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Refer to the Material and Process Compatibility page for information on materials compatible with this tool.
Equipment Status: Set as UP, PROBLEM, or DOWN, and report the issue date (MM/DD) and a brief description. Italicized fields will be filled in by BNC Staff in response to issues. See Problem Reporting Guide for more info.

StatusUP
Issue Date and Description


Estimated Fix Date and Comment

Responding StaffFrancis
iLab KioskeLog: SubmiteLog: View/EditReport Problem

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iLab NameC - Xactix E1 Xenon Difluoride (XeF2) Etcher
iLab KioskBRK Etch Core
FICShared
OwnerFrancis Manfred
LocationCleanroom - S Bay
Max. Wafer4"/100 mm
Internal Page/wiki/spaces/BNCWiki/pages/6236532
Staff Page

Overview

TypeMaterialsRestricted MaterialsAvailable GasesMax RF PowerWafer Size
Vapor-Phase EtchSi, Mo, Ge, SiGe, W
XeF2, N2NA4 inch (100 mm) SEMI Specification

Note: Some literature mentions XeF2 etching of Ti, TiN, Ta, TaN, and TiW. Etching of these materials is temperature dependent, negligible etching occurs below 50 degrees C for any of these materials. Our system does not have heated chuck capability and is not suitable to etch these materials.

General Description

At room temperature and atmospheric pressure Xenon Difluroide (XeF2) is a white solid material.  It sublimates directly into the required vapor-phase etchant, not requiring plasma or other activation, under vacuum at room temperature and about 4 Torr pressure.  XeF2 etch is a dry, isotropic, etch desirable for many MEMS release applications.  Not needing plasma activation minimizes damage to other materials on the wafer and offers broad flexibility in process design. The XeF2-Si reaction is exothermic and the process engineer should be mindful of potential thermal issues. 

For silicon, the etch proceeds as:
2XeF2 + Si => 2Xe + SiF4


Commonly Etched Materials
MaterialSelectivity to Si

Si

1:1
Mo2:1
GeSame or Faster than Si
SiGeSame or Faster than Si

Etch rates depend on the amount of exposed silicon, and will depend on a particular sample.

Commonly Used Low or Non Reacting Materials
MaterialSelectivity to Si

Thermal SiO2

1000:1
Low Temperature SiO21000:1
Si3N4>1000:1
GoldLow Amount of Attack Under Certain Conditions
CopperLow Amount of Attack Under Certain Conditions

SiC

Low Amount of Attack Under Certain Conditions
Non Reactive Materials
MetalsCompounds
AlPZT
NiMgO
CrZnO
PtAlN
GaGaAs
Polymers and Organics
Photo ResistsPDMS
C4F8Silica Glass
Dicing TapePP
PENPET
ETFEAcrylic

Sample Requirements and Preparation

The Xactix E1 Xenon Difluoride etcher will accept small clean and dry pieces up to full 100 mm (4 inch) diameter wafers.  Small pieces can be placed directly on the sample surface with no mounting required.

XeF2 etching is a chemical diffusion process, as such, the etch rate and etch profiles are dependent on loading effects. Any silicon that you do not intend to etch should be covered to prevent consumption of XeF2 resulting in etch process variation. A test run with the exact same sample size and pattern is required to determine specific etch rate for a process.  The following effects have been observed:

  1. Etch rate differences can be observed between the center and the edge for wafers with large etched areas.
  2. Exposed silicon at the edge of the wafer can consume XeF2 and 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 depending on the feature size or to the proximity of two adjacent features.


The surface preparation prior to etch is important in achieving consistent and repeatable etch results. 

  1. Note that surface roughness increases with time/etch depth in XeF2 etch chemistry.
  2. Etch roughness can also occur through a mechanism similar to micromasking resulting in irregular etch fronts.  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. For well controlled silicon etching, samples are dipped into a BOE solution for 10 s to remove any native oxide prior to the XeF2 etch,
  3. All samples need to be dehydrated directly before placement in the etch chamber, either with IPA or a hotplate dehydration bake at 120" C for 10 min. The presence of adsorbed water on samples will result in the formation of both gaseous HF and a silicon flouride polymer on the sample surface. This polymer layer will reduce or completely stop etch progression, and will not be removed in either solvent soaks or O2 plasma.

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

Standard Operating Procedure

XeF2, SiF4 and any other F-containing species present in this etching process are both toxic and corrosive. Inhaling them can result in chemical burns to respiratory tracts. 

