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Operating Instructions- Ox-35

Here are operating instructions for the Ox-35 etcher. They come courtesy of superuser Alex Piggott. Yunhan Chen expanded the instructions to include procedures for recipe set up.

 

OX-35 Oxford III-V Etcher Instructions

Version: 2.0

Last Edit: by Yunhan Chen

 

General Overview

The OX-35 Oxford etcher is an inductively-coupled plasma (ICP) reactive ion etcher (RIE), designed for etching III-V semiconductors. The OX-35 is currently approved to etch GaAs, InGaAs, InAs, InP, InGaAsP, and GaP only. Please contact Jim McVittie (mcvittie@stanford.edu) for approval to etch other substrates.

There are several important points regarding usage of this etcher:

  1.   Only 4” wafers can be etched using this machine. Any smaller chips must be mounted on a carrier wafer for etching. Any carrier wafers that are not etched by the chemistry can be used; Si and SiO2-on-Si wafers are acceptable with most chemistries.
  2.   5mm EBR (edge-bead removal) is required on all wafers due to the wafer clamp. The backside of the wafer must also be clean.
  3.   Never run the etcher without a wafer in the chamber, especially during cleaning. The plasma will damage the wafer clamp if it is exposed.
  4.   Never place a wafer back into the etch chamber after running a process without physically resetting it! The wafer may shift, leading to breakage.
  5.   When evacuating the load lock, ensure “Soft Pump” is selected (green check mark) to ensure that the wafer does not shift at the start of pumping.

The measured bias voltage may drift significantly if an oxide carrier wafer is used, but this is not indicative of actual process variations.

 

Picture and Location

The tool is located at D section of the lab, next to amtetcher and oxford-rie.

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Basic Instructions for Etching a Wafer

  1.   Log in: system→password. Username = “super user”, password = “ox-35-su”
  2.   Vent the loadlock: system→pumping
    1.   Press the “Stop” button (stops pumping the loadlock)
    2.   Press the “Vent” button (vents loadlock)
      Image3 
  3.    Wait for the purge cycle (pre-vent, pump down, and final vent) to complete. The load lock pressure will rise to atmospheric pressure before the machine pumps the load lock back down to base pressure. The cycle should take 1.5 – 2min. You do not have to wait until the vent timer reaches zero, just until the final pump cycle reaches approximately atmospheric pressure (770 torr). The gauge is inaccurate above ~200 torr, and may only reach ~500-600 torr.
  4.   Open the load lock and load your wafer.
    1.   Wafer flat must face the two screws on the platter
    2.   Make sure the wafer is correctly positioned, and firmly pressed against the screws. Even an offset of 1-2mm can cause wafer breakage.
    3.   The wafer may shift after being placed in the etch chamber, and returned to the loadlock. Always re-position the wafer after running a recipe.
  5.   Evacuate the loadlock: system→pumping
    1.   Ensure “Soft Pump” is selected (green check mark)
    2. Press “Stop “ button (stops venting the loadlock)
    3. Press “Evacuate” button (pumps down the loadlock)
  6.   When prompted by the system, enter the wafer name.
  7.   Load recipe: Process→Recipe
    1.   Make sure the system is in “automatic” mode. If it is in manual mode, change it to automatic.
    2.   Press “Load”, click “Yes” when asked whether to overwrite, and choose the desired recipe.
    3.   You can now edit the steps in the left-hand panel. These settings will only change temporarily unless you manually save the recipe.
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  8.   Run the recipe: in Process→Recipe, press the “Run” button. The system will automatically load the wafer, run the recipe, and unload the wafer. 
  9.   Vent the loadlock: system→pumping
    1.   Press the “Stop” button (stops pumping the loadlock)
    2.   Press the “Vent” button (vents loadlock)
  10.   Open the loadlock and retrieve the wafer. NEVER place the wafer back into the etch chamber without physically resetting it! The wafer may shift, leading to breakage.
  11.   Evacuate the loadlock: system→pumping
    1.   Ensure “Soft Pump” is selected (green check mark)
    2.   Press “Stop “ button (stops venting the loadlock)
    3.   Press “Evacuate” button (pumps down the loadlock)
  12.   Log out: system→password, then press “verify” without entering a username. The program will then enter the “View Only” mode.

