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Karl Suss MJB-3 Mask Aligners

 

Table of Contents:
I: Introduction
II: Machine Specifications
III: System Components
IV: Operating Instructions
V: Exposure Parameters
VI: Troubleshooting
VII: DO's and DON'Ts
VIII: Check-off requirements

I. Introduction

Mask aligners are used in photolithographic processes to transfer a pattern from a mask to a photoresist coating on the substrate. The MJB-3s are suitable for handling wafers up to 3 inches in diameter and can be used for other samples with dimensions not exceeding 3 inches in any direction.

A mask aligner is used to expose a photoresist-coated substrate to ultra-violet light through a photo mask, which casts a shadow on the surface to define the photoresist mask. Different photoresists are sensitive to light at different wavelengths. It is important to select an aligner with an output wavelength optimized for your resist.

II. Machine Specifications

Left MJB-3

  1. Vacuum contact exposure capability for smallest possible gap between mask and substrate

  2. configured for 220nm exposure wavelength

  3. supports substrates up to 3" in diameter

  4. supports masks up to 4"

Right MJB-3

  1. equipped with IR back-side alignment system

  2. configured for 320nm - 365nm exposure wavelengths

  3. supports substrates up to 3"; however, alignment must be done within 1" of the center of the substrate

  4. supports masks up to 4"

III. System Components

  1. Front Control Panel

    1. POWER Button-When the machine is powered, the POWER button is illuminated

    2. CONTACT Indicator ----The CONTACT indicator is illuminated whenever both the contact lever and the separation lever are in the contact position. It is not possible to perform alignment when the contact indicator is lit.

    3. SEPARATION Indicator--- This indicator is illuminated when the contact lever is in the contact position and the separation lever is in the separation position. Exposure is not possible in this condition unless the aligner is equipped with a PROXIMITY button.

    4. EXPOSURE Button ---Pressing the EXPOSURE button initiates exposure and illuminates the button until exposure has been completed. The exposure time is determined by the setting on the exposure timer.

    5. VACUUM MASK Button --- In order to avoid damage to masks which may inadvertently be left in the machine, the mask vacuum is always on whenever the machine power is OFF.

    6. SOFT CONTACT Button - The SOFT CONTACT button is used to select soft contact exposure mode. In this mode, the substrate is pressed against the mask only by mechanical pressure during exposure. The vacuum under the substrate remains on.

    7. EXPOSURE TIMER --- The exposure timer is located on the right side of the front panel. In order to set the timer, two controls are used: an inner knob marked "s", "10s", "m", "10m", "h", and "10h" is used to set the multiplier, and an outer ring which is used to move the timer pointer. The scale for the timer pointer is graduated from 0 to 3. The exposure time is determined by multiplying the pointer setting by the multiplier set on the inner knob.

  2. Alignment Stage

    1. Transport Slide

    2. Alignment Micrometers-The Y and Theta alignment micrometers are located on the front of the alignment stage while the X micrometer is mounted on the right side. The range of adjustment in X and Y is 6 mm and the pitch of the micrometer lead screws is 1 mm (coarse adjustment) and 0.05 mm (fine adjustment). The Theta (rotation) micrometer has a range of 300 with a pitch of 0.5mm for the MJB 3 standard model.

    3. CONTACT Lever - Controls the Z-axis movement of the chuck. Located at the lower left side of the stage.

    4. SEPARATION Lever-is also located at the lower left side of the stage. The lever is used to mover the substrate in and out of contact with the mask in order to perform alignment.

    5. Mask Holder

    6. Variable Thickness Adjustment

    7. Nitrogen Purge

  3. Microscope

  4. Manometer

IV. Operating Instructions

Loading the mask

  1. The maskholder should be attached to the mask aligner by a thin vacuum tube. If the maskholder you want to use is not connected to the vacuum tube, connect the tube now.

