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Xactix: A XeF2 dry, chemical etcher

The Xactix e-1 is a XeF2 (xenon difluoride) isotropic silicon etcher. XeF2 is a vapor phase etch, which exhibits nearly very high selectivity of silicon to photo-resist, silicon dioxide, silicon nitride and aluminum.

Introduction

The Xactix e-1 is a XeF2 (xenon difluoride) isotropic silicon etcher.  XeF2 is a vapor phase etch, which exhibits nearly very high selectivity of silicon to photo-resist, silicon dioxide, silicon nitride and aluminum.   The e-1 Series can be used to etch silicon wafers, up to 6” in diameter, wafer pieces, die, or other structure into the etch chamber.  Details of the process sequence are controlled and captured by the control software. The etch progress can be monitored through the transparent chamber lid. 

The tool is located in the first aisle upon entering the SNF from the gowning room left to the wbflexcorr-3and-4.

 

 

Processing Tid-Bits

1.      XeF2 etching is a chemical diffusion process.  The etch rate and etch profiles can be highly dependent on loading effects.  For example, etch rate differences can be observed between the center and the edge for wafers with high open areas. 

2.      Exposed silicon at the edge of the wafer can lead to more etch rate non-uniformity.  This can be from photoresist edge bead removal or simply form loss of photoresist on the vertical edge of the wafer.

3.      Patterns with differences in feature sizes can show different etch rates dependant on the feature size or to the proximity of two adjacent features.

4.      The surface preparation prior to etch is important in achieving smooth etch surfaces. Etch roughness occurs through a mechanism similar to micromasking in dry etch. 

5.      XeF2 etches native silicon dioxide, but at a much lower rate than silicon.  Any surface contamination or variation in removal of the native oxide can lead to surface roughness.

6.      The XeF2-Si reaction is exothermic.  A delay step can be used to cool the wafer between etch cycles to minimize any observed thermal issues.


Operating Procedure

Initial System Check List

1.   Since the standby state does not have the vacuum pump running, you will need to wait until systems reaches base pressure once you have enabled the tool on Badger.

2.   If the system goes in to a gauge calibration routine, the total time is just over one hour. 

2.1.      It is best to let the system complete the gauge calibration.  Contact the SNF staff and they can adjust your time to remove the gauge calibration charges.

 

Status Check:

1.   Do not use if an IN USE sign or MAINTENANCE sign is present.

2.   Check for reservations, problems notes, and to see if it is already enabled by another user.

3.   The system is available if the initial system and status checks are good.

4.   Enable the system on Badger.

 

Logging on the System

1.   Each user is required to log in to the system using their Badger ID.

2.   Input your Badger ID and password.

3.   The KB button beside the Username and Password field can be used to access the on-screen keyboard.

4.   Press the LOGIN button to access the MAIN MENU screen.

Xactix Login

 

Loading a Sample

1.   Press Load/Unload sample on the Main Menu screen.

1.1.      Press YES if you’re sure, NO if you’re not. The system begins chamber purges and flushing cycles to evacuate the chamber, shown below.

NOTE:  If the chamber ventilation shroud covering the chamber is not pulled fully forward, a request to close the shroud will be displayed before venting the chamber. Move the shroud forward and acknowledge the prompt.

Xactix Main Menu

2.   When the chamber is vented, the dialog box below will appear and you can open the chamber lid. The lid will rest open on the stop behind chamber.

 

3.   Be sure to use the correct substrate holder according to the rating of your wafer (clean, semi-clean or gold).

 

4.   Place your sample on the substrate holder.

5.   Inspect the O-ring.  If necessary, wipe down the O-ring with a dry wipe.

6.   Close the lid.

6.1.      Make sure the lid is fully closed and the lid is seated against the O-ring.

7.   Press Done on Pop-up screen. 

7.1.      The system will go through a purging cycle prior to chamber pump-down. As a side note, the Examine button only pumps the chamber down, without purges, so that the system can be quickly vented to load the sample. This is very useful when examining a sample away from the system to prevent moisture from accumulating in the chamber. However, it is always necessary to press Done before etching the sample.