Etching a Sample

Activating the System

  1. Use the kiosk or Wiki links to make your logbook entry.
  2. Log into iLab and start your reservation to activate the tool.

Logging onto the System

  1. When starting the Xactix system the prompt to the right will be displayed.
  2. Enter the username and password below.
    1. Username: user
    2. Password: user

Loading / Unloading a Sample

  1. Press the Load/Unload Sample button on the Main menu screen. 
  2. The system will launch a verification window. Press YES yes to continue, NO to abort chamber vent.
    2. Verification prompt is provided since the purge/vent process can be time consuming and inconvenient if accidentally started.
  3. The Load/Unload Sample process window will load and begin 3 chamber pump / purge cycles to evacuate the chamber.
  4. 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.

  5. Load / Unload your sample and close the lid.
  6. Press Examine or Done button.
    1. Examine button -  Pumps the chamber down quickly, without purges, so that the system can be quickly vented again to load the sample.  This is useful when examining a sample away from the system, and prevents moisture from accumulating in the chamber. In general better results are obtained by etching in one run rather than several shorter runs where the sample is removed and then replaced.
    2. Done button -  The system will go through a purging cycle prior to chamber pump-down. It is always necessary to press Done before etching the sample. 

    3. Do not leave the chamber lid open longer than necessary. Any moisture accumulated will negatively affect the etch process.


  7. After pressing Done, and the chamber pumps down, it will go back to the main screen, and the Machine Status information box at bottom left will show “Ready”.

Entering the Etch Menu Screen

  1. Select Etch Menu button from the Main Menu Screen.
  2. A prompt asking for a lot number will appear.
    1. Enter an alpha-numerical lot number which will be recorded with your etch data. This entry does not affect your process in any way.

  3. Press the Done button
  4. The Etch Menu will show on screen. The name of last used recipe is shown on top left.

Load an Existing Recipe

  1. Click the current recipe pull down to expand.
  2. Select the desired recipe.
  3. Verify the # of Cycles, Etch Time, XeF2 Pressure, and N2 pressure values are correct for your process.

Editing Process Parameters

  1. Set the four process parameters for your etch process.
    1. # of cycles -  The depth 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, opening to chamber and etching for a set amount of time and evacuation of the main chamber and expansion chambers.
    2. Etch Time - This is essentially the exposure time of the sample to XeF2 etchant. More precisely the etch time counter starts when the valve between the charged expansion chamber and the evacuated process chamber opens, and stops when the valve between the process chamber and the vacuum pump opens.
      1. Although it is sample dependent, it has been found that XeF2 is typically fully depleted after 30 seconds of exposure to the Si sample.

      2. Since the XeF2 reaction with Si produces more moles of gas (2XeF2 + Si -> SiF4+ 2Xe) as it reacts, you will see the chamber pressure rise as the reaction continues (keep in mind that you may see a normal leakup of the chamber also).  When the available XeF2 has been consumed, the pressure in the chamber should not stop rising.  If you watch the Chamber Pressure carefully throughout an exposure, you should see a point where the etch chamber pressure essentially stops rising.  You can use this time, plus 10%, to set your cycle time since any additional time is essentially wasted.

    3. Xef2 Pressure - To introduce a consistent amount of XeF2 into the main chamber, a set pressure charge of XeF2 is delivered to the expansion chamber before exposing the sample to the etchant. XeF2 has a vapor pressure of ~4T at 25 C.
      1. Assuming all XeF2 is consumed in each cycle, this parameter determines etch depth per cycle.
      2. Typical values used for this parameter at our facility are between 1 - 2.5 Torr.
      3. It is recommended to set the XeF2 pressure below 3 Torr. The system must stabilize the expansion chamber pressure at this set point before proceeding. Setting higher pressures may result in very slow cycle times, or failure to proceed.
    4. N2 Pressure - XeF2 is added to the expansion chamber first, followed by N2.  In this way, an accurate and reproducible mixture of gas can be delivered to the sample.
      1. The addition of N2 improves the selectivity to silicon nitride.
      2. The addition of large amount of nitrogen can dilute the etchant and help achieve very slow etch rates. 
    5. Etch Mode - Select between Normal and Normal with delay under the Etch Mode pull-down.
      1. Normal with delay allows the user to add a delay time between cycles so the wafer can cool down.

Saving a Recipe

  1. To Save a new recipe based on currently displayed process parameters, click the Save button. The save new recipe window will appear.

