Petta Lab Raith EBL Guide

Bart H. McGuyer

July 9, 2007

This document is a guide for performing elec-tron beam lithography (EBL) with the PRISM Raithe LiNE system, and is primarily aimed for makingsmall devices on GaAs. The majority of this docu-ment is adapted from “Jeff’s (Fairly Comprehensive)Raith Usage Notes” by Jeff Miller from the MarcusGroup at Harvard.

Jump to the end of this document for a condensed,single-page overview.

Important Rules

• Always handle your chip with carbon tippedtweezers–even on hot plates.

• Never let solvents dry on a chip, otherwise it willbe ruined.

• Use a teflon beaker for quick transfer betweensolvents.

• MORE RULES?

1 Pre-Raith Preparation

Device Design

Use DesignCAD or LayoutEditor to design your de-vice. Make sure all polygons are closed, and thatthey surround the areas you want exposed. In gen-eral, make two EBL layers in your device: one layer(“Gates”) that includes all the smallest features in-side one 100 µm x 100 µm write field, and anotherlayer (“Connector”) that connects the first layer tothe photolithography features and spans multiplewrite fields. See figure 1 for an example. This way the

smallest features can be written with optimal beamsettings for their size, and then the larger surround-ing features can be written quickly with a larger aper-ture and more beam current. In addition, separatethe smallest features inside a 10 µm x 10 µm writefield from the rest of the Gates layer by breakingthe polygons in two; this allows the smallest featuresto be exposed quickly in succession, preventing stagedrift error.

For DesignCAD, save as DXF. Use either LayoutE-ditor or LinkCAD on PRISM’s mask computer in theJ-wing atrium of the E-Quad to convert the DXF fileto GDS II. A USB stick is very handy. I keep one inmy orange tote-box. For LayoutEditor, just save asGDS II.

Figure 1: EBL layers in the author’s Spin Shuttle de-vice. The Gates layer is the black pattern in the cen-ter with alignment marks spaced 90 µm apart. TheConnector layer is the black surrounding layer. Theouter gold and green layers are for photolithography(metal and mesa-etch, respectively). REDO THISWITH MORE IMAGES!!!!

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2 1 PRE-RAITH PREPARATION

Preparing the Chip

To prepare your chip, first get all your supplies inorder in your orange clean room tote-box. At a min-imum you will need tweezers (carbon, wafer, andmetal fine-tip), a timer, and a sample carrier box.You’ll also need five labeled small plastic beakersfor chip processing; one each for trichloroethylene(TCE), acetone, isopropanol (IPA), PMMA devel-oper (MIBK:IPA 1:3), and liftoff (dirty acetone andmetal).

To make chip processing both easier and safer foryour chip, use a small white 5mL Teflon beaker tohold the chip. Drill small holes in the beaker for easytransferring into liquid, and cut the beaker height atleast in half.

You will also need your own short-term supply ofEBL resist. We use 950K molecular weight poly-methyl methacrylate (PMMA), 4% in anisole. Soni-cate a small vial in acetone and IPA, and let it drycompletely. Label it with the date and fill it 3/4full with PMMA. Store this bottle upright in yourtote-box, and keep it wrapped with parafilm. KeepPMMA off the threads of this bottle, otherwise it willcreate gunk that will fall inside. You should make anew bottle when you begin a new device.

For 50nm and smaller sized gates, its best to use asingle layer of PMMA. Here is the recipe for reliablydepositing a single 200 nm layer of PMMA:

200 nm PMMA Recipe

3 Solvent Clean: If this is the first EBL layer, per-form a three solvent clean with the chip in your Teflonbeaker:

1. Sonicate 5 minutes in trichloroethylene (TCE)

2. Sonicate 5 minutes in Acetone

3. Sonicate 5 minutes in Isopropanol (IPA)

4. Blow dry with N2

NEVER let TCE or acetone dry on the chip!Rapidly transfer the chip between beakers. Whenblowing dry, do not let any puddles of solvent evapo-rate on the chip surface. If you contaminate your chip

Figure 2: Spin speed versus film thickness curve forMicrochem 950k MW PMMA in Anisole. We useA4, or 4% PMMA in Anisole. For 4000 rpm, thefilm thickness is 200 nm.

at any time later in this recipe, you have to start allover again with the three solvent clean.

