Experiment M43: Electron beam lithographyV-042014

Experiment M43: Electron beam lithography

Room 01.578 (Staudtstr. 7, Building B2, 2nd floor)

Contact supervisor by email: BetreuerV43@physik.uni-erlangen.de

ATTENTION: Send the report on the preparatory work to the supervisor one day in advance (before 14:00) inpdf format.

Bring a USB-Stick with free memory to copy your experimental data.

The experiment aims at fabricating chromium micro structures on a glass sample by electron beam lithographyand wet chemical etching of chromium.Previous knowledge of the handling of the SEM is required (Experiment B43) and is part of the preparation!

1 Preparatory work:

1. Electron beam lithography (e-beam lithography) :

a) Very briefly summarize the necessary components and the working principle of a SEM, and describethe changes which have to be made to use the SEM for electron beam lithography.

b) Recapitulate the aberrations in the SEM and their effect on the resolution of the SEM/e-beamlithography.

c) Briefly compare the thermal electron source and the field emitter concerning their advantages anddisadvantages for e-beam lithography.

d) Describe the problem arising from the use of glass as sample material. How it can be overcome fore-beam lithography?

2. Micro structuring process:

a) Briefly describe the fabrication of micro structures by e-beam lithography and lift-off processing.

b) Briefly describe the fabrication of micro structures by e-beam lithography and etching.

c) Explain the following parameters and their influences on the lithography process:

• Acceleration voltage (see B43)

• Electron dose

• Beam current

d) Explain the proximity effect concerning its origin and its outcome.

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Experiment M43: Electron beam lithographyV-042014

2 Recommended Reading:

[1] B. Bercic,Introduction to electron beam lithography , seewww.optik.uni-erlangen.de/odem/download/v43 electron beam lithography.pdf

[2] M.A. McCord, M.J. Rooks, Chapter 2: E-beam lithography, in: Handbook of Microlithography, Micro-machining and Microfabrication, Volume 1: Microlithography ,ed. P. Rai-Choudry, SPIE, 1997, Chap.2.1-2.4, 2.5.2 and 2.7.1, seewww.optik.uni-erlangen.de/odem/download/v43 ebeamlitho.pdf

[3] JEOL, A Guide to Scanning Electron Microscope Observation, seewww.optik.uni-erlangen.de/odem/download/v43/v43 guideSEM.pdf

[4] D. Drouin, A. R. Couture, D. Joly, X. Tastet, V. Aimez, R. Gauvin, CASINO V2.42-A Fast and Easy-to-use Modeling Tool for Scanning Electron Microscopy and Microanalysis Users, SCANNING Vol. 29,92-101 (2007), seewww.optik.uni-erlangen.de/odem/download/v43 casino08.pdf

3 Experiment: Electron beam lithography

Wear safety goggles and gloves when handling chemicals for development and etching. Use thebasin dedicated to this application.

The sample used in this experiment consists of a glass substrate (thickness 1.5mm) and a 60nm layer ofchromium. The sample is spin coated with the electron beam resist PMMA (A2, thickness 50nm) at 4000rpm.

Default settings for lithography:

• Place the beam blanker in the beam path of the SEM and centre it. Use Ctrl+b to open or close thebeam blanker.

• Select aperture stop 4 (50µm), acceleration voltage Uacc = 20kV and probe current 6 · 10−10A.

• Move sample to z position 15mm and adjust coarse focus to 15mm.

• Select SEM1 Modus (no electronic image shift).

3.1 Simulation by CASINO:

Simulate the electron distribution in the sample for the acceleration voltages Uacc = 1kV , 5kV , 10kV , 20kV ,40kV using CASINO. Compare the following systems:

(a) 50nm PMMA, 60nm Cr, substrat SiO2

(b) 500nm PMMA, 60nm Cr, substrat SiO2

concerning the distribution of the electrons (primary and back scattered) and the absorbed energy. Answer thefollowing questions in your report with the help of these simulations:

• Which is the minimum acceleration voltage to completely penetrate the resist layer in the two cases ?

• Explain how the resist profile after development, the resolution, and the proximity effect are affected byincreasing the acceleration voltage.

• Explain how the resolution changes with increasing the resist layer thickness.

