Raith e_line Electron Beam Lithography

Transcription

1 Raith e_line Electron Beam Lithography Standard Operating Procedure 1 (For an un-patterned sample) Revision: 7.0 Last Updated: Feb.18/2015, Revised by Mohamad Rezaei Overview This document will provide a detailed operation procedure of the Electron Beam Lithography system. Formal Training is required for all users prior to using the system. Revision History # Revised by: Date Modification 4 Nathanael Sieb 02/18/09 Updated format, modified contact info, and added more procedure details 5 Nathanael Sieb 01/18/10 Update for Nathanael Sieb 09/20/11 Added 3-point focus alignment 7 Mohamad Rezaei 02/18/15 Updated with loadlock and new software info. Document No. 4DSOP000X 1

2 Table of Contents General Information Clean Room Supplies Prerequisites Designing the Patterns Spin Coating PMMA... 5 Operation Procedure Start the system Copy GDS File to Raith Lithography Monitor Load the Sample Start the Column Optimize the Column Settings Set-up the Coordinate System Origin Correction Angle Correction Three-point focus alignment (optional) Write Field Alignment Locating a Mark or Particle Alignment Procedure Exposure Parameters Measuring Beam Current Setting Exposure Parameters Height Control (optional) Adjusting focus and working distance (Adjust W) Setting up Height Control datasets

3 9.3 Monitoring CCD peak detection Controlling and correcting the sample height using Process Mode Using Focus correction during Positionlist scan Exposure Unload the Sample Exit the System Reference and Files Contact Information

4 General Information Raith e_line (Version 6.0) is an ultra-high resolution electron beam lithography tool. The system is designed with thermal field emission (TFE) Schottky source, a crossover-free beam path that provides extremely high beam current density. The column voltage varies from 100 ev 30 kev. The stage is laser interferometer-controlled with travel range of 100 x 100 mm. The stage movement has a precision of 2 nm XY position at any working distance, writefield size and SEM magnification. Users can define the write field from 0.5 µm to 2.0 mm with automated calibration and selection. There are six apertures on the system: 7.5, 10, 20, 30, 60, and 120 µm. The electron beam current is controllable by choosing the appropriate aperture. 1. Clean Room Supplies 1. Electron beam resist: 950K PMMA 5%, 3%, and 2% (in Anisole); ma-n Developer: MIBK:IPA = 1:3, AZ Developer Diluted 1:1 3. Laurence Spin coater: Spin speed up to 4000 rpm 4. Digital Hot Plate: 180 C (for PMMA) 2. Prerequisites We ask users to be experienced in use of SEM before using the e_line system. If you never used SEM before, you should receive the SEM training and be completely able to use it before you plan to use the e_line system. Formal training is required to use the system. Before you receive training, you need to contact the acting clean room manager or a staff member to complete an application form. 4

5 3. Designing the Patterns Raith uses GDS files in the exposure system. If you don t have GDS software in your computer, you can use Design CAD or other drafting software to draw your own patterns. But later on, you need to convert it into a GDS file by the following steps: From Design CAD, export your design as a DXF file Using Link CAD (installed on the design computer) convert to GDS file Save the GDS file on a USB flash drive for transfer to the Raith computer. 4. Spin Coating PMMA Prior to lithographically writing your sample, you must spin coat a layer of PMMA on your sample (or substrate). The layer thickness depends on what patterns you want to make. For instance, patterns made via lift-off will require thicker layers of PMMA than chemically etched patterns. Some spin speed curves for PMMA are shown in the figure below. The negative resist ma-n 2403 has different spin curves. 5

6 Operation Procedure The Raith Lithography monitor is on the Right and the Column monitor is on the Left. The joystick on the far left is used to move the stage. The following operation steps are for writing a pattern on an un-patterned sample. For an overlay operation procedure, please use Raith e_line Standard Operation Procedure 2 1. Start the system Start the column software by double clicking Raith EO (on the Column Monitor) and log in as user 4dlabs and password 4duser. Also, ensure that the control program RemCom32 is running. Start the Raith lithography software by double clicking e_line (on the Lithography Monitor) and log in with your own account (after training, you will get your account). 2. Copy GDS File to Raith Lithography Monitor Insert the USB flash drive with your GDS file into the Lithography monitor (the USB cable is on the computer desk, located between 2 monitors). 6

