Split Field Microscope and Alignment Standard Operation Procedure

TABLE OF CONTENTS:

1. 2. 3. 4. 5. 6. 7.

SUMMARY ....................................................................................................................................2 INTRODUCTION .........................................................................................................................3 LOCATION OF EQUIPMENT, ACCESSORIES, TOOLS, AND SUPPLIES ..................3 PERSONAL SAFETY EQUIPMENT .......................................................................................3 MATERIAL COMPATIBILITIES............................................................................................3 PRIMARY HAZARDS AND WARNINGS ..............................................................................3 OPERATIONAL PROCEDURE................................................................................................4 Procedure 1. Initial settings: ...........................................................................................................4 Procedure 2. Test vacuum system (without mask or wafer present):...........................................6 Procedure 3. Mask installation: ......................................................................................................6 Procedure 4. Place wafer on vacuum chuck and coarsely align with mask: ...............................7 Procedure 5. Split-field microscope use: .....................................................................................10 Procedure 6. Split-Field Microscope Adjusting (Optional):.......................................................11 Procedure 7. Mask alignment .......................................................................................................12 Procedure 8. Exposure ..................................................................................................................16 Procedure 9. Unloading wafer ......................................................................................................16 Procedure 10. Unloading mask.....................................................................................................17 Procedure 11. Restore normal positions and turn off power ......................................................17 8. HELPFUL HINTS, COMMON QUIRKS, AND TROUBLESHOOTING .......................17 9. ENVIRONMENTAL HEALTH AND SAFETY ISSUES.....................................................18

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1. SUMMARY Equipment Picture

Oriel Split-Field Microscope and 3-inch Aligner

Specification

Split-field, high resolution microscope used in conjunction with 3-inch wafer aligner to align a secondary mask with alignment marks already on wafer from previous mask. 3-inch wafer and 4-inch mask only EE/CSE B017, Photolithography room, UW (access through B025) Access Prof. Bruce Darling, Dept. of Electrical Engineering (206-5434703, [email protected]). Technical Prof. Bruce Darling, Dept. of Electrical Engineering (206-5434703, [email protected]). Emergency Prof. Bruce Darling, Dept. of Electrical Engineering (206-5434703, [email protected]). Date Rev. Authors 1999-06-06 0 Brian Skare, Shuliang Li

Materials Location Contact

This Document

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2. INTRODUCTION This standard operation procedure is constructed to provide information to a user on the Oriel Split-Field microscope and aligner. The Oriel Split-Field microscope is used in conjunction with a 3-inch aligner to allow precise mask positions on a 3-inch wafer. The split-field microscope is essentially two microscopes in one. The two lens apertures can be moved a variable distance apart (with a minimum separation distance of 1 inch from centers of the lenses). Each side of the microscope is displayed juxtaposed on the same screen. A TV monitor and viewing devise is connected to the microscope, so not only can the view from both lenses be seen through the microscope eyepieces, they can also be projected on the TV monitor. The benefit of the split field microscope is that two separate portions of the wafer can be viewed at the same time. This makes aligning the mask with the wafer a much easier job than with just a single microscope, since two sets of alignment cross hairs can be viewed and adjusted simultaneously. The aligner itself serves the purpose of allowing easy adjustment of the wafer position so that successive masks can be precisely aligned with micrometer accuracy with respect to a stationary mask. Three micrometers serve to adjust the position of a vacuum chuck on which the wafer sits – two micrometers adjust the forward/backward movement and twisting of the wafer, while one micrometer adjusts the horizontal left/right motion of the wafer. 3. LOCATION OF EQUIPMENT, ACCESSORIES, TOOLS, AND SUPPLIES The Oriel split-field microscope is located in the photolithography room (room B017) of the Electrical Engineering and Computer Science Building of the University of Washington, along the southwest wall, underneath the cabinets. (Access to room B017 is first through the gowning room, B025). 4. PERSONAL SAFETY EQUIPMENT No true personal safety equipment is required for the use of the split-field microscope. However, because handling of wafers and masks is required and because the microscope is located in the photolithography clean room, full clean room gowning is required (lab coat, long pants, closedtoed shoes, shoe covers, hair net, safety goggles, and gloves). 5. MATERIAL COMPATIBILITIES The material compatibility for the split-field microscope and aligner are rather straightforward. The only materials needed are a 3-inch wafer and a 4-inch mask. 6. PRIMARY HAZARDS AND WARNINGS In general, the only real operation hazards have to do with the fragileness of the silicon wafers and glass masks. If either the masks or wafers are mishandled or dropped, there is a potential for them to break. The shards of silicon and glass could pose obvious safety risks by causing cuts in the skin. The fragments that remain can also pose a problem to clean room status. Other than those possible safety risks, no hazardous situations should arise from typical operation of the split-field microscope and aligner. Therefore, this section is intended to remind the user of important steps to ensure proper operation of the equipment. This is a list of split-field microscope and 3-inch aligner do’s and don’ts—general guidelines:

