to 3" in diameter however alignment must be conducted within 1" of the center of the substrate
supports masks up to 4"

Understanding Exposure Control

Before using the mask aligner to expose your sample it is important to understand how the mask aligner controls exposure and what exposure parameters are actually important.

Wavelength

The ultraviolet light in the mask aligner is provided by a high pressure mercury arc lamp. This lamp produces light with a wide range of wavelengths, but it is particularly strong at certain spectral lines that are characteristic of mercury. Depending on the optics installed, the mask aligner will deliver some of these wavelengths to the sample more efficiently than others. Regardless of which mask aligner you use and what settings you use, your sample will be exposed to light at a variety of wavelengths.

This is a link to a page that has information on HP mercury lamps and a chart that shows which wavelengths are strongest.. This is the company that makes the lamps we use in our mask aligners.

http://www1.ushio.co.jp/catalog/1100/006.html

Intensity

The intensity of the light produced by the lamp will vary with various factors including the electrical energy that the power supply delivers to the lamp and the age of the lamp.

If you operate the aligner with the power supply in one of the CI (constant intensity) modes, the power supply will adjust the electrical power so the lamp will always produce the preset illumination intensity. In CP (constant power) mode the mask aligner delivers a constant electrical power to the lamp. Constant power mode does not result in a constant light output due to degradation of the lamp over time and other variables.

Each CI channel must be calibrated for the desired wavelength and illumination intensity. This should ONLY be done by the cleanroom staff. When the CI channels are calibrated and a channel is selected (by pressing the CI1 or CI2 membrane switch) the power supply will display the light intensity (in mW/cm²) that is being produced.

Note: The display will show the intensity only during the exposure, the rest of the time it will show 0.0. If the display still shows watts, press the DS (display select) membrane switch to change to mW/cm² (the DS switch does not work on the left MJB-3).

If you operate the mask aligner in CI mode your wafer should always receive the same exposure dose per second for every exposure. The only variable will be the exposure time.

WARNING: It is possible to change the illumination intensity and power settings to levels that can damage the lamp and cause it to explode. At operating temperature the pressure inside the lamp can be as high as 6.9 MPa (70 atm). If the lamp explodes the mask aligner may be damaged and hot quartz fragments may strike the user. The lamp contains several grams of mercury that are vaporized when the lamp is operating. A lamp explosion will release hot mercury vapor which will cause untreatable and irreversible neurological damage if it is inhaled.

Controlling Exposure

To achieve consistent results when performing photolithography it is important to deliver a consistent amount of energy to each sample. If the CI mode has not been calibrated correctly you will not have consistent light intensity for every exposure. You must compensate for variations in intensity by varying your exposure time to maintain a consistent energy per unit area during each exposure. Recall that one watt is equal to one joule per second. Therefore you can find your exposure energy by multiplying the light intensity (in mW/cm²) by the exposure time (in seconds). This will yield exposure energy (in mJ/cm²).

To measure the light intensity on your sample you can use one of the handheld intensity probes. These probes measure intensity in a narrow wavelength spectrum so you must be sure to use the correct probe for the proper wavelength for your photoresist.

Important Notes

Selecting the CI1 or CI2 setting does not control the wavelength of light that your sample is exposed to. The mask aligner emits light at a number of wavelengths at all times.
The intensity measured by the power supply on the CI1 or CI2 setting may be wrong. You can use the handheld probes to check the intensity.
The CP setting on the power supply does not produce light at a constant intensity. You must measure the light intensity with a handheld probe to determine the light intensity.
The lamp power and intensity settings in the lamp power supply should not be adjusted. The only thing that you should do to the power supply is to change the mode to CP, CI1, or CI2 by pressing the corresponding button.

Operating Instructions

Introduction

Before exposing your sample, it is important to know the exposure parameters that are required by the photoresist you are using. In particular, you need to know what wavelength of light your photoresist is sensitive to and how much energy is required to activate the resist. You must use a mask aligner that is configured to emit the wavelength required by your photoresist. The MJB 3 on the left is configured to emit light at 220 nm, while the one on the right is configured to emit light in the 320 - 365 nm range. Both mask aligners also emit longer wavelengths.

Before you load the substrate, you should perform a test exposure using the "Exposing the Substrate" procedures below. During the exposure you will be able to see the light intensity. This will allow you to adjust your exposure time if necessary.

For more detailed information about special mask aligner features, please contact MiRC technical staff, or look at the operating procedures from the manual, which may be found in one of the table drawers near the MJB 3s, or follow the link at the top of the page.

Loading the Mask

The maskholder should be attached to the mask aligner by thin vacuum tubing. If the maskholder you want to use is not connected to the vacuum tubing, connect it now.
Place the maskholder on the table with the round groove facing up. (Note: If the mask holder is placed on a very smooth surface it will stick to the surface when mask is in place and the mask vacuum is turned on)
Make sure that the MASK VACUUM switch on the front panel of the mask aligner is out.
Place the mask on the maskholder so that the side with the pattern is facing up (away from the maskholder). The mask should be centered from side to side and aligned with the edge of the mask holder that is away from you.
Press the MASK VACUUM button in. This will activate the mask vacuum to secure the mask to the maskholder.
Insert the maskholder into the slots above the stage. The maskholder should be turned so that the side with the mask is facing down.
Gently tighten the knobs to clamp the maskholder in place. The knobs only have to be tight enough to prevent the mask from moving.

Loading the Substrate

Adjusting the mask aligner for the thickness of the substrate is very important. If your substrate is not positioned correctly the mask pattern will not be accurately transferred to your wafer. This is the most complicated part of using the mask aligner because you have to make many adjustments.