7 You may copy the pattern into the folder C:\e_LiNE\User\*username*\GDSII if you wish. 3. Load the Sample Carefully remove the sample holder from the desiccator. Sample holder Use soft plastic tweezers around the sample holder. Metal tweezers are not allowed since they may damage the sample holder. Put your sample on the sample holder, and then fasten the clip. If your sample has no marks to identify the U, V coordinate system, make a scratch in the left corner of the sample as indicated. Sample Left Corner Clips Faraday Cup Use nitrogen to blow off the front and back side of the sample holder, in order to remove any dust. 7

8 Open the load lock door and set the sample holder on the mounting rods of the load lock door and carefully close it, sealing the door shut using the thumb screw. 8

9 In order to begin the loading process, enable the Raith window. Click on the Load Lock icon followed by the Load Sample button in the right hand vertical panel. By default, the following prompt message will be displayed on the PC monitor: Press OK The Load procedure will be carried out step-by-step and the progress bar will indicate the Activation Progress. 9

10 The button indicates the currently active step, the button indicates a completed step. You can stop the Load procedure at any time by pressing the STOP button. The software asks the following question: Press Yes to proceed The following window message will be displayed 10

11 Make sure the load lock door is closed then press OK After a while the following message will be displayed on the PC monitor: Open the load lock valve: o Lift the load lock valve locking pin o Turn the locking lever from Locked to Open and pull out to open the loadlock valve Insert the transfer rod into the stage chamber. WARNING: Make sure the load lock valve is open otherwise the transfer rod would be bent by pressure! Press OK then the stage will lift sample holder off the transfer rod 11

12 After a few minutes the following message will be displayed on the PC monitor: Remove the transfer rod from the chamber Close the load lock valve: o Lift the load lock valve locking pin o Push in the locking lever, turn it clock-wise until it has snapped into place Press OK During the loading process, a number of pop-ups will prompt the user for action. Press Yes, enter a sample name of your choice, press OK, press NO, and press OK 12

13 At this stage, the sample holder will be visible in the SEM window If high-resolution patterns (<100 nm) are required, then turn off the mini-environment fan. 4. Start the Column After your sample is loaded, click Stage Control monitor). on the right side menu bar (using Lithography In the Stage Control window, select the Drive tab and select. Enter 26 mm into the Z coordinate and press Start to drive the stage to the required height. 13

14 In order to apply the acceleration voltage (EHT), select Column Control on the right side menu bar. Select the aperture size and EHT by clicking the in front of the desired data set. (Raith has already saved several different sets of apertures and accelerating voltages for you to choose) Then press to activate the selected mode. NOTES: (1) The acceleration Voltage (EHT) is selectable: KV. For greater than 20 KV, you need to select the SE2 detector. Otherwise use the InLens detector. (2) To select the detectors, click Detector on the SEM Control panel (Column monitor), and then choose the one want to use. (3) There are 6 apertures that can be chosen in this system: 7.5, 10, 20, 30, 60, 120 µm. If you are not happy with the apertures available on the Column Control window, you can choose a different size of aperture. To do so, click Aperture on the SEM Control panel (Column monitor), and then select the one you want from the Aperture Size list. 14

15 (4) The EHT can be changed by double clicking on the EHT value on the bottom information bar in the Raith EO software (Column monitor). Enter your desired accelerating voltage in the EHT Target window. 5. Optimize the Column Settings If you already know the clip position which your sample is in, then on the Stage Control window go to the Positions tab. Select the clip which your sample is in and then click Go. The system will automatically drive to your sample position. If you do not know the position of the clips or of your sample, then use the joystick to find your sample and save the position for your future use. Turn on the beam by clicking Beam On/Off on the Lithography monitor. Find the left corner of the sample where you previously made a scratch. In the Raith EO software, use the following buttons on the top menu bar to obtain good focus and stigmation on the sample surface: magnification/focus, stigmation, and bright- 15