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1. Make sure the flat of the wafer is lined up with front edge of the vacuum chuck when putting on any masks, especially the first one. If the first mask is crooked, this will cause much future frustration because all following masks will have to be crooked by the same amount to align correctly, which can make alignment much more difficult than necessary. 2. Make sure that the “Vacuum Leak” knob is opened significantly before switching the “Main Vacuum” switch to the “On” position to avoid large pressure difference on mask. It is desired to bring the gauge reading UP to 5 in. Hg by closing the “Vacuum Leak” knob. Otherwise the mask may be broken.

3. Do not turn main vacuum switch into “On” position when the wafer is not VERY close to the under surface of the mask. If this is not followed, it will bow the mask and will possibly cause damage to the mask. 4. Do not adjust aligner when the mask is in contact with the wafer. This would scrape the wafer against the chrome possibly severely damaging the mask. 5. Watch the fiber-optic cables from the TV monitor to the microscope while moving the microscope head – the cables tend to catch a bit, so it’s best to be careful not to pull on the cables too much. 6. The split-field microscope is located in the photolithography room. Both silicon wafers and glass masks are fragile and be broken if handled too roughly. Breaking of a silicon wafer or mask in the main photolithography room can pose a problem to the cleanliness of the room and everything inside, so use caution while handling the wafers and masks. 7. OPERATIONAL PROCEDURE Note: If this is the first mask being applied, no steps with the split-field microscope need be followed. Only the checklists in Procedures 1 through 4 and 9 and on need to be followed, (i.e. Procedures 5 through 8 can be skipped for the first mask). Procedure 1. Initial settings: Location Display unit (underneath the TV monitor) Aligner Vacuum Control Box

Item Left intensity knob Right intensity knob All aligner micrometers Vacuum Leak knob Vacuum Control knob Main Vacuum Switch Atmosphere/Purge Switch

Setting 9 9 5 open closed “OFF” N/A

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Note: Refer to Figures 1 and 2 for diagram and terms used to describe the Oriel Alignment Fixture and vacuum control box in the following procedures. Figure 1: Oriel Alignment Fixture*

*Figure 1 was adapted from drawing originally made my Dr. R. B. Darling for EE527, University of Washington, Spring quarter 1999 Figure 2: Vacuum Control Box

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Procedure 2. Test vacuum system (without mask or wafer present): •

Lift mask door frame to locked position (about a 25° angle from the horizontal)

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Open the Vacuum Control knob (turn counter-clockwise) – should not hear a hissing sound

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Start lowering the mask door frame from locked position – should start to hear a small hissing sound of the air being sucked through the vacuum chuck, then lower all the way back down

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Turn off the Vacuum Control knob (turn clockwise)

Procedure 3. Mask installation: •

Release all 4 clamps on the mask holder (only 3 are present at the time of this composition) – pull all clamps towards the center of the respective edge to release. (Sometimes they are difficult to release, so some force is required; however only move them far enough such that the tips of the clamps clear their respective pins)

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Lift mask holder straight up off the pins (place in a clean and convenient location in the room)

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Lower the vacuum chuck down by turning the thumbscrew to the left. (Until you get a good idea of how the wafer will sit vertically with respect to the mask, it’s best to simply lower the vacuum chuck all the way down until the thumbscrew stops turning; but DO NOT apply significant force).