Prebake: Bake the chip at 170◦ C for 2 minuteson a hot plate next to a spinner. Set the hot platetemperature in advance to save time.

Spin PMMA: Program a spinner with the followingrecipe:

1. 5 seconds at 500 rpm, 500 rpm/s spinup.Apply PMMA during this step.

2. 60 seconds at 4000 rpm, 1000 rpm/s spinup.

Find a good chuck, preferably with an o-ring sealto hold the chip. Clean out the spinner if the lastuser left a horrible mess (if your chip comes looseduring the spin, it will become contaminated withthat mess).

Use a junk chip to test the spinner! Now is the timeto find out if spinner is going to be troublesome. Ifyou real chip comes loose during a spin, you have tostart all over again with cleaning the chip.

Use a plastic pipette from our lab to administerthe PMMA. Blow it off with nitrogen, and inspectfor cleanliness. Try to only take PMMA from yourbottle, and do not return any unused PMMA fromthe pipette into the bottle. Get rid of bubbles by

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making a few sacrificial drops into the spinner bowlright before putting a drop on your chip.

Spin your chip and put on the PMMA during the500 rpm cycle. Be nice and clean the spinner whenyou are finished.

Bake: Immediately bake the chip at 170◦ C for 15minutes on the hot plate next to the spinner. Thissets the resist layer by removing the anisole.

Now you have a chip prepared with a single 200nmlayer of PMMA, ready for EBL. As a final step, if yourchip does not have any small features to focus on,place a small amount of silver paint (SEM quality)on the corners of your chip. Do not dilute it, andwait at least 15 minutes for it to dry before loadinginto the Raith. I use a sharp wooden dowel fragmentto apply the paint. Use parafilm to seal the silverpaint bottle after it is closed, as it is very volatile.

2 Raith Procedures

Raith Overview

The Raith system is a modified Leo SEM, and is con-trolled by the two computers in figure 3. The moni-tor on the right (“Leo” or “SEM”) controls the SEM.The monitor on the left (“Raith”) controls everythingelse, such as the laser stage, and also steals control ofthe SEM as needed, such as during exposure.

Both computers should always be on, thoughPRISM asks that we turn the monitors off when themachine is not in use. If at any time the Raith soft-ware crashes, you can safely restart the Raith com-puter. However, do not restart or shut down the Leocomputer; if you have trouble with it, contact an ad-ministrator. The Windows XP account for both com-puters is the same: username “User” and password“eLiNE.”

To use the Raith computer, login to the e LiNEprogram with your account and open the Raith Pro-tocol HTML file with the shortcut on the desktop.For the Leo computer, Smart SEM and RemCon32should always be running. To start or restart SmartSEM, login with the XP username and password. IfRemCon32 is off, contact an administrator.

Figure 3: Raith e LiNE and Leo SEM monitors.

Figures 4 and 5 highlight the most commonly usedfeatures in the e LiNE and Smart SEM software.

There are three additional mask design computersin the outer Raith room. Their Windows XP logonis “User” with no password. They each have a copyof the Raith e LiNE software, including the GDS IIeditor for working with device designs. Login withyour account or the training account with password“raith.” Please note that these computers, like theRaith and Leo computers, have no internet access.

When you are finished with the Raith, don’t for-get to turn off the gun EHT with the Leo computer.NEVER turn off the gun itself; it should always re-main on. Log off the e LiNE program, but leave theSEM software running. Turn off both monitors, andfinish filling out the logbook.

Loading a Design on the Raith

It is best to load your design on the Raith computerand setup positionlists in advance. Reserve sometime before your exposure to use the Raith computerfor this purpose.

First, inspect your GDS II file with a mask designcomputer in the Raith room. Fix any conversion er-rors as needed.

Use the USB hub next to the Raith monitor toload your GDS II file in your e LiNE directory(C:\e LiNE\User\username\GDSII\filename). Lo-

4 2 RAITH PROCEDURES

Figure 4: Raith e LiNE program main screen (withTemplate 1280), including zoom of the top menu.

gin to the e LiNE software and open the GDS IIDatabase window. Load your device file, and selectonly the EBL layer(s) to be exposed. Edit the file asneeded. Save, close, and re-open the file in-betweensteps. Click Set Dose under Dose in the Modify menu,and set the dose factor to 1.0 for all gates.