 

8.   During the pumping cycle, a “click” sound will be heard that indicates that the ventilation shroud can be moved.

Running a Recipe

1.   Select ETCH MENU from the Main Menu Screen

1.1.      A prompt asking for a lot number will appear.

 

1.2.      Enter an alpha-numerical lot number which will be recorded with your etch data.

 

1.3.      Press the DONE Button.

2.   The following Etch Menu will appear.

 

Xactix Etch Menu

 

3.   Under the CURRENT RECIPE pull-down, select the required recipe.

4.   Verify the # of Cycles, Etch Time and XeF2 Pressure values are correct.

4.1.      If any values are not correct change them by either moving the slider or typing in the correct value in the required field.

 

4.2.      Useful observation: Use has shown that after about 30s, the XeF2 is depleted.

5.   Select between NORMAL and NORMAL WITH DELAY under the ETCH MODE pull-down (normal with delay allows the user to add a delay time between cycles so the wafer can cool down).

6.   Press the START ETCH Button.

7.   Progress of the etch can be monitored following the Etch Time Completed and Cycles Completed bars along with the Elapsed Cycle Time and the ETA.

Changing the Number of Cycles During an Etch.

1.   The CHANGE CYCLES button can be used at any time during the etch to modify the number of cycles to the etch.

2.   The top row of arrows will add cycles in this order, (right to left); ones, tens, hundreds, and thousands; the lower row of arrows, having the same values, will remove cycles.

2.1.      Click on the designated arrow for the desired number of cycles needed to be added to the etch in-progress.

2.2.      Example, an extra 125 cycles have been added to the etch by clicking the right most arrow (ones) five times, the next right most arrow (tens) twice, and the 2nd from left most arrow (hundreds) once.

3.   Press OK to return to the ETCH MENU screen.

 

Xactix change cycles

 

Unloading a Sample

1.   Press Load/Unload sample on Main Menu screen.

1.1.      Press YES if you’re sure, NO if you’re not. The system begins chamber purges and flushing cycles to evacuate the chamber, shown below.

NOTE:  If the chamber ventilation shroud covering the chamber is not pulled fully forward, a request to close the shroud will be displayed before venting the chamber. Move the shroud forward and acknowledge the prompt.

2.   When the chamber is vented, the dialog box below will appear and you can open the chamber lid. The lid will rest open on the stop behind chamber.

3.   Unload your sample.

4.   Inspect the O-ring.  If necessary, wipe down the O-ring with a dry wipe.

5.   Close the lid.

5.1.      Make sure the lid is fully closed and the lid is seated against the O-ring.

6.   Press Done on the Pop-up screen. 

6.1.      The system will go through a purging cycle prior to chamber pump-down.

7.   NOTE:  Examine button only pumps the chamber down, without purges, so that the system can be quickly vented to load the sample. This is very useful when examining a sample away from the system to prevent moisture from accumulating in the chamber. However, it is always necessary to press Done before etching the sample.

 

Xactix Examine pump down

8.   During the pumping cycle, a “click” sound will be heard that indicates that the ventilation shroud can be moved.

 

 

Viewing Detailed Etch Information

1.   The Detailed Etch Information log may be viewed by clicking on the VIEW LOG button on the Main Menu.

 

1.1.      The log file is a database that is queried by beginning date and ending date. The “Today”: button will automatically set the dates to the current month, day, and year. The lot number, username, recipe, note’s keyword(s) and/or etching mode used can further specify your search. Wildcard characters (*,?) may be used to fully specify the search criteria.

 

 

Creating or Changing a Recipe

1.   Creating or changing recipes is done from the ETCH MENU screen.

2.   Make the appropriate recipe changes

3.   Save the recipe by clicking on the save button located near the top of the “perform etch screen”.

4.   The save new recipe screen will appear.

Xactix Recipe Save

 

5.   Global Recipes are starting recipe templates available to all users. 

6.   Use the toggle switch in the middle of the screen users can save the new recipe to their Personal Directory or the accessible Global Recipes.

7.   Type in the name of your new recipe and Click on the SAVE button.

 

A Note on Etch Recipe Parameters

Number of cycles

Since the e1 Series is primarily a pulsed xenon difluoride etching system, the duration of etching is controlled by the number of cycles. A cycle consists of the xenon difluoride sublimating to the set pressure in the expansion chamber, etching for a set amount of time and evacuation of the main chamber and expansion chamber.