  2. Global Recipes are starting recipe templates available to all users, please do not overwrite these.

  3. Use the toggle switch in the middle of the screen to set to Personal Recipes. Users should save their new recipe to this location.

  4. Type in the name of your new recipe and Click on the SAVE button. You will be returned to the Etch Menu Screen.

    1. Please start the file name with your Purdue alias. (example: jshepar-Recipe1)

Run a Recipe

  1. From the Etch Menu Screen, and after etch recipe and parameters have been verified, press the Start Etch button.

  2. Progress of the etch can be monitored using the tools in the lower right corner of the Etch Menu screen.

  3. When the etch completes normally, the system will load the Main Menu screen and you will see the message Etch completed, press for details displayed and flashing.

Stopping an Etch Before Recipe Completes

  1. From the in progress Etch Menus Screen, the Stop button is used to end an etch recipe prematurely.
    1. Pressing the Stop button once initiates a soft stop. The tool will complete the current cycle and then stop executing the recipe and return to the Main Menu screen.
    2. Pressing the Stop button twice initiates a dialog box asking for input.
      2. Pressing Yes, Hard Stop will stop the recipe immediately.
      3. Pressing No, Keep Waiting will revert to a soft stop described above.

Changing the Number of Cycles During an Etch

  1. Pressing the CHANGE CYCLES button at any time during an in progress etch will prompt the user with the Change # of Cycles menu.
  2. The dialog shows # of Cycles remaining currently.
  3. The top row of arrows will add cycles in this order, (right to left); ones, tens, hundreds, and thousands.
  4. The lower row of arrows, having the same values, will remove cycles.
  5. Shows the result if changes are applied.
  6. Press OK to apply changes, Cancel to exit without changing.

Example: In the image to the right, 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. 

Viewing Detailed Etch Information

  1. The Detailed Etch Information log may be viewed by clicking on the View Log button on the Main Menu screen.
    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.
    2. The lot number, username, recipe, note’s keyword(s) and/or etching mode used can further specify your search.
    3. Wildcard characters (*,?) may be used to fully specify the search criteria. 
    4. If the boxes are left blank, clicking the search button will reveal all history info, displayed in a tabular form. 
  2. Database search results will be displayed in tabular format after pressing the Search button.
    1. The export data button allows you to export all of the information shown in the table to a tab-separated text file.
    2. The information for a particular process can be accessed by selecting the row of information desired and clicking the “Detailed Info...” button at the bottom.

Unloading a Sample

  1. Press the Load/Unload Sample button on the Main menu screen and follow prompts.
  2. 76878801.

Finishing Your Session

  1. Verify that you have removed all materials from the chamber by looking through the chamber lid.
  2. Make sure the chamber lid is closed, and the system is pumped down.
  3. From the Main Menu screen, click the Log Out button to exit the Xactix software.
  4. Terminate your session in iLab.
  5. Clean up the workspace as necessary.

Questions & Troubleshooting

How long should my cycle time be?
Cycle time should be set to allow all of the XeF2 vapor to react with exposed silicon. For large areas of exposed silicon, XeF2 will quickly react with the exposed areas, and the cycle time may be reduced. For a smaller chip, cycle times will need to somewhat longer to allow all of the XeF2 to react fully.

In order to tell that all XeF2 has reacted, the chamber pressure may be monitored. The etching reaction proceeds as 2XeF2 + Si -> SiF4 + 2Xe, meaning 2 moles of reactant gas will be present at the beginning of the etch step, and 3 will be present after the etch has completed. Therefore, the pressure in the chamber will increase somewhat while the etch is proceeding, and will level off after the etch has completed.

The etch rate on my sample seems nonuniform?
Uniformity is an inherent issue with XeF2 etching. There is a limited amount of etchant released in each cycle, and the local etch rate will depend on the amount of exposed silicon and available XeF2 vapor. In general, large exposed areas of silicon will show the worst uniformity, and regular patterns of small holes (with the rest of the wafer covered) will show the best uniformity. Minimizing unneeded exposed silicon will allow for faster and more uniform etching of the pattern.


Process Library


References

Isotropic Etching With Xenon Difluoride - Presentation from Xactix giving an overview of Xenon Difluoride etching, including selectivity and etch rate information (Internal Resource)

Unique Capabilities of Xenon Difluoride for Releasing MEMS - Xactix (Internal Resource)

"A behavior of Xenon difluoride etching on poly-silicon sacrificial layer for fabrication of MEMS device" - Takeshi Taniguchi et al., 2002. (Internal Resource)

XeF2 Etching of Silicon

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