Set the write-order of your gates as you want themto be exposed. In the Raith GDS II editor, push O toview the current ordering and then click on the gatesin the order you would like them to be written. Thisway, the gates that are most sensitive to stage driftcan be written consecutively.

Setup the working area, which is the portion of thedesign you would like to expose. If you want to ex-pose less than the write field size, such as a dose test,then just set the working area to be the area you wantexposed. If you want a working area larger than onewrite field, then it simplifies the position calculationif you specify a working area that is an integer mul-tiple of write fields. Make sure that the critical areaof your pattern is not divided between different writefields. NEEDS WORK: I will understand this moreafter I make a trial “real exposure.” *Positioning ofworking area . . .

For a dose test, create a new positionlist. Edit yourdevice to include diagnostic parallel lines and dots.

Figure 5: Leo Smart SEM program main screen.

Drag your device into the positionlist. Edit it andset (U,V) to (2,2) mm. Set the working area to (-50,50) µm in both U and V. Set the dose factor to 1.00.Matrix copy the entry to create a duplicate entry at(U,V) = (2.1, 2) mm. Set this entry’s dose factor to1.04, and reduce the working area to (-5, 5) µm inboth U and V. Matrix copy this entry, creating a 31item array spaced by 25 µm in U and with incremen-tal steps of 0.04 in the dose factor. This will createexposures with dose factors of 1.00 to 2.20 in steps of0.04. Save this positionlist in your directory.

For the real exposure, create a new positionlist andfill it . . . . NEEDS WORK: I will understand thismore after I’ve given it a try. Need details aboutworking area vs. write field size, Connector vs. Gatesexposure, correct positioning of exposures . . .

Beware that if you make a positionlist on anothercomputer, it will look for your file in the directory itused on that computer.

Now you’re ready to use the Raith for an exposure.

Load Sample

Use your carbon tweezers to place your chip on theRaith sample holder. See figure 6 for chip orientation.Gently clip it in place, and make sure the sample islying flat on the holder. Use the nitrogen gun on the

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Figure 6: Raith e LiNE general purpose sampleholder. This is the orientation of the holder as viewedwith the Leo CCD camera inside the Raith.

wall to generously blow off the sample holder, andinsert it in the loadlock. The holder should lie flat onthe two supporting rods, and the holder’s tab shouldrest well on the spike when pushed down. Inspectthe loadlock door o-ring, and close the loadlock door.Screw it tightly closed, but don’t over-tighten.

Log on to the e LiNE software on the Raith com-puter. Click the second stop light to open the Naviga-tor Loadlock window. Click Load Sample, then ViaLoad Lock. A series of pop-up windows will guideyou through the loading procedure, which should takeabout 8 minutes. Please note that the load lock turbopump will start after you click Ok on the window ask-ing if the Loadlock gate valve is closed.

The miniature load lock turbo pump will ramp upquickly over a few minutes. You should always beable to hear it, and it steadily increase in pitch duringpump down. If the turbo becomes silent for severalminutes, then it is probably having a hard time dueto dirt on the load lock door’s o-ring. If this happens,abort the loading procedure and inspect the o-ring.If you still have trouble, contact an administrator.

A window will eventually ask you to open the loadlock gate valve. Please double-check that the greenVac Ready light at the load lock is on before openingthe gate valve. Be gentle when inserting and remov-

ing the transfer rod, using only the handle to push orpull.

At the end of the loading procedure the Raith com-puter will ask you some questions: Reset UV Adjust-ment: No. Enter Voltage: 30 kV. Enter ApertureSize: 10 µm. Enter Working Distance: (doesn’t mat-ter). No values for stigmation or aperture found indatabase: Wait before clicking Ok! PRISM asks thatwe wait until the system vacuum is below 2 x 10−6

Torr before turning on the EHT. Once you click Okthe Raith computer will set up the column and turnon EHT. So wait, and then click Ok. The ramp uptime is about 1 minute for 30 kV.

Write down the time in your notebook, so that youwill know how long the beam has been on before yourexposure. Two more windows will appear: Load pro-cedure finished successfully: Ok. Please specify sam-ple name: Cancel. Close the Navigator Load Lockwindow.

Jeff Miller’s instructions recommend using “FindHome” at this moment to reduce stage drift. Openthe Find Home window and click the two lightningbolts to find home in the X and Y directions. Thestage will drive to the end of its range and back in theX and then the Y direction, which will take about 1minute. You could also find home in the Z direction,which takes longer.