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Carefully remove mask from plastic carrier.

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Inspect mask to see which side has the chrome on it. A good mask will be nearly impossible to determine which side has the chrome on it by looking directly at the side. One of the easiest ways to do actually tell the difference is to slant the mask so you’re not looking straight at it and look at the edge. Look to see if you can see through the glass to the chrome (meaning the chrome is on the bottom -- it will be a bit distorted looking), if you can’t see the distortion, flip over the mask and look at the other side. Figure 3 illustrates this. Figure 3: Chrome on Mask Orientation

Another method to use if the edges aren’t clearly distinguishable is to point the surface of the mask towards a light source and look for a shadow of the chrome on the glass in an opening in the chrome like with an alignment window. Again look at both sides of the mask, when you see the shadow, that means the chrome side is facing the light source. •

Place mask, chrome-side DOWN onto the foam rubber mask gasket with the large perpendicular, corner feature in the chrome pointing towards the RIGHT, REAR corner of the aligner. Make sure it is in contact with the three alignment pins sticking through the gasket. Figure 4 shows this mask placement. Figure 4: Mask Placement on Gasket

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Replace the mask holder and lock into position using the clamps (push clamps towards their respective pins until they are surrounding the pins) Procedure 4. Place wafer on vacuum chuck and coarsely align with mask:

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Make sure that all parts of the vacuum system are OFF

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Using tweezers, place wafer on wafer holder such that the flat of the wafer is facing the front of the vacuum chuck. Be careful while placing the wafer as it will often capture a pocket of air under the wafer and will float across the vacuum chuck.

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VERY IMPORTANT (especially so for the first mask): align the flat of the wafer as parallel as possible to front of the vacuum chuck, and as close to centered on the chuck as possible. Doing this will prevent many future headaches caused by poorly aligned wafers. Figure 5 shows how the wafer should be aligned. Figure 5: Wafer Placement on Vacuum Chuck

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Turn on the vacuum by turning the vacuum control knob counter-clockwise. The vacuum gauge should read about 25 in. Hg.

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Start to lower the mask door frame so that it is no longer in the locked position (so the vacuum can turn on.

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Test vacuum by using tweezers to GENTLY push the edge of the wafer; if it doesn’t move, the vacuum is working properly.

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Lower the mask door frame all the way down; a substantial gap should be visible between the chrome on the underside of the mask and the surface of the wafer.

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While watching an opening in the chrome (typically one of the 4 view windows), start to raise the vacuum chuck by turning the thumbscrew to the RIGHT. Raise the wafer until the gap between the chrome on the mask and the surface of the wafer is VERY SMALL (too great of a distance can cause damage to the mask in later steps).

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Check the placement of the wafer through the view windows on the mask. There should be about the same amount of the wafer showing through the right, left, and rear sides of the mask. If the wafer doesn’t appear to be well centered in the mask, adjust the micrometers on the aligner straight backward, forward, right, or left until about equal portions of the wafer can be seen through the mask.

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Turn on the vacuum leak by turning the “Vacuum Leak” knob significantly (several turns) counter-clockwise

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Turn on the “Main Vacuum” switch. The mask should noticeably be sucked against the wafer, and the vacuum gauge should read close to 0 if not fully 0. (It is VERY IMPORTANT not to try to adjust the vacuum chuck position when the mask is in contact mode – this could cause damage to the chrome on the mask and render the mask unusable.)

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Start slowly closing the “Vacuum Leak” valve. The vacuum gauge should start increasing slowly. Continue closing until the gauge reads 5 inches Hg.