NOTE: You will need to make many adjustments to the Z knob. This knob adjusts a very delicate mechanism. You should ALWAYS rotate the contact lever toward you AND move the separation lever toward you BEFORE you turn the Z knob. This will release the pressure on the Z knob mechanism and prevent damage. NEVER turn the Z knob when the wafer is in contact with the mask. Turning the Z knob clockwise increases the separation between the mask and the wafer chuck (Turning the Z knob clockwise increases the reading which corresponds to the thickness of the substrate).

These steps describe hard contact mode (this is the best compromise between accuracy and speed).

To load your substrate, rotate the contact lever toward you.

Make sure the X, Y, and theta micrometers are adjusted to to place the transport slide at the center of it's range of motion.

Gently pull the transport slide out from the right side of the stage.

Place your substrate on the transport slide chuck. Center your substrate on the chuck. If you are going to align to the wafer flat it is usually best to have the wafer flat toward you. Gently push the transport slide all of the way into the stage.

Move the separation lever away from you. You can not rotate the contact lever away from you to raise chuck unless the the separation lever is away from you first.

SOWLY rotate the contact lever away from you and watch your wafer as it rises towards the mask. The contact lever will rotate 180 degrees until it is pointing away from you. You should not see your wafer touch the mask or feel resistance while turning the contact lever until the contact lever has rotated AT LEAST 160 degrees. If you feel resistance before you have rotated the contact lever 160 degrees away from you, STOP! Rotate the contact lever toward you AND move the separation lever toward you, turn the Z knob clockwise some more, and try again.

The goal is to feel a little resistance just before you have rotated the contact lever away from you 180 degrees. This will press your wafer against the mask with the correct force to give good accuracy. When the wafer is in this position it will be pressed firmly against the mask. You will NOT be able to align your wafer. Move the separation lever toward you. This will lower your wafer about 150 um.

Look through the microscope and focus on your wafer through a clear area in the mask. Turn the X or Y micrometer a little. You should see your wafer move. If your wafer does not move, it is still pressing against the mask too much. You must lower the chuck (this means rotate the contact lever toward you AND move the separation lever toward you), turn the Z knob farther clockwise, and try again.

NOTE: If your mask does not have any large clear areas on it you should use a different mask that does have clear areas for the contact adjustment, and then put your mask back in for the exposure. All 4-inch masks have the same thickness (0.060 inch), so if the contact pressure is correct for one mask it will be correct for any mask. If your contact pressure is too hard you can break your wafer, the mask, or the mask aligner. If your contact pressure is not hard enough you can have poor resolution and alignment.

A good test for correct contact pressure is to turn the X or Y micrometer a little when the separation lever is toward you. Your wafer should move.
If you turn the X or Y micrometer a little when the separation lever is away from you, your wafer should not move .

You should only have do make this adjustment for your first substrate. Usually all your substrates will be the same thickness (you should know the thickness of all your substrates).

Performing Alignment

The two red buttons on the microscope manipulator release the X and Y movement of the microscope. The microscope will not move unless you press the buttons. One button allows the microscope to move in the X direction and the other allows the microscope to move in the Y direction. If you press both buttons you can move the microscope in any direction. Do not try to move the microscope without pressing the button(s).

CAUTION: Do not twist the microscope manipulator so the wire wraps around the shaft. This will damage the wire.

The substrate will not be able to move if it is in contact with the mask. Move the separation lever toward you to separate the substrate from the mask. Then, use the X, Y, and theta knobs to align the substrate with the mask.

Exposing the Substrate

Move the separation lever away from you (the contact lever should already be rotated away from you).
Set the timer on the front panel of the mask aligner to the desired time. The small black knob in the center selects the time units. You can choose seconds, tens of seconds, minutes, tens of minutes, or hours. The time unit on the left side of the knob (the one that is completely right side up and next to the little black triangle) is the one that is currently selected. The big clear knob adjusts the time from 0 to 3 in the time units selected.
Stay clear of the microscope and press EXPOSE on the mask aligner control panel. The microscope will lift and move towards you and the substrate will be exposed for the time selected. You must have safety glasses on during the exposure to protect your eyes from the UV light and possible lamp explosion. The lamp power supply will display the intensity of the exposure light in mW/cm² or the power to the lamp in watts depending on which display you have selected. You can perform a test exposure without a substrate loaded to check the exposure intensity, then load the substrate and calculate the exposure time using the exposure intensity that was measured.

Removing the Substrate

Move the separation toward you and rotate the contact lever toward you. This will take the substrate out of contact with the mask and lower the substrate chuck back onto the transfer slide.
Gently pull the transfer slide out from the right side of the stage. NOTE: If you feel ANY resistance when pulling the transfer slide STOP! Check the position of the X, Y, and theta micrometers and return them to the center position. If you are near the maximum adjustment the side of the transfer slide will rub against the chuck housing.
Remove the substrate from the chuck.
Load your next substrate OR if you are finished gently push the transfer slide to the left.

Removing the Mask

Be sure to do this before you turn off the power or your mask will fall off the holder onto the chuck.

Loosen the knobs on the side of the maskholder slot.
Carefully slide the maskholder out of its slot and place it mask side up on the table.
Release the VACUUM MASK button on the front panel of the mask aligner. This will release the vacuum that is holding the mask on the maskholder.
Remove the mask from the maskholder.
Leave the mask holder knob loose. The next user will have to loosen them anyway to put in their mask.