Initial Setup and Focusing

Drive the stage to the Faraday Cup by opening theCommand tab in Stage Control. Select Faraday Cupon Holder and click Go.

Figure 7: Stage Control window.

6 2 RAITH PROCEDURES

Set Z for your desired working distance with theDestination tab in Stage Control. Select XYZ coor-dinates and Absolute, and enter 30.5 mm for Z andclick Go. The stage will drive up from 18.0 mm to30.5 mm, which happens to be the maximum allowedZ. This sets the working distance to about 6.4 mm.This is what we use for our 30kV exposures.

For a different working distance, drive Z to anothervalue. If needed, fine tune the working distance laterby focusing the SEM on the sample, reading the work-ing distance from the SEM computer, and adjustingZ as necessary.

Turn on the Joystick with the Motor Control win-dow. Set the high speed to 9 and the low speed to 2.Use this window to turn the joystick back on if theRaith software disables it, such as after write fieldalignment. Toggle between high and low speed withthe upper left button on the joystick.

Turn off the lights for easy viewing; it’s time to usethe SEM. Switch to SEM on the Leo computer andun-blank the beam on the Raith computer, if neces-sary. Adjust brightness and contrast to the mid 40’s%. Zoom out until you see the Faraday cup. If theimage is still blank, double-check that the detector isset to SE2 and not in-lens.

Open the SEM Control Panel in the Tools menu,or with Crtl-G. This window is incredibly useful foradjusting the beam and changing the SEM scanningmode; I keep it open all the time. Select the Aperturetab and click Beam Shift (figure 8). Click the “0”where the sliders intersect. This ensures that thereis no SEM beam shift. Note that Fine/Coarse willchange how much the sliders move per click in theSEM Control Panel. Make sure to click Mag/Focusafter changing beam parameters, so that using theLeo mouse later will not change your beam settings.

Adjust focus, aperture alignment, and stigmationon the Faraday cup. This will save time during fo-cusing on your chip. Toggle focus wobble in the SEMControl Panel for aperture alignment. Write downthe aperture and stigmation values; this will help ifyou accidentally change them later, or if the Raithsoftware crashes.

MORE HERE ? ———–By now the beam has been on for a good amount

of time, so we can measure the current. Zoom inside

Figure 8: SEM Control Panel, Aperture tab. To zeroa setting, click the “0” where the X and Y slidersintersect.

the Faraday cup to at least 200 kX. Open the Currentwindow and measure the current a few times, untilyou land on a representative value. Write this downin the Log and your notebook.

Figure 9: Exposure window.

Now setup the exposure parameters. Doing thisstep here will help minimize the time between focus-ing and exposure. In the Microscope Control window,set Magnification to 1 kX and Field Size to 100 µm.Open the Exposure window (figure 9) and click thecheck boxes to enable Lines and Dots if needed.

Click Calculator to open the Exposure ParameterCalculation window (figure 10). Flip through theArea, Line, and Dot tabs. Set the area dose to 250

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Figure 10: Exposure Parameter Calculation window.

µC/cm2, line dose to 1500 pC/cm, and dot dose to(???). Set the step sizes to 0.0064 µm, and click themiddle calculator icons to update the dwell times.Check that the beam speed is less than about 4 mm/sto avoid breaks and other problems. Click Ok.

Now drive to your chip while watching the SEMimage. Follow the sample holder’s structure all theway to the sample clip. If your sample is far awayand you cannot do this, then blank the beam and driveby CCD to a point close to the chip. Then un-blankthe beam, zoom in, and proceed slowly to the chip.

Once you reach the chip, stay near the edge andzoom in to at least 1kX to avoid exposing the chip.Blank the beam when you’re not using it. Travelalong the chip’s edge to find the three corners youwill use for 3-point focus. Save these locations in the3 Points tab of the Adjust UVW window, to savetime later. Afterwards use an XY lightning bolt todrive back to the lower left corner of your chip, andun-blank the beam.

Zoom in on silver paint or an alignment mark andadjust focusing, aperture, and stigmation as best youcan. Write down your new aperture and stigmationvalues when you’re done.