16 ness/contrast. The primary function (e.g., magnification) is activated with the left mouse button and the secondary function (e.g., focus) is activated with the right mouse button. The parameters are changed by holding the mouse button down and moving the mouse. Find a small particle on the surface and obtain good focus and stigmation at a higher magnification (~100 kx). In the SEM Control window, click on Aperture Align, and mark the Focus Wobble. Move the mouse in the X and Y directions to minimize the image shift during the wobble sequence. Turn off the focus wobble and recheck focus and stigmation Move to an unexposed region of your sample using the Step tab in the Stage Control window. To create a contamination dot, select Spot Short in the Raith EO software. This function will create a 5 s long exposure. You must quickly turn the beam on by selecting Beam On/Off in the e_line software. Alternatively, a longer exposure can be achieved by selecting Spot Long (middle mouse button). This function will expose the sample until Long Spot is selected again. A good dot should be circular with a diameter of less than 20 nm at 10 kv, 30 µm apertures. An example is shown in the image below. 16

17 6. Set-up the Coordinate System These steps create a UV coordinate system for the sample. 6.1 Origin Correction Use joystick move to the left corner of the sample, and zoom to ~100 x. Select Adjustments on the right side menu bar. In the Adjust UVW (Global) window, click on the tab Origin Correction. Ensure, that it is on Global; if it is on Local, click on the button once to change it to Global (left bottom corner of Adjust UVW window). Select Adjust. The lower left corner is now defined as the origin of this UV coordinate. 17

18 6.2 Angle Correction In the same Adjust UVW (Global) window, click on the tab Angle Correction, and then click on Read (Label 1) for Label 1. The actual XY coordinates are displayed. Switch back to low magnification and move to the lower right corner of the sample. Zoom to 600 x and click on Read at for Label 2. Select Adjust to calculate the transformation angle between XY and UV coordinate systems. (NOTE: the computer assumes the +U direction is along the vector from point 1 to point 2.) 6.3 Three-point focus alignment (optional) In the Adjust UVW (Global) window, go to the menu and Options and select Enable automatic focus correction and Correct Focus by working distance. Click Ok. In the Adjust UVW (Global) window, click on the tab Adjust W, then click on Read, make sure sample is focused, and click Ok. Click Adjust. W will now be updated to the actual working distance. In the Adjust UVW (Global) window, click on the tab 3-Points. In the lower-left corner of the sample, focus on a particle. Click on Read, beside P1. Type in the exact current UV coordinates. Activate the checkbox next to P1. In the lower-right corner of the sample, focus on a particle. Click on Read, beside P2. Type in the exact current UV coordinates. Activate the checkbox next to P2. 18

19 In the upper-right corner of the sample, focus on a particle. Click on Read, beside P3. Type in the exact current UV coordinates. Activate the checkbox next to P3. Select Adjust to calculate the focus plane. Test the alignment by using the lightning icon next to UV in P1, P2, and P3 to drive to those locations. Check that the focus is preserved. You can also create a stage position entry in the position list to test. Note, use the UV template, NOT the UVW template. 7. Write Field Alignment Now the beam needs to be aligned to the UV coordinate system: 7.1 Locating a Mark or Particle Move the stage back to the lower left corner of the sample. You can click the lightning icon beside Label 1 in the Adjust UVW (Global) to quickly do this. Locate a small reference particle with the aid of the crosshairs on the Column PC display at ~1000x. Select Microscope Control on the Lithography monitor. In the Microscope Control window select the appropriate magnification and write field size (e.g., 1000 x, 100 µm) and then press to activate. Open the File menu and select New positionlist. Open the File menu and select New image. 19

20 Scan the image one time on the lithography computer. 7.2 Alignment Procedure Select Automation on the right side menu bar. In the Scan Manager window, double click Writefield Alignment Procedures and then double click Manual. Drag-and-drop 100 um WF - Manual ALWF 25 µm marks into the Positionlist. Select the Positionlist item, and then press Scan. While holding the Ctrl key down, use the left mouse button to select and move the cross to the required position. Once you have reached the new position, release the Ctrl button and a blue cross will be displayed at the selected position. The crosshairs should be placed in the position corresponding to the center of the image on the SEM computer. Click on Continue, and the stage will move into the next corner for you to perform the same mark alignment. Move the crosshairs to the same point that was used in the first scan. Click Continue and perform the cross alignment once more. Then the software will ask you if you want to accept the changes, click Accept. Repeat the align write field procedure with a scan size of 5 um, then again with 1 um. 20