  2. Place the maskholder on the table with the round groove facing up.

  3. Make sure that the MASK VACUUM switch on the front panel of the mask aligner is released.

  4. Place the mask on the maskholder so that the side with the pattern is facing up (away from the maskholder).

  5. Press the MASK VACUUM button in. This will activate the mask vacuum to secure the mask to the maskholder

  6. Insert the maskholder into the slots above the stage. The maskholder should be turned so that the side with the mask is facing down.

  7. Tighten the knobs to clamp the maskholder in place.

Loading the Substrate

  1. Move the separation lever back if it is not already there.

  2. Move the contact lever forward if it is not already there.

  3. Rotate the Z knob counterclockwise all the way if you do not know the correct setting for your substrate. If you have used a substrate that is the same thickness as the one you are using now, set the Z knob to the setting that you used when exposing that substrate.

    • WARNING: The step above is extremely important. If the Z setting is too high, the substrate will be forced through the mask when the substrate is moved into the contact position. This will break the mask, it will probably break the substrate, and it will damage the mask aligner.

  4. Pull the transport slide out from the right side of the stage if it is not already out.

  5. Place the substrate on the transport slide chuck. The substrate must cover all of the exposed vacuum holes on the chuck. If the substrate is not big enough to cover all of the holes, cover the chuck with a piece of blue tape and use a pick to poke one or more vacuum holes in the tape. The substrate will need to cover all of the exposed holes so that it doesn't slide.

  6. Push the transport slide all of the way into the stage.

  7. Turn the contact lever back. This will lift the wafer into position. Note that you will not be able to pull the transfer slide out when the contact lever is turned back.

Performing Alignment

  1. The two red buttons on the microscope joystick release the X and Y movement of the microscope. When neither button is being pressed, the microscope will not move. One button allows the microscope to move in the X direction and one button allows the microscope to move in the Y direction. Do not attempt to move the microscope without pressing the appropriate button(s).

  2. The substrate will not be able to move if it is in contact with the mask. Push the separation lever back at least part way to separate the substrate from the mask. Then, use the X, Y, and theta knobs to align the substrate with the mask. You may need to use the separation lever to move the substrate into contact with the mask to see it clearly with high magnification objectives. If you do this, you will have to move the lever back before you can move the substrate again.

Adjusting the Substrate Height

If you did not know the correct Z setting for your substrate when you loaded it into the aligner, you should have lowered it all the way before loading your substrate. Now, you will probably need to increase the setting to bring the substrate into contact with the mask. Follow the procedure below carefully to avoid damage to the mask, substrate, and mask aligner.

  1. Make sure that the separation lever is back all the way.

  2. Make sure that the contact lever is forward all the way.

  3. Rotate the Z knob clockwise until the substrate is in contact with the mask. You can use the microscope to check the distance between the substrate and the mask. Focus the microscope on the mask. Then focus it on the substrate and note the distance the microscope moved. This is the distance between the substrate and the mask. If you raise the substrate too far, you may break the mask so be careful.

  4. Once the substrate is in contact with the mask, you may want to note the current Z knob reading. Next time you expose a substrate of this size, you can just set the Z knob to this number. When reading the Z knob, be certain to note the number displayed in the hole in the knob as well as the number on the outer ring of the knob.

Exposing the Substrate

  1. Move the separation lever back and move the contact lever forward if they are not already there.

  2. 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 (seconds, tens of seconds, minutes, tens of minutes, hours, or tens of hours). The time unit on the left side of the knob (the one that is completely right side up) is the one that is currently selected. The big clear knob adjusts the time from 0 to 3 in the time units selected.

  3. Set the lamp power supply to the correct setting for your process. See the section Understanding Exposure Control above.

  4. 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. DO NOT LOOK INTO THE UV LIGHT. The mask aligner is exposing the substrate to ultra-violet light, which is damaging to your eyes. During the exposure, the lamp power supply will display the intensity of the exposure light in mW/cm2. 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.