 

List of Available Gases

The following gases are available for use:

  •           BCl3
  •           Cl2
  •           HBr
  •           CH4
  •           H2
  •           Ar
  •           O2
  •           N2
  •           SF6


Recipe Management

Recipe management screen can be accessed by pressing Process→Recipe located at the top left of the screen. The left side of the screen shows the recipe that is currently loaded to the program. The panel on the right side is the Step Library. 

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Step Library

Step Library stores process steps that can be used to compose a recipe. Several standard steps are stored here.

  •   Press “New” or choose an existing step and press “Edit” to open the Process Step Editor.
  •   After editing the step parameters and name, click “OK” to save or “Cancel” to discard the changes.
  •   Choose an existing step and press “Copy” to duplicate this step, or press “Delete” to delete this step from the Step Library. DO NOT delete existing standard steps in the step library.
  •   When creating a new recipe or edit an existing recipe, steps can be directly copied from the Step Library. However, please note that once a step is copied to a recipe from the Step Library, any edit on this step in the Step Library will not affect the step in the recipe, and vice versa.

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Create a new recipe 

  •   Press “New” in the Current Recipe panel, click “Yes” to clear current recipe. A blank recipe is then created.
  •   Click on the empty step in the recipe box, click “Edit Step” to open the Process Step Editor. Edit the name and parameters of this step and press “OK” to save.
  •   You can also drag a step in the Step Library, to the recipe box. This step will then be copied to the currently recipe. Click on this step in the current recipe box and press “Edit Step” to edit this step. Any changes will not affect the step that is saved in the Step Library.
  •   Create a repeating loop
    1.   Click on an empty step and press “Repeat Step”
    2.   Enter the number of times that the loop will repeat
    3.   Close the loop by insert a loop step by pressing “Loop Step”
    4.   Any steps between Repeat Step and Loop Step will be repeated the number of times that has been set in the Repeat Step.

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  •   Delete a step by click on the step and press “Delete Step”. Press “Yes” to confirm.
  •   Edit the Recipe Name and press “Save” to save the current recipe.

 

Edit an existing recipe

  •   Press “Load”, click “Yes” when asked whether to overwrite, and choose the desired recipe.

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  •   This recipe is now loaded to the currently recipe panel.
  •   Click on the step you want to edit. And choose from the Step Commands as needed. If press “Edit Step”, the Process Step Editor will be open. The step parameters can then be changed.
  •   You can drag a step from the Step Library and release on top of the step you want to replace in the current recipe window. Click “Yes” to confirm replacing steps.
  •   Before pressing “Save” button, change the Recipe Name to avoid overwriting the original recipe.

 

Standard Recipes (incomplete)

Before etching any wafers, one should run a cleaning recipe, followed by ~15 minutes of seasoning with the intended etch recipe. There should be a 1:1 correspondence between the time spent cleaning, and the time spent running any other recipes. After particularly dirty recipes (e.g. those using methane), even more cleaning is recommended. A blank Si wafer can be used for cleaning, but the same substrate should be used for seasoning as the carrier wafer for the actual etch.

  •   OPT - Chamber Clean: standard chamber cleaning recipe

          O2 (100 sccm), SF6 (20 sccm), 20 mTorr, 70 W RF power, 2000 W ICP power

          ~10min run is recommended

          This recipe is extremely effective at etching oxides; use a Si dummy wafer instead when running this recipe.