Tips for focusing and adjusting the beam: Openthe SEM Control Panel and play with noise reduc-tion and scan speed. Continuous averaging withscan speed 3 or 4 usually works well. Reducingthe scan window size sometimes helps. Make sureto select Fine control. When making adjustments,hold down the mouse button and slide the mouseslowly from side to side. Find the end-points ofmotion where you know the image is worse, and then

Figure 11: Raith e LiNE joystick with axes.

move to the center. Move slowly to find the sweetspot. Note that the higher the magnification, themore fine your adjustments become. Be careful toswitch back into mag/focus mode before zooming orfocusing, otherwise you will change your aperture orstigmation values. Also be careful about accidentallyswitching between Fine and Coarse adjustment.

Coordinate System Setup

Before you can write your pattern, you need to setupthe Global UVW coordinate system for your chip.

Open the Adjust UVW window. Make sure that“–> Local” is displayed in the lower left of the win-dow, otherwise toggle the button until it is. If youdon’t see “Focus correction!” in red at the bottom ofthe window, open Options in the Edit Menu, enableautomatic focus correction, and click the “correct byworking distance” radio button.

For a blank chip, the coordinates setup isn’t veryprecise. Turn on crosshairs in the SEM, and positionthe lower left corner of your chip in the center. Open

8 2 RAITH PROCEDURES

Figure 12: Angle Correction tab of the Adjust UVWwindow. Note that “–> Local” and “Focus correc-tion!” are displayed in the lower left corner.

the Origin Correction tab in the Adjust UVW win-dow. Click Adjust to set the current location as theorigin. Move to the Angle Correction tab (figure 12).Position the lower edge of your chip in the crosshairs,and click Read in the first line. Drive to the lowerright corner of the chip. (If you saved that locationin the 3 Points tab, then click the lightning bolt nextto the XY coordinates for the corner. Un-blank thebeam after driving.) Position the lower edge of thechip in the crosshairs again, with the same magnifi-cation. Click Read in the second line, and then clickAdjust. The UV coordinate system is now rotatedsuch that the U axis points to the right along thebottom edge of your chip. Double-check that the ori-gin is still correct by clicking the Origin Correctionlightning bolt, and then re-adjusting as necessary.

For a patterned chip, focus on an alignment markas best you can. Zoom in and center it in thecrosshairs. Click Adjust under Origin Correction toset this as your origin. In the Angle Correction tab,Read this location into the first line. Drive to an-other alignment mark that you would like to be inthe positive U direction from your origin, say in thelower right corner of the chip. Center the alignmentmark in the crosshairs, and Read this location intothe second line. Press Adjust to update the UV co-ordinate system. Double-check that the origin is stillcorrect by clicking the lightning bolt under OriginCorrection, and re-adjust as necessary.

Now that the UV coordinate system is setup, ttshould be possible to drive to other locations on your

Figure 13: 3-Points tab of the Adjust UVW window.Note the two columns of lightning bolt, one for XYand the other for UV coordinates.

chip, such as alignment marks, using UV coordinatesin the Destination tab of the Stage Control window.

If at any time you drive the stage away from thechip, for example, to the Faraday Cup to re-measurecurrent, make sure that you double-check and correctany origin shift.

QUESTION: Coordinate system scaling errors?

Focus Correction

Good focus is critical for small features and repro-ducible exposures. To achieve good focus, we use3-point focus correction. This method yields threeresults: (1) The working distance will be set to thePMMA surface to within 1 µm. (2) You will see thesize and shape of the beam before you expose, whichwill let you know when to stop focusing and help toverify consistency between sessions. (3) Finally, youwill reliably correct for the tilt of your chip surface,so that the focus is correct anywhere you write onthe chip.

If instead we focused on silver paint or on align-ment marks 200 nm below the PMMA surface, ouraccuracy would be only around 50 µm or worse. Sucha large error would increase the beam spot size toaround 100 nm, and prevent exposure of any smallfeatures! So, let’s begin.

QUESTION: Focusing with mesa-etched sample?Open the 3-Points tab in Align UVW (figure 13).

Double-check that “Focus correction!” is displayed inred at the bottom of the window. Choose three non-

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colinear points for focusing. If this is a blank chip,then three corners with silver paint will suffice. For apatterned sample, choose the center of three distantalignment marks.

During initial setup you may have saved XY coor-dinates for three corners of your chip in the 3-Pointstab. If so, click the lightning bolt next to the XYcoordinates of the first location you want to focusat. This drives the stage to that location. Zoom in,un-blank the beam, and begin focusing. For chip cor-ners, make sure to drive in as far as possible towardsthe middle of the chip to focus, so that you avoid thethicker resist layer near the edge.