  5. When the exposure is finished, press the CP button on the lamp power supply, then push the down arrow button until the light is flashing.

Removing the Substrate

  1. Make sure the separation lever is back and turn the contact lever back. This will take the substrate out of contact with the mask and lower the substrate chuck back onto the transfer slide.

  2. Pull the transfer slide out from the right side of the stage.

  3. Remove the substrate from the chuck.

Removing the Mask

  1. Loosen the knobs on the side of the maskholder slot.

  2. Carefully slide the maskholder out of its slot and place it mask side up on the table.

  3. 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.

  4. Remove the mask from the maskholder.

V. Exposure Parameters

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.

Intensity

The intensity of the light that reaches the sample 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 being supplied to the lamp in an attempt to keep the intensity measured on a built-in intensity meter constant. In CP (constant power) mode the mask aligner delivers a constant electrical power to the lamp. This does not result in a constant light output due to degradation of the lamp over time and other variables.

When the mask aligner operates in one of the CI modes it displays a light intensity (in mW / cm2) on the display. It is important to understand what this number really means. The mask aligner measures the light intensity inside the mirror housing between the UV lamp and the sample it is exposing. The intensity is not measured according to any particular scale and the measurement is not selective to a specific wavelength. This measurement is scaled by a calibration factor and displayed on the power supply display. This display is calibrated by MiRC staff to correspond to the actual intensity measured by a wavelength selective probe at the time of calibration. Over time the relative intensity of the light at different wavelengths will change and the measured intensity will be accurate because the change in overall intensity (measured by the meter) will not be proportional to the change in intensity at the calibrated wavelength.

Controlling Exposure

To achieve consistent results when performing photolithography it is important to deliver a consistent amount of energy to each sample. This does not mean that you must have consistent light intensity for every exposure. Light intensity will vary over the life of the lamp. 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 it is best to use one of the handheld intensity probes. These probes measure intensity in a narrow wavelength spectrum and are usually more accurate than the built-in meter in the mask aligner.

Important Notes

  1. 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.

  2. The intensity measured by the power supply on the CI1 or CI2 setting may be wrong. You should use the handheld probes for reliable measurements.

  3. 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 current intensity.

  4. 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.

VI. Troubleshooting

Q:The UB light is not ON after I pushed the Exposure button, what should I do?

A: Check whether the CONTACT button / light is on.

If CONTACT button is not on, check the SEPARATION and CONTACT levers

Q:I can't see any patterns through microscope, what should I do?

A: Make sure you turn on the microscope on the panel on your right side.

Q:When I move the CONTACT lever forward, my substrate contacts with the mask too tight. What do I do?

A: Lower the Z-axis a little

Q:When I do the alignment, the substrate moves. What do I do?

A: Make sure your substrate covers all the holes on the chuck; if your sample is too small, you can use some plastic tape / blue plastic to cover some of the holes. See staff for blue plastic.

VII. DO's and DON'Ts

DO NOT

    look into the UV light when during exposure.

DO NOT

    use the machine if another user has placed a 'machine down' on it; ask the technical staff to verify that it is working.

DO

    lower the z-axis before you load the substrate.

DO

    measure the light intensity every time before you use the machine.

DO

    turn off the microscope after you are done

DO

    push the CP button, and push the down arrow button until the light flashes after you are done. This puts the power supply into standby.

DO

    ask the trainer, cleanroom / technical staff, and other user questions about anything that is not clear or that you do not understand

VIII. Check-off requirements

To be checked off, you must be able to do, to demonstrate and/or to know the following items:

  • Locate the relevant components of the system

  • Know the difference between the left and right MJB-3 aligners

  • Know the basic exposure control principles

  • Load and unload a mask

  • Load and unload a substrate

  • Align a sample

  • Set exposure time and expose a sample

  • Measure UV light intensity

  • Deal with the errors covered in the Troubleshooting section.