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Note: For terms and locations of split-field microscope parts in Procedures 5 and up, refer to Figure 6 below. Figure 6: Oriel Split-Field Microscope*

*Figure 6 was scanned and then adapted from “Fig 2-1 MICROSCOPE DIMENSIONS” from page 11 of “HIGH RESOLUTION SPLITFIELD MICROSCOPE Model 85100” Instruction Manual. Procedure 5. Split-field microscope use: •

Turn on the power to the split-field microscope and its TV monitor. 10

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Slide the whole split-field microscope into position over the mask and wafer such that the light spots emitted on the wafer/mask by the microscope are approximately over the innermost edges of the view windows. If the microscope is locked in position, release the lock by pushing knobbed lever (located on the right side of the microscope by the microscope rail) up and towards the main microscope unit.

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Watch the fiber-optic cables as the microscope head is being moved – they tend to catch at little bit, and pulling to much on them could potentially damage either the cables of their connections.

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Adjust the microscope as needed so that the edges of the view windows can be seen through the microscope eyepieces. See Procedure 6 for adjusting the microscope.

Procedure 6. Split-Field Microscope Adjusting (Optional): 1. Changing the horizontal lens spacing: Course adjustment: •

Unlock the lens position of the desired lens by turning the locking screw slightly counter clockwise.

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Push that lens into a new position.

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Place the lens back into locked position by turning the locking screw clockwise until it stops (but do so without applying a lot of force).

Fine adjustment: •

With the lens in locked position and while looking through the eyepieces to determine the proper location of the lens, adjust the lens adjusting screws. The lens adjusting screws are located on the lower side of the main microscope head and beside their respective lens.

2. Front and rear adjusting of the microscope: • While looking through the eyepieces to determine the proper front/rear positioning, turn the adjuster knob (located on the lower left corner of the microscope base). Turn the knob clockwise for movement of the microscope head backwards and counter-clockwise for forward movement. 3. Adjusting location of the split-field line of the microscope: •

Turn the split field knob (located on the right side of the microscope head, about half-way up).

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Turn CLOCKWISE to show more of the LEFT field than the right. If it is turned all the way clockwise only the left field will be shown and the right field will be missing.

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Turn COUNTER-CLOCKWISE to show more of the RIGHT field than the left. If turned all the way, only the right field will be shown.

4. Focusing the split-field microscope: •

The main focuses are the large, black knobs on the right and left side of the microscope head. It adjusts the main focus on both lenses simultaneously.

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If the left lens and right lens can’t be well focused at the same time, focus the left lens well first.

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A fine focus for the right lens is located under the microscope head next to the right lens. Adjust it as necessary.

Procedure 7. Mask alignment Note: This procedure uses a dark field mask (Figure 5) as example. Light field mask alignment is similar. 1. Align left and right alignment windows: •

Make sure the mask isn't in touch mode with the wafer, which is called “contact mode”.

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In the left microscope field, find the left alignment window on the mask. Usually, not both left and right windows will appear in the two split fields at the same time.

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Figure 7 Align left window

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Adjust the micrometer 1, 2 and 3 (see Figure 1) accordingly to align one corner of the left window mark with the mask window so that the mark is a little bit inside the window. See the table below to see how to use 3 micrometers to move wafer.

Table 1 wafer mvement by adjusting micrometer 1 and 2 Wafer movement Twist clockwise Twist counter-clockwise Straight forward Straight backward

Micrometer 1 (Left) Clockwise Counter-clockwise Clockwise Counter-clockwise

Micrometer 2 (Right) Counter-clockwise Clockwise Clockwise Counter-clockwise

Table 2 wafer movement by adjusting micrometer 3 Wafer movement Move left Move right

Micrometer 3 (Right most) Clockwise Counter-clockwise

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In the right split field, find the right alignment window.

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Do the similar procedure to align the right window mark.

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Repeat these steps until both left and right window are aligned.

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Note: Spacing between window corners is larger than that of crosshair pairs. It is not very necessary to adjust the horizontal lens spacing to let both them viewable at the same time, especially when doing a batch of same wafer alignments. 2. Align coarse crosshairs: •

Find the coarse crosshair windows. Adjust the horizontal lens spacing as necessary to let both of them viewable at the same time (refer to Procedure 6). They should show up like Figure 8. It is normal that the two windows usually don't line up on a horizontal line.