If you did not save locations before now, then ei-ther drive to a location with the joystick, or enterapproximate UV coordinates, zoom in, and click thelightning bolt next to the UV coordinates to drivethe stage. In either case, when you reach an areaand start focusing, save the XY coordinates in theAdjust UVW window so that you can come back ata later time.

At the first location, fine tune focus, aperture, andstigmation. Write down the new values for apertureand stigmation. Once you have tuned as much as pos-sible, drive away from any features to a blank PMMApatch and zoom in to around 300 kX. Wait a few sec-onds for the stage to settle, then burn a dot. Thisfirst dot will probably need to burn for 30 second to1.5 minutes to be visible due to poor focus. Onceburned, zoom out to around 150 kX and focus on thedot.

PICTURE of out of focus dot!PICTURE of in focus small dot!The dot is in focus when it looks more like a dough-

nut, with a darker region in the center. Drive thestage away again, zoom in to around 300 kX, andburn another dot for a shorter amount of time. Focuson this dot. It should be smaller and sharper. Zoomin to fine focus more if needed, but beware that look-ing at a dot with high magnification will change thedot over time.

Continue this process of moving and burning dots.You should be able to burn dots in 5 to 10 secondsor less. When the 5 second dot diameter is around20-30 nm, then the working distance is correct forthis location. However, if you have trouble making

the dot smaller, or if the dot is not circular, thenfine-adjust aperture and stigmation on a new seriesof burned dots. If necessary, turn on the crosshairsto help check if the dots are circular. Write down thenew values for stigmation and aperture.

Once satisfied, read the XY coordinates into an en-try of the 3 Points window. Type in the UV coordi-nates exactly as they are in the Coordinates window,otherwise you will modify the UV coordinate system.Click the check box for that line. Write down thecoordinates and working distance (from the Leo com-puter) in your notebook, in case the Raith computercrashes. Repeat this procedure of burning dots attwo other focusing locations, and save their coordi-nates. Once all three entries are full and checked,click Adjust.

Unfortunately, you need to double-check the focuscorrection. Click the lightning bolt next to a UVcoordinate. You should see some of your previouslyburned dots. Burn a new dot, and check focus. Ifeverything is ok, then move to the next focusing lo-cation. Otherwise, uncheck the box for this location,repeat the burning dot procedure, update its entry,and re-check its box. Do this for the other two focus-ing locations. If you made changes, then click Adjustand double-check the focus correction again.

Write Field Alignment

The final step before exposing your chip is to calibratethe beam deflection with stage movement, known aswrite field alignment. Open the Microscope Controlwindow and check that the magnification is set to1000 X and the Field Size is set to 100 µm.

Open the Align Write Field window, which displaysthe alignment parameters and has a Reset buttonin case you need to start the alignment procedureover again. Open the Raith Protocol HTML file, andnavigate to your account, the current date, and writefield alignment. Update this page to keep track ofthe changes you make to the write field parameters,and to check for convergence of the parameters.

Under File select New Positionlist. Open the ScanManager window (figure 14), and expand Align WriteField Procedures and then Manual. Turn on thecrosshairs in the SEM, and position a small detail

10 2 RAITH PROCEDURES

Figure 14: Scan Manager window.

in the crosshairs. Zoom in to double check that thefeature is centered in the crosshairs. Freeze the imageon the SEM.

Drag the Manual ALWF 25 µm marks entry inScan Manager into the positionlist. The position willbe set to the current UV coordinates. Right click andchoose Scan. The Raith will take over the SEM andmake a slow scan. Position your cursor to match thecrosshairs in the slow scan. When the crosshairs turnpurple to indicate alignment, press and hold Ctrl andthen click and drag the cross to the feature you cen-tered in the SEM. Click Continue, and repeat twomore times. The Raith will ask if you want to ac-cept the changes. Click yes if everything went ok.If successful, repeat this alignment to double-checkthat the crosshairs line up in all three slow scans,and make fine adjustments if needed. After you aredone, delete the entry in the positionlist. You willneed to un-blank the beam and switch back to In-ternal to use the SEM. You may need to turn thejoystick back on in Motor Control.