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Experiment M43: Electron beam lithographyV-042014

3.2 Dose test exposure:

To become completely exposed, the electron beam resist has to absorb a certain amount of energy by secondaryelectron interactions. Therefore, a test exposure with varying electron doses is performed. The parameters(Uacc, working distance, write field, sample structure) in this test exposure have to correspond to those of thefinal exposure. Due to the proximity effect, the electron doses for areas, lines (single pixel width), and dots(single pixels) have to be obtained separately.

a) Test pattern design: Use the GDSII-Editor (Elphy Quantum Software → GDSII Database) in the lithog-raphy software to design your pattern for the test exposure. The test pattern should consist of simpleshapes which are repeated with a scaling factor for the electron dose inside the pattern. The shapes shouldbe of the following elements:

• Area or line with line width > 0nm

• Line with line width 0nm

• Dots

Consult the Elphy Quantum manual for creating a design. Only use layer 0. Keep in mind that a largerpattern will result in a longer writing time and a smaller pattern will be harder to find after development.The overall size of the pattern must not be larger than the writing field (see table 1) you are going to use.

b) Preparation for exposure - part 1:

(i) Mount the sample on the sample holder near the alignment sample. Take care not to introduce a tiltto the sample. Create a conductive connection between the chromium layer and the sample holder(and therefore the earth potential) with the help of copper tape. Load the sample into the system.Make sure the sample will not be exposed to the electron beam once the beam is turned on (e.g.drive to the LithoLoad position). Switch on the SE-detector, turn on the beam, and open the beamblanker.

(ii) Select the desired write field in Elphy Quantum Software →Microscope Control (see table 1). Confirmwith SET.

(iii) Align the SEM in order to achieve a proper imaging of the surface and drive to the alignment sample.Align the SEM to achieve a high quality imaging of the alignment pattern.

(iv) Do the U-V adjustment (see Sec. 4.2) at the alignment pattern and perform a write field alignmentfor the selected write field size (see sec. 4.3). Note down the correction values for zoom, rotation,and shift.

Answer the following questions in your report:

• Why and when is the write field alignment necessary?

• What are the correction values you got and what is their meaning?

c) Preparation for exposure - part 2: During driving, take care not to expose the sample to the electron beammore than necessary. If possible, blank the beam during driving1.

(i) Drive to the lower left corner of the sample. Align the SEM to obtain a high quality imaging of thesample surface (!). Do the U-V adjustment (see Sec. 4.2) at the sample using the lower left and thelower right corner of the sample.

(ii) Drive to the U-V origin and align the SEM to obtain a high quality imaging of the sample surface.

(iii) Drive near to the position where the exposure will be done (approx. two write field sizes away)by entering the U-V coordinates in Elphy Quantum Software → Stage-Control → Destination. Thesoftware drive command will automatically blank the beam. Align the SEM to obtain a high qualityimaging of the sample surface again. Verify the quality of your alignment by generating a contamina-tion spot (see Sec. 4.4). The alignment should be improved until the spot is circular with a diametersmaller than 150nm.

1REMEMBER: Don’t drink and drive!

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Experiment M43: Electron beam lithographyV-042014

(iv) Drive to the gap between inner and outer sample holder. Do not realign the SEM focus orastigmatism there ! Measure the probe current and note it down.

(v) Switch the SEM control to extern and change the magnification to the respective magnification foryour selected write field size. Blank the beam.

Answer the following questions in your report:

• Why is the U-V adjustment necessary?

• What beam current did you measure?

• Why do you have to align the SEM near the exposure position? Present a picture of the finalcontamination spot.

d) Set up the exposure:

(i) Open a position list to set up the exposure script (Elphy Quantum Software → File → New Posi-tionlist). Drag&Drop the GDSII structure in the position list. With Mouse-r → Properties openthe properties dialogue and enter the exposure parameters. Select layer 0 as Exposed Layer. TheWorking Area size must not be larger than the write field size. Change the Position to the desiredU-V coordinates. The dose values have to be changed in the dialogue window Exposure Parameters→ Calculator. Enter the measured probe current and the following starting values:

• Area dose 100µC/cm2

• Line dose 1200pC/cm

• Dot dose 0.2pC

Use the calculator button behind the respective Dwell Time to calculate the dwell time according tothe displayed formula. Note down the U-V position of your test exposure.

e) Exposure, development and characterization:

(i) Make sure that you have selected the correct magnification and that the SEM is switched to externalcontrol (extern).

(ii) Start exposure by pressing the button Scan Selection. Attention: Do neither touch the SEMnor the software during exposure.

(iii) After exposure (exposure window closes,) unload the sample (see Sec. 4.1). Mark one corner byscratching before unmounting the sample to ensure proper realignment of the sample.

(iv) Develop the sample for 60s in a 1 : 3 mixture of MIBK and isopropyl alcohol and rinse afterwardsfor 60s in pure isopropyl alcohol.