21 Repeat the 1 µm align write field procedure until the Factor zoom correction values are between to Check this by clicking on the Raith Protocol Tool icon at the bottom of the lithography monitor s screen and clicking on the WF Alignments tab to check the results. If desired, the automatic alignment procedures are found under Scan Manager Automatic with Images. NOTES: (1) A writefield shift > 10 µm likely indicates a problem with the writefield alignment. You may want to start over and possibly seek assistance. (2) After the alignment, do not scan an image on the lithography monitor at a magnification other than 1000 x; doing so will reset the writefield alignment. 8. Exposure Parameters 8.1 Measuring Beam Current Before you start setting the exposure parameters, you need measure the beam current first. In the Positions tab, select Faraday Cup and click on Go. Turn on the beam by clicking Beam On/Off on the Lithography monitor. Use the crosshairs to center the Faraday cup on the Column monitor. Be careful to not disturb the beam focus, alignment or stigmation settings. Zoom in until the entire screen is black (within the cup). Click on Exposure on the Lithography monitor. In the Beam Current window ensure <current position> is selected and press Measure. The beam will switch to a small spot completely con- 21

22 fined within the Faraday cup and the picoammeter will measure the current. Perform this step a couple of times to ensure that the current is fairly stable. 8.2 Setting Exposure Parameters Open a new positionlist. Open the Design window and then open your database. Drag and drop your design into the positionlist. Right-mouse-click the position and select Properties from the context menu Click on the Layer icon Click Working Area and select the layers to be exposed. and select Complete Pattern (or another required working area). Enter the correct position for the pattern to be written at. Remember that the position entered here will correspond to the center of the first write field. Open the Patterning Parameter and then press the Calculator button to set up step sizes and dwell times according to the beam current and the required clearing dose. 22

23 Calculate the Dwell Time for each index tab displayed (Area, Curved Elements, Line, and Dot). For reasonable results, the beam speed should not exceed 10 mm per second. If it does, increase the Step Size and recalculate. NOTE: If a different dose is required (e.g., for non-pmma resists or other accelerating voltages), then remember to enter those values first. 9. Height Control (optional) This section describes how to control the height of a sample surface via the Height Sensing software module in combination with a Laser Height Sensing tool (LHS) in order to keep the electron beam in focus. The Height Sensing software module consists of the following main dialogs: o Height Control to choose and activate a proper height control method for your sample or application. o CCD Control for monitoring and adjusting the CCD peak detection as well as for calibrating the CCD camera. 9.1 Adjusting focus and working distance (Adjust W) As a precondition for the reliable use of the height control functions of the LHS, the working distance measured by the electron and the working distance calculated by the software must be synchronized (Adjust W procedure). Focus to any point on the sample. In the Adjust UVW (Global) window, click on the tab Adjust W, then click on Read, make sure sample is focused, and click Ok. Click Adjust. W will now be updated to the actual working distance. 23

24 9.2 Setting up Height Control datasets A height control dataset serves to store and recall all height control settings for a sample or special application. Click on Height Control icon Click on to set up a new height control dataset for the current sample or application. The New Sample dialog will be opened. Type in a name for the dataset by using a significant name for your sample or application. The height control dataset is stored with the current settings of the Height Control dialog. For use and editing, it can be selected of the Sample: combo box. Any change of the settings can be added to the dataset by clicking on. 9.3 Monitoring CCD peak detection For the use of the LHS it is presumed that the CCD peak detection works reliable. Therefore, it is possible to monitor the peak detection. To monitor the peak detection, the laser light distribution can be read out periodically and will be displayed in the CCD Control dialog graphically. By default, the CCD Control dialog will be displayed as minimized window in the desktop area of the eline software in lithography monitor. 24