If for some reason the feature in the SEM imagedoes not show up in the slow scans, then right clickthe Manual ALWF 25 µm marks entry in Scan Man-ager and select Duplicate. Right click the duplicateentry and select Properties. Under the Mark Proce-dure tab, reduce the Placement from WF center from30 µm in U & V to a smaller value, like 10 µm. Clickok. Drag and scan this entry into the positionlist,and proceed like normal. If this doesn’t fix the prob-lem, try reducing the Placement again. Once youare done, delete the new entry in the Scan Manager.

Then retry the normal Manual ALWF 25 µm marks,and continue to the next step.

Repeat the alignment procedure for the 5 and 1 µmManual ALWF entries in Scan Manager. Drive theSEM to a smaller feature before dragging these en-tries into the positionlist. Ideally, use burned dots forthe final 1 µm step. In between ALWF steps, doublecheck that the write field alignment parameters areconverging in the Raith Protocol file.

As a final step, use 1 µm Automatic ALWF withImages. (For a patterned sample, you can also use 1µm Automatic Align Write Field with GDS II.)

Double check that the changes to the write fieldparameters were small in the Raith Protocol file.More specifically, the zoom factor should be between0.99975 and 1.00025, corresponds to an acceptablezoom error of 25 nm or less for a 100 µm writefield.The shift error should be just a few nm, and the ro-tation error should be a few 0.001’s of a degree.

Write down the Align Write Field window’s param-eters in your notebook. The values for zoom shouldbe around 0.9197 for U and 0.9156 for V. The valuesfor shift should be in the 10’s of nm or less, and thevalues for rotation should be a few degrees or less offof 0◦ or 90◦.

At this point you can choose to re-measure the cur-rent at the Faraday Cup and update the exposuredwell times. If you do, be sure to double-check theUV origin after returning to the sample, since thelarge stage movement may cause an offset. If neces-sary, fix any origin offset.

Exposure

Now it’s time to expose a pattern. Before writingon a real 2DEG chip, expose a dose test on a junkchip of the same material. Once you demonstratethat your pattern works repeatably for some range ofdose factors, then it’s time to write the real device ona 2DEG chip.

Load the saved positionlist for your first exposure.Make any last minute modifications as needed. Dou-ble check the layer(s), dose factors, working area, UVcoordinates, and exposure parameters. If everythingchecks out, then click Scan All in the Scan menuto expose all entries in the positionlist. The Raith

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will take over from here until the exposure is fin-ished. You can pause or stop the exposure at anytime with the buttons in the pop-up window. Ifyou want to make sure that the Raith is exposing,double-check that the X and Y-Deflection bar-graphson the High Speed Pattern Processor are moving,that the DMM current reading is fluctuating, andthat the Beam light in the Beam Blanking Controlleris slightly flashing. A dose test exposure should takearound 45 minutes, and a Gates layer exposure shouldtake only a few minutes.

If this is your real chip, then you’ve just exposedthe Gates layer. Before exposing the Connector layer,change aperture size to 120 µm. Measure current atthe Faraday Cup, and roughly focus on an alignmentmark. Double-check that the UV origin is correct;the rotation and scaling should still be ok. ALIGNWRITE FIELD AGAIN? Load the positionlist forthe Connector layer, double-check, and expose justlike the Gates layer. NEEDS WORK: I will betterunderstand the Connector layer exposure once I’vedone it.

Unload Sample

Once your exposure is finished turn off EHT on theLeo computer, but leave the gun on! Open the Nav-igator Loadlock window and click Via Loadlock andthen Unload Sample. Follow the pop-up instructions.In about 8 minutes, your chip will be unloaded. Makesure you close the load lock door. Place the sam-ple holder in its protective case on the prep table.Use your carbon tweezers to remove your sample, andclose the sample holder case.

Log off the e LiNE software but leave the SEMsoftware running. Close any miscellaneous windowsyou left open. Turn off both monitors, and finishfilling out the Log.

3 Post-Raith Processing

These next steps need to be performed in immediatesuccession. You can wait before developing, but oncedeveloped, you need to finish processing.

Develop your chip in MIBK:IPA 1:3 for 60 seconds,rinse in IPA for 30 seconds, and gently blow dry withnitrogen. Use your Teflon beaker and labeled plasticbeakers. Do not swirl or sonicate. Set the nitrogenflow rate with the regulator in our hood to a gentlespeed.