(v) Remount the sample and load it into the SEM. Do the U-V adjustment (see Sec. 4.2) similar to theU-V adjustment before the exposure by using the same corners.

(vi) Drive to the test pattern by entering the U-V coordinates of your test exposure in Elphy QuantumSoftware → Stage-Control → Destination.

(vii) Obtain images of the different features in your test exposure. Determine the correct dose factors.Compare the structures sizes.Hint: If you are unable to find the pattern with Uacc = 20kV , reduce the acceleration voltage toUacc = 10kV . The write field alignment for Uacc = 20kV will not be valid for Uacc = 10kV . Therefore,the length scales will not be correct. You have to do a new write field alignment at Uacc = 10kV touse the measuring tools in the software. Do not overwrite your old alignment before noting down theold correction parameters. Otherwise, you will have to do a new write field alignment at Uacc = 20kVbefore the next exposure step. You can see and edit the correction parameters in the window ElphyQuantum Software → Write Field.

In your report, describe the results of the test exposure of the different shapes. Answer the followingquestions:

• Which dose factors or which dose will you need for the exposure of dots, single pixel lines, and areas?

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Experiment M43: Electron beam lithographyV-042014

• Show examples of the proximity effect by comparing the size and the form in the design and in theexposed pattern for different shapes.

3.3 Final exposure:

In this section, you will design one grating obligatorily and other patterns voluntarily. These designs will beexposed with the determined dose and etched into the chromium layer. With the help of a laser, the grating’sfeature sizes will be verified.

a) Design a grating in the GDSII editor. The geometry, the period, and the line width can be chosen freely.The smallest feature size must not be smaller than 2µm. At least 10 periods must be in the write field.Note down the period and the line width of your grating. You are free to design additional other patterns,but keep in mind that the writing time increases with the exposed area.

b) Repeat the Sec. 3.2 c) to prepare the exposure.

c) Exposure: Drag&Drop the GDSII structures in the position list and enter all necessary parameters (Sec.3.2 d)) for each structure. Calculate the required dose by multiplying the dose factor with the dose fromSec. 3.2 d) and use it as exposure dose. Perform the exposure and unload the sample. Develop and rinsethe sample (Sec. 3.2 e) (iv)).

d) Transfer the resist pattern into the chromium layer by etching the resist free chromium: Place the samplefor 60s in the chromium etch and rinse for 30s in H2O afterwards.

e) Reload the sample and acquire SEM pictures of the fabricated structure. Measure the feature sizes.

f) Unload the sample and mount it on the x-y stage in front of the laser (λ = 633nm). Do not look directlyinto the laser beam. Illuminate the grating with the laser beam. Measure the separation of severaldiffraction orders (e.g. +1&−1, +1&0, 0&−1 and +3&−3, +3&0, 0&−3).

Describe the fabrication process and show your results in the report. Answer the following questions:

• How big is the deviation between the designed pattern and the feature sizes measured in the SEM afterfabrication?

• Calculate the diffraction angles of the grating for the different diffraction orders. Which errors can occurduring the measurement as it is carried out in the experiment and how big is their influence?

• Calculate the grating period from the diffraction angle and compare it to the period measured in the SEMand the one in the design. Take into account the measurement errors. How well was the reproduction ofthe designed period?

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4 Basic Operation Instructions:

4.1 Unloading the sample:

ATTENTION: Loading and unloading of the samples must be undertaken only in the presenceof the supervisor. Do not tilt the sample.

1. Move sample to home position (z = 39mm, [x, y] = [LithoLoad ]) :

a) Lower sample stage with the help of the z-control to z = 39mm.

b) Drive stage to LithoLoad -Position:Elphy Quantum Software → Stage-Control → Command → LithoLoad → Go.

2. Decrease filament heating slowly (!) to 0µA.

3. Turn of acceleration voltage (red button).

4. Switch off SE detector (switch position off) and the SE-collector (switch position off) .

4.2 U-V Adjustment in the Elphy Quantum Software:

The motorized sample stage inside the SEM defines the global coordinate system which is named the X-Y coordinate system. Typically, the sample is mounted with an arbitrary rotation and position to the globalcoordinate system leading to troubles if a certain position should be found after reloading the sample. Therefore,a local coordinate system (U and V axis) is assigned to the sample. The coordinate system is defined by itsorigin and the angle between the U -axis and the X -axis. To determine the angle between the U -axis and theX -axis, two distinctive points on the U -axis are selected. Afterwards, one distinctive point is selected as origin:

1. Elphy Quantum Software → Adjust U-V → Angle adjustment

2. If you perform the first U-V adjustment on the sample, find a distinctive point on the sample. Otherwise,find the distinctive point you selected the first time as point 1.