25 Proceed as follows to monitor and evaluate the CCD peak detection: Maximize the CCD Control dialog. Click on to activate the continuous CCD polling. The polling will be performed at pre-defined intervals e.g. every 3.0 s. You can define the polling interval via Height Control dialog Height Control tab Polling rate. The CCD peak detection will be displayed as a frequency function of the laser light distribution: 25

26 The CCD peak is a calculated value and represents the current position of the sample on the X-axis. The frequency distribution of the laser light distribution indicates all reflected light signals of the laser beam. It should be as thin as possible. 9.4 Controlling and correcting the sample height using Process Mode In the Focus Correction mode, the working distance will be corrected during a patterning process or Positionlist scan after the stage was moved from one position or one stitch field to another one. At a single-field patterning this correction will be executed after driving to the destination directly before starting the patterning. At a multi-field patterning this correction will be executed in front of every stitch field. When using the Focus Correction mode the working distance has to be in the range of W = 9 mm to W = 11 mm. You can one of the below (9.4.1 or 9.4.2) methods to control and correct the sample height Using Focus correction during Positionlist scan Enable the height control function via Extras Settings Drive Scan Enable Height Control. Generate a new Positionlist or open an existing one via the main menu bar File New positionlist/open positionlist... Change to the Height Control dialog Height Control tab. 26

27 In the Upper threshold input field, enter an upper threshold for the working distance. Up to this value, the correction of the focus will be performed. Select by working distance: The system will adjust the working distance of the electron or ion optics. Click on Process Mode. The process Mode mode is activated. Focus the electron beam at the desired height. Take care not to expose any position where a patterning will be placed later on. Click on New Scan to take a reference scan of the current position. The corresponding LED illuminates green to indicate an active reference scan. 27

28 Start the Positionlist scan. The focus will be corrected during the positionlist scan. 10. Exposure If you want to write the same pattern with a different dose, you can set the dose factor in the position list. Multiple patterns can be added to the positionlist by following step 8.2 or by using the PLS processor to create a matrix of patterns. Add Beam Shutdown if desired from the Automation section to the end of the positionlist. Now, it is time to write your pattern! To do so, select the positions on the positionlist to be scanned (e.g., select them all) and click Scan to start the exposure. 11. Unload the Sample Click EHT in the lower-right corner of the Column monitor screen, and click on EHT off. Turn on the mini-environment fan if it was previously turned off. On the lithography menu (lithography monitor) Click on LoadLock dialog On the sample exchange window click on the Unload sample By default, the following prompt message will be displayed on the PC monitor: 28

29 Press OK The Unload procedure will be carried out step-by-step and the progress bar will indicate the Activation Progress: The button indicates the currently active step, the button indicates a completed step. You can stop the Unload procedure at any time by pressing the STOP button. By default, the following prompt message will be displayed on the PC monitor: Make sure that the load lock door is closed then press OK After a few minutes the following message will be displayed on the PC monitor: 29

30 . Open the load lock valve: o Lift the load lock valve locking pin o Turn the locking lever from Locked to Open and pull out to open the loadlock valve Insert the transfer rod into the stage chamber. WARNING: Make sure the load lock valve is open otherwise the transfer rod would be bent by pressure! Press OK After a few minutes the following prompt message will be displayed on the PC monitor: Remove the transfer rod from the chamber Close the load lock valve: o Lift the load lock valve locking pin o Push in the locking lever, turn it clock-wise until it has snapped into place Make sure the load lock valve is closed then press OK After a few minutes the following message will be displayed on the PC monitor: 30

31 Press OK The Unload procedure is completed and you can remove the sample holder from the load lock. Return the sample holder into the desiccator. The system vacuum can be degraded by dust or dirt from the load lock chamber. Remember to close the door to the load lock chamber after you have removed the sample holder. 12. Exit the System Press File and then Exit to close the e_line control program (Lithography monitor). The Raith EO software is generally left running. In the end, do not forget to fill in the Log Book. Reference and Files Raith e_line Manual and training notes. Contact Information Questions or comments in regard to this document should be directed towards Mohamad Rezaei in 4D LABS at Simon Fraser University, Burnaby, BC, Canada. 31