If you want, now is a good time for a quick opticalinspection under the microscope.

UV/Ozone Cleaning: Expose the chip in UV for60 seconds in the Samco UV-1 in B4. Please see theQMM UV/Ozone Cleaner instruction sheet for moreinformation. Immediately load in the evaporator af-ter this step.

Evaporate 50 A of titanium at 0.5 A/s followed by150 A of gold at 0.5 A/s. Do degas your sources, withTi last, and wait for the evaporator pressure to dropbefore the real evaporation. Keep and reuse yourown evaporator boats. Please see the QMM SharonVacuum Evaporator instruction sheet for more infor-mation.

Liftoff: Place your chip in your liftoff beaker, andfill with a good amount of acetone. Cover tightlywith aluminum foil and let it sit for over two hours(preferably overnight). After two hours, it may helpto break up the metal layer by spraying your chipwith acetone from the dispenser bottle. Do not letacetone dry on the chip, or it will be ruined. Donot use your Teflon beaker during liftoff, as it maybecome contaminated with metal flakes.

Once you are sure that the extra metal is removed,spray your chip with IPA and gently blow dry withnitrogen.

After Liftoff: Your chip should now have gates. Ifyou exposed a dose test, go image the chip in theSEM, take pictures, and choose appropriate dose fac-tors for your features. If you made a device, then–asJeff says–measure, publish, and graduate.

12 3 POST-RAITH PROCESSING

Quick Outline

3 Solvent Clean the chip for the first EBL layer:

1. Sonicate 5 minutes in TCE

2. Sonicate 5 minutes in Acetone

3. Sonicate 5 minutes in IPA

4. Blow dry with N2

Prebake for 2 minutes at 170◦ C.

Spin on a 200 nm layer of 950k PMMA A4:

1. 5 seconds at 500 rpm, 500 rpm/s spinup.Apply PMMA during this step.

2. 60 seconds at 4000 rpm, 1000 rpm/s spinup.

Bake for 15 minutes at 170◦ C. Apply silver paintto chip corners, if needed.

Raith EBL: Load sample and set Z to 30.5 mm(WD ∼ 6mm). Tune up the Raith. Expose at 30kV, with area dose 250, line dose 1500, and dotdose XXX. Use a 10 µm aperture and 0.0064 µmfor gates, and 120 µm and 20 nm step size forthe connector layer. For gates, focus on burntspots. For the connector layer, just roughly fo-cus on alignment marks. Expose a dose test todetermine appropriate dose factors.

Develop for 60 seconds in 1:3 MIBK:IPA.

Rinse for 30 seconds in IPA.

UV/Ozone Clean for 60 seconds.

Evaporate 50 A of Ti and 150 A of Au at 0.5 A/s.Degas your sources, with Ti last, then wait forthe pressure to recover before evaporating.

Liftoff in acetone for at least 2 hours, preferablyovernight. Spray with IPA and blow dry withN2.

Raith EBL Steps

Load Sample

Initial Setup and Focusing: Turn on joystick.Focus and adjust beam on Faraday Cup. Mea-sure current, set zoom to 1 kX and Field Size to100 µm in Microscope Control, and set exposureparameters. Drive to chip and focus.

Coordinate System Setup: Set Origin and adjustAngle Correction. Double-check origin.

Focus Correction: Perform 3 Point Focus Correc-tion for Gates layer, but just focus on alignmentmarks for Connector layer. Double-check focusand origin.

Write Field Alignment: Perform 25, 5, and 1 µmmanual write field alignment, followed by 1 µmautomatic write field alignment with images.Check convergence in Raith Protocol file.

Exposure: Load positionlist. Double-check layers,working area, write-ordering, UV coordinates,dose factors, and exposure parameters. SelectScan All.

Unload Sample

This column doesn’t look as helpful as the other one(?!?!).

13

RANDOM NOTES:-Don’t know where to put these notes yet.-minimum step size is 0.0016 µm, but this creates

communication problems for the Raith-If you have trouble with the SEM software, say

for example you tried auto stigmation and canceledit, which nukes the working distance, then try clickingthe Emission button in the SEM Control Panel.

-If the SEM image is rotated, Tools ¿ Goto Panel¿ Rotate/Tilt – leave all entries unchecked and set to0.

-To save aperture and stigmation values, use thefirst stop light (Navigator Column) which allows youto store the values