  •   AR - GaAs Etch (High eR) - BCl3/Cl2: GaAs etch recipe

          BCl3 (5 sccm), Cl2 (5 sccm), Ar (20sccm), 2 mTorr, 30 W RF power, 250 W ICP power

          Etches 150nm diameter holes at ~130nm per minute

Process Monitoring and Machine Qualification

 

Frequency

The Ox-35 qual runs two wafers through the GaAs etch process. We use the Qual_BCl3_Cl2_HighER in the library. One wafer with a 1.6um 3612 Photoresist pattern and one wafer with a 10,000A SiO2 coating are etched for 15 min each. Nanospec measurements are taken before and after etching to calculate the PR and SiO2 etch rate respectively. To calculate the PR:Si selectivity, the two wafers coated in photoresist are then stripped of resist so that the Silicon step height may be measured via Alphastep.

Procedure

SUMO Wafer #
(all Si 4" wafers)
Coating Pattern
(using SUMO mask)
-  10,000A SiO2 gold
 -  1.6um 3612 PR pattern, 1min bake @ 110C  gold

 


Pre-Etch Wafer Measurements

All pre-etch measurements are taken on Nanospec. Reference the SUMO Characterization How-To's for Nanospec operating procedure.

For the SiO2 wafer, use the Oxide on Silicon Recipe (#1). For PR Wafer, use the Positive Resist on Silicon Recipe (#10). Use 10x magnification for all measurements (Objective 1 on Nanospec).

  1.  Use the reference wafer to calibrate before measuring.
  2. Take readings for the Center, Top, Flat, Right and Left positions of the wafer. Readings should be taken about 15mm from the wafer edge.
        a.  For PR wafer, measure the PR thickness, which is darker in color than the Si. Accidentally measuring the Si will result in a <100A reading.
  3. Calculate the averages.
  4. Record results on the SUMO Qualification Log Sheet. 

  

Etch Process

Before loading the wafers,  season the chamber so that the chamber surface chemistry is brought to equilibrium with the process being run. This enables the actual process to stabilize quickly after the plasma is turned on. This seasoning also leads to more repeatable etching result.

  1. Season the chamber for 15 minutes using the Oxide etch BCl3 / Cl2. Follow the standard operating procedure as outlined above, or on the procedure print-out at the tool. Be sure to check the program parameters while seasoning and adjust the resistor/capacitor settings as necessary.
  2. Run a separate etch for each of the two wafers. Each etch is 15 minutes.

Recipe = GaAs Etch : Qual_BCl3_Cl2_HighER

    •  BCl3 (5.0 sccm)
    • Cl2 (5.0 sccm)
    • Ar (20.0 sccm)
    • Power : 30W (Fwd), 250W (ICP)
    • Pressure: 2 mTorr
 

Post-Etch Wafer Measurements

  1. Measure post-etch thickness for all wafers using the same Nanospec as before and the same respective Nanospec analysis recipes.
  2. Take readings for the Center, Top, Flat, Right and Left positions of the wafer. Readings should be taken about 15mm from the wafer edge.
  3. Calculate the etch-rate for PR and SiO2 by subtracting the post-etch thickness from the pre-etch thickness and then dividing by etch time.
  4. To calculate selectivity, measure the Silicon step height of PR wafer.
  5.              a.   Strip the wafers of PR using the Strip.rcp recipe on Matrix.
                 b.   Measure the step height at each of the 5 points via Alphastep. Readings should be taken about 15mm from the wafer edge.
                 c.   Calculate the average step height.
                 d.   Calculate the amount of Si lost and divide by etch time to determine the Si etch rate.
  6.  Calculate the selectivity by dividing the PR etch rate by the Si etch rate.

 

Reported Data

Qual data may also be found on the Badger comment log. The following data is reported for the Ox-35 qual:

  • PR Etch Rate (3612)
  • Si Etch Rate
  • PR:Si Selectivity
  • SiO2 Etch Rate
  • SiO2:PR Selectivity

Machine Status States

  • Red: A major sub-system of the tool (RF power supply, mass flow controller, vacuum pump, etc.) is not functioning up to standard.
  • Yellow: A sub-system of the tool not common to all recipes is not up to standard. For example, a specific gas is being replaced.
  • Green: System is up to standard.

 

Process Monitoring Results

 

 

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