3. Choose the appropriate magnification and acquire a single image by the Elphy Quantum Software.

4. Read X-Y coordinates of point 1 by selecting flag 1 and positioning it in the SEM image at point 1.X/mm and Y/mm now show the X-Y coordinates of point 1 (position of flag 1 although the flag will not bedepicted in the image).

5. Drive to a distinctive second point (or point 2 ), choose the same magnification as at point 1, and acquirea single image by the Elphy Quantum Software.

6. Read X-Y coordinates of point 2 by selecting flag 2 and positioning it in the SEM image at point 2.X/mm and Y/mm show now the X-Y coordinates of point 2 (position of flag 2 although the flag will not bedepicted in the image).

7. Press the button Adjust. The calculated angle should now be green. The calculated angle is the anglebetween the U -axis and the X -axis where the U -axis is defined by point 1 and point 2.

8. Drive back to point 1 by using the Flash-button behind the coordinates of point 1 and acquire a singleimage.

9. If the image does not show point 1, the steps 2 to 8 have to be repeated.

10. Elphy Quantum Software → Adjust U-V → Origin Correction.

11. Drive to point 1 and press the button Adjust to select point 1 as origin.

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Experiment M43: Electron beam lithographyV-042014

Figure 1: Single alignment structure for write field size 100µm (left side) and placement of available alignmentstructures inside alignment pattern (right).

4.3 Write field (WF) alignment in the Elphy Quantum Software:

The write field is the area on the sample being scanned by the electron beam during lithography or imageacquisition. It is determined by the deflection angle of the scan coils and the height of the sample surface. Inorder to achieve correct length scales on the sample surface, the write field definition in the software has to bealigned to an existing structure of the desired size. This is done by scanning several points in the alignmentstructure with known distance. A high accuracy can be achieved by doing this iteratively.

1. Elphy Quantum Software → Microscope Control → select desired write field size (see table 1) → pressbutton SET.

2. Select SEM1 at the SEM to deactivate the Image-Shift.

3. Drive to the sample with the alignment pattern and adjust the U-V coordinate system to the alignmentpattern (see Sec. 4.2).

4. Adjust magnification at the SEM to the respective one (see table 1).

5. Drive to the respective alignment structure in the pattern, centre it, and acquire an image with thesoftware.

6. Elphy Quantum Software → File → New Positionlist to open a new position list.

7. Elphy Quantum Software → Scan Manager → Align Write Field Proc. → Manual → select respectivealignment script. The scripts are sorted by the size of the scan area for a single mark (Scan) and theoverall write field size (field). For the first iteration, select a large scan area.

8. By double clicking the desired script. the parameters can be checked. Mark Proc. → enter/checkplacement of the marks (see table 1) and select all four marks. Bear in mind the mark sequence depictedhere. Press button ok to accept the changes before changing to another tab!

9. Drag&Drop the script into the position list. The centre of the image will be the centre of the write fieldto be aligned.

10. Press button Scan Selection to start the interactive (!) write field alignment script.

Microscope Control Write field Mark placement Magn. WD

400x100um,14mm 100µm 37µm 400× 14mm200x200um,14mm 200µm 74µm 200× 14mm130x300um,14mm 300µm 111µm 130× 14mm80x500um,14mm 500µm 186µm 80× 14mm40x1000um,14mm 1000µm 375µm 40× 14mm

Table 1: Possible write field sizes and the required magnifications at a working distance of 14mm.

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a) An overview image is acquired.

b) A small image at the presumed position of the first mark is acquired and depicted together witha green cross-hair. By pressing Ctrl-left and Mouse-left together, the cross-hair can be moved.The cross-hair changes its colour to blue after it is moved to indicate that the software accepts thispoint as valid data. Only valid data will be taken into account for the write field alignment. Positionthe cross-hair in the center of the alignment mark 1 (cross). Press button Continue to continue whenthe cross is blue and positioned correctly.

c) All other marks have to be processed likewise.

d) At the end of the procedure, the write field correction values (Zoom/Shift/Rot.) are displayed andhave to be accepted.

If the marks are not inside the scan area, either increase the size of the scan area (size+placement shouldnot exceed write field size) or move the cross-hair in the direction where the mark should be. Repeat theprocedure until all marks are centred in their respective scan areas. To increase the accuracy of the writefield alignment, the procedure has to be repeated several times with decreasing size of the mark scan area.