Etch Time

When the valve between the main chamber and expansion chamber is opened the pressure equilibrates and the etching process begins. The etch time is the time between the opening of the valve between the expansion chamber and the process chamber and the opening of the valve between the process chamber and the pump.

XeF2 Pressure

In order to introduce the proper amount of xenon difluoride into the main chamber a set pressure charge of xenon difluoride must be delivered to the expansion chamber. Because xenon difluoride has a vapor pressure of ~4T at room temperature the upper limit for the XeF2 pressure is approximately 4T. Due to the slightly elevated temperature inside of the etcher cabinet, you may be able to get considerably higher XeF2 pressures, however.

N2 Pressure

Nitrogen can be added into a recipe to improve selectivity. The pressure obtained in the expansion chamber likewise controls the amount of nitrogen introduced into the process chamber. The above variables can be set either by moving the white slider on the scroll bar or by tapping or depressing on the arrows at the top and bottom of the scroll bar to increment each value or by direct entry through the keyboard. Additionally, a range button is included for the number of cycles, etch time and nitrogen pressure so that the user can input higher values than the default range allows. Due to equipment characteristics, it is necessary to add at least 4.0 Torr.


How to become a qualified user

 

1) Read all material on the SNF website concerning the wet bench, including Background, Process Capabilities, Operating Procedures and Process Monitoring.

2)  Contact a qualified user of the Xactix to arrange to ‘shadow’ them while they use the tool before requesting the final session. You are responsible to be with that labmember for the full time they are operating the tool, and it would be intelligent to ask questions and try to become as familiar as possible with the tool during this ‘shadowing.’  You may have to shadow the qualified user more than one time.
If you do not know a Xactix user, contact the user list (xactix at snf dot stanford dot edu) or check Badger for upcoming user reservation.  It is up to the user if they are willing to have you shadow them.  If you have trouble finding someone to shadow, please feel free to contact the SNF training contact. 

3) Contact the SNF training contact on the Equipment Summary page to schedule a final session. At this session, typically lasting 30 minutes, you are expected to demonstrate your ability to run the tool safely and appropriately without any input. 
Frequency 

Process Monitoring and Machine Qualification

Process

The qualification process takes place to ensure the equipment is functioning properly and works to test its accuracy for etching, based on the inputted desired process.

Frequency

These qualification runs are to be completed on a monthly basis, and information regarding the results of such shall be posted for the public to review.

Monitoring may also occur as needed/requested by user reports and concerns or after major repairs.

Procedure

Wafers For Processing

  • Start with two L test Si wafers with 1um of 3612 photoresist patterned with the SUMO mask.

Measure the Wafers Before Etching

  • Measure photoresist thickness using one of the Nanospecs.  Be sure to use reference wafer before testing to calibrate the tool.
  • Take readings for the Center, Top, Flat, Right and Left positions of the wafer.
  • Use the statistics option to get averages.
  • Readings should be taken about 15mm from the edge.
  • Record results.

Etch Process

  • Run 20 cycles of 30 seconds each with no added N2 and no delay as the standard process.
  • Run two wafers sequentially.

Analyzing the Resulting Etched Wafer

  1. After the wafer has run through its process and removed from the machine, it must be analyzed to observe the etching patterns.
  2. Measure photoresist thickness using the same Nanospec.  Be sure to use reference wafer before testing to calibrate the tool. 

            a. Take readings for the Center, Top, Flat, Right and Left positions of the wafer.

   3. Take note of the amount of photoresist etching that occurred on the wafer through measurements using Nanospec. Record these measurements from several etching areas on the wafer.

   4.  Using an optical microscope with measurement capability (such as the one in the lithography area near the karlsuss) measure the lateral etching of the silicon. 

            a. This can be done by measuring the distance from the edge of the photoresist to the edge of the underlying silicon (we’ll save an image from the first qual). 

            b.  Make measurements at 5 points on the wafer (flat, top, left, right, and center). 

   5.  Record the etch distance at each location.

   6.  Announce the etch results on badger and update the trend chart on the tool’s website.

   7.  A further look into the etching pattern can be determined by use of SEM. This can help analyze the shapes of which the etching process allowed to take shape on the wafer.

Responsibility

This monitoring will be handled on a monthly basis by the students of SUMO, but users are encouraged to run a qualification process if they feel the tool is not in good running order. The staff should be informed of the results retrieved from such a run so as to add to the previously collected data.

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