Figure 8 Coarse crosshair alignment

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Get horizontal components of wafer crosshairs about equal distance from edge (d1=d2 in Figure 8) by twisting adjustment.

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Move wafer straight forward or backward until they are aligned in horizontal slots (Figure 9).

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Figure 9 Coarse crosshair alignment

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Move wafer left/right to align the two crosshair marks in the enter (Figure 10). Small twisting adjustment may be necessary. Figure 10 Coarse crosshair alignment

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3. Align fine crosshairs: •

Move to fine crosshairs and do the similar alignment (Figure 11).

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Keep the microscope field on fine crosshair when the fine alignment is done.

4. Contact mode checking and fine adjustment: Note: Remember "NEVER ADJUST WAFER IN CONTACT MODE". It will damage the mask and wafer. •

Set to contact mode by turning on main vacuum switch on vacuum control box (Figure 2). It makes the mask to touch the wafer. If vacuum gauge on vacuum control box is not 5 psi, adjust the vacuum leak knob.

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Check if there is any change of the alignment. If not, keep the mask in contact mode and the whole mask alignment is finished.

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If alignment is distorted after switch to contact mode, switch back to non-contact mode by turning off main vacuum switch. Do fine adjustment on the wafer position in “NON-CONTACT MODE" and check it in contact mode repeatedly until fine alignment is obtained in contact mode

Procedure 8. Exposure 1. Slide microscope head all the way to the left. Be careful of those optical and electrical cables. 2. Slide the aligner to rear lock position and do exposure. See standard operation procedure for exposure. Procedure 9. Unloading wafer 1. 2. 3. 4. 5. 6.

Slide the aligner back to front lock position. On vacuum control box (Figure 2), turn main vacuum switch off. Turn off vacuum control knob (clockwise). Turn vacuum leak knob on more (counter clockwise). Reading of pressure gauge will drop slowly. When pressure reach to 0, while "PRESSING RELIEF BUTTON DOWN" by right thumb, lift the mask door (Figure 11). Lock it at lock position. 7. Remove the wafer with tweezers. 8. Close mask door.

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Figure 11 Open mask door while pressing relief button

Procedure 10. Unloading mask 1. 2. 3. 4.

Release clamps on mask holder. Take off mask holder. Carefully remove the mask. Never touch the surface with finger. Put mask holder back and lock the clamps.

Procedure 11. Restore normal positions and turn off power 1. 2. 3. 4.

Lower the vacuum chuck down by turning the thumbscrew to the left. Set all 3 micrometers to 5 (middle position). Turn off power of microscope and TV monitor. Make sure vacuum control knob and main vacuum switch are off. But leave vacuum leak on (knob turned counterclockwise).

8. HELPFUL HINTS, COMMON QUIRKS, AND TROUBLESHOOTING One of the best sources of information on the split-field microscope is its actual instruction manual (which is located in the cabinet above the microscope). Human resources are also invaluable in learning new equipment, so it is good to have an experienced person give new users a brief run through of the operation. There might be a case where the first mask was put on at an angle, which throws off all future masks. So, it might be discovered that the twist needed to match the new mask with the mask features already on the wafer is greater than the micrometers can correct. In such a case return the micrometers to their nominal positions, turn off the vacuum, and manually twist the wafer on the vacuum chuck to an angle that is more suitable for matching the stationary new mask. Every now and then the whole microscope head will be accidentally knocked out of skew with the aligner (i.e. the microscope head is twisted with respect to the stationary aligner). If such a

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case occurs, it can be corrected by straightening the microscope head. Loosening the Allen screws on the base of the microscope slightly and then physically twisting the microscope head back into alignment can do this. Make sure to tighten the Allen screws back up afterwards though. 9. ENVIRONMENTAL HEALTH AND SAFETY ISSUES No true environmental heath and safety issues should arise if proper caution is used while handing the materials. The main safety issue that could arise would only occur if the wafers or masks are mishandled and break as a result. If the wafers or masks are broken, the fragments that remain could possibly be fairly sharp and cause cuts in the skin if touched.

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