11. After finishing the write field alignment, the correction values have to be saved (Note them also in yourlab book): Elphy Quantum Software → Microscope Control → Save.

Changing the working distance or the acceleration voltage inevitably leads to a change in the write field size.In this case, the write field alignment has to be redone for the new working distance/acceleration voltage ifthe correct write field size is required under the new conditions. If the magnification is changed at the SEM,the write field size is scaled with the magnification change. This change will not be taken into account by thesoftware and has to be applied to length measurements afterwards manually.

4.4 Contamination spot:

Contamination spots are useful tools for the alignment of focus and astigmatism. A contamination spot is anartificial punctiform contamination on the surface of the sample. It is generated by fixing the electron-beamto a certain position for a certain amount of time. The shape of the spot corresponds to the electron densitydistribution in the beam. The imaging quality of the electron source can be inspected by contamination spots.Spots can be generated only if the imaging of the sources is already good.

1. Adjust focus and astigmatism to achieve a good SEM image.

2. Select maximum magnification (300000x ) at the SEM.

3. Press button SPOT (left to the magnification knob): The scanning of the beam is deactivated and thereforethe screen gets blank.

4. Monitor the probe current. During growing a contamination spot, the probe current must decrease. Ifthe probe current decreases only slowly (or not at all), the focus/astigmatism adjustment are not goodenough to grow a contamination spot.

5. Keep the electron beam at least 30s-60s in the fixed mode and do not shake the electron column or changeanything at the SEM during this time.

6. Press button PIC (left to the magnification knob) to return to the normal scanning mode, and reduce themagnification to 80000x. A bright dot should be visible now. If it is not visible, adjust brightness andcontrast, or try acquiring an image with the software. Also, the focus/astigmatism adjustment might notbe good enough to grow/observe the contamination spot.

7. Improve the focus and astigmatism adjustment at this spot. Grow another one to verify the improvement.

The SEM used here can achieve spots down to a diameter of 100nm - 150nm.

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4.5 Short software manual CASINO

Menu CASINO

4.5.1 Create a simulation:

(I) Setting up the Micoscope : Open the dialogue Microscope and

Simulation Properties by pressing the menu button Microscope

Setup (1):

(a) Energy of the primary electrons.

(b) Select if multiple energies should be simulated in one run.Enter energy step size Step and maximum energy End.

(c) Number of primary electrons. The larger the number, themore significant the results. Attention: A larger number ofelectrons also requires more memory and longer simulationtime.

(II) Creating the sample: Open dialogue Edit Layers by pressingthe menu button Sample Definition (2):

(a) Press to add a new layer. Add all necessary layers includingthe substrate before proceeding.

(b) By double clicking the name, the chemical composition canbe changed (see point III).

(c) To change the layer thickness, double click the thicknessvalue. The thickness is entered in [nm]. Attention: Thelowest layer should stay Substrate.

(d) Both checkboxes Use Substrate and Multi-Layer should beselected.

(III) Chemical composition: The chemical composition can be changed directly by entering the chemi-cal elements (including valence number) into the field Composition (e + f). Attention: User Defined

Density and User Defined Distribution (g) must not be checked in this case!The composition can also be changed by selecting a material from the database (h). Enter the Name (e.g.

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PMMA) and the chemical composition will be taken from the database. Here, User Defined Density

and User Defined Distribution (i) have to be checked.

4.5.2 Start simulation: To start simulation, press menu icon Begin Simulation (3). The window title showsthe progress of the simulation in percentage during simulation.

4.5.3 Show data: The left panel will show a folder with the simulated data for each energy once the simulationis finished. You can choose from several data representations:

Main folder: Paths of the primaryelectrons with colour coded energy loss.

Main folder: Paths of the primaryelectrons with colour coded energy lossand all collisions. BSE are depicted red.

Distribution → ZMax: Penetration depth ofthe primary electrons

Distribution → Energy by Position:Distribution of the absorbed energy in

the sample.

The menu icon Display Options (4) opens the properties dialogue for thedisplay of the electron paths:

a) Colour coding the energy loss.

b) Show all collisions.

c) Show all electrons leaving the sample (BSE) in red.

The menu icon Distribution Display Options (6) opens the properties dialogue for the histogram.The menu icon Automatic Rescale (5) resets the displayed area.

4.5.4 Save results: Use the menu icon Save Display (7) to save the various displayed distributions as im-ages. Check that the image was saved correctly. The simple image manipulation program PaintShopPro(psp.exe) is installed on the computer.

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