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Plasma-Therm ICP

Table of Contents:
I: Introduction
II: Machine Specifications
III: System Components
IV: Etched Materials and Precursor Gases
V: Operating Instructions
VI: Creating and Editing Recipes
VII: Troubleshooting
VIII: Suggested Practices
IX: DO's and DON'Ts
X: Check-off requirements

I. Introduction

An ICP (inductively coupled plasma) etcher can be used to etch silicon, silicon dioxide, silicon nitride, and some polymers (must have special permissions to etch polymers). This ICP is equipped with two chambers. The right chamber is dedicated to deep silicon trench etching using the Bosch process, while the left chamber is used for silicon dioxide, silicon nitride and polymer etching.

Etching is performed with a RF (radio frequency) induced plasma of various gases. The plasma generates free ions which are accelerated towards the surface to be etched. Through chemical reactions and bombardment of the wafer with the reactive ions, etching is accomplished. An ICP is different than an RIE because it uses two power supplies to generate plasma. One power source is used to generate a dense plasma (~10x more reactive species than RIE), while the second power source accelerates the ions towards the etching surface. This combination increases the anisotropy of the etched feature as compared to conventional RIE.

The Bosch process alternates between an etch and deposition step to create very high-aspect-ratio structures in silicon (~20:1). The cycle begins by istropically etching the silicon with SF6. Then, C4F8 is used to conformally deposit a fluorinated polymer coating on both the trench bottom and sidewalls. The process then switches back to etching with SF6, which etches the polymer off of the trench bottom faster than the trench sidewalls. Thus, the silicon is etched on the trench bottom while the sidewalls are protected by the polymer. The protective sidewall polymer layer is eventually etched away, so the process switches back to the deposition step, and the entire process is repeated. Typically: deposition time ~4 sec, etch time ~8 sec, ~0.5 um per cycle.

II. Machine Specifications

Right chamber gases for etching silicon:

  • SF6
  • C4F8
  • Ar

Left chamber gases for etching oxides, nitrides, and polymers:

  • Cl2
  • BCl3
  • H2
  • O2
  • CF4
  • Ar
  • He

Both chambers are pumped with turbo pumps for low-pressure operation.

III. System Components

  • Heat exchanger
  • Gas cylinders
  • Pumps
  • 2 power supplies
    • RF1: Power applied across plasma. Provides energy to the ions and accelerates them to the surface.
    • RF2: Power applied to a coil that is located in the upper portion of the plasma chamber. The power to the coil generates the dense plasma needed for etching.
  • BOTH chambers use helium backside cooling.
    • The left chamber has an annular clamp ring to hold wafers in place.
    • The right chamber uses an electrostatic chuck.
  • This system does not have a cleaning process
  • This system accepts 4" wafers ONLY. Smaller samples must be mounted on a 4" carrier wafer. Larger wafers must be cleaved into smaller pieces and mounted on a 4" carrier wafer.
  • NO METAL OR SU-8 IS ALLOWED IN EITHER CHAMBER

IV. Etched Materials and Precursor Gases

  • Right Chamber

    • Si / poly-Si - SF6, C4F8, Ar

  • Left Chamber

    • SiO2 - CF4, O2, H2
    • Si3N4 - CF4, O2, H2
    • Si - Cl2
    • Polymers, etc. - O2, CF4, Cl2, BCl3, Ar

V. Operating Instructions

  1. Loading a sample

    1. Sign in to the machine via the access controller box.
    2. Press the STANDBY button at the bottom of the screen if it is not already lit.
    3. Click Utilities, Loadlock, Vent on the menu bar.
    4. The loadlock pressure will be displayed in the diagram. The loadlock will indicate a pressure of "VACUUM." Wait for the pressure to reach "ATMOSPHERE" and turn sky-blue, then open the loadlock. If the system does not display "ATMOSPHERE" after approximately two minutes, contact MiRC technical staff. NOTE: if you are processing a transparent substrate (e.g. sapphire/alumina, GaN, etc., you must contact MiRC staff to help you with this, as the system uses a laser to determine if there is a sample in the loadlock. It will not run processes if the laser passes through your substrate -- it will record no sample as being present. The staff can solve this problem. Your process will not run if you do not.
    5. Load sample into the center of the transfer arm, with the wafer flat to the rear.
    6. Close the loadlock cover

  2. Operating

    1. Click Process, Batch on the toolbar. If you do not already have a recipe and a batch, see the Recipes and Batches section of this document first.
    2. In the batch editor window select File, Load. select the batch you want to load. Ensure that the batch was loaded; if it was, it will be shown in the lower right hand side of the main screen.
    3. Close the Batch Editor windosw.
    4. Press the READY button at the bottom of the screen.
    5. After READY parameters have been applied, hold the loadlock cover down and press the RUN button at the bottom of the screen.
    6. Watch the helium cooling conditions on the bottom left hand side of the screen. The top display is a measure of the pressure in the chamber. The middle display is the pressure of the helium on the back side of the wafer, and the third display is the helium flow rate.
    7. After the helium and process gas flow rates have been stabilized, the plasma will ignite. Note the color of the plasma. In the right chamber, if the plasma is a darker purple (rather than a bright white), the process should aborted and MiRC staff notified immediately.

  3. Aborting a Process

    1. You may abort a process while it is running.
    2. Select the Abort key on the bottom of the screen.
    3. The system will ask "Abort Process?" and give you a YES and NO button. Select YES.
    4. The system will then ask "Abort Batch?" and give you a YES and NO button. Select NO.

  4. Unloading

    1. After the process is over, press the STANDBY button at the bottom of the screen.
    2. If your batch does not automatically vent, click Utilities, Loadlock, Vent.
    3. Inspect the chamber for any wafer chips and your wafer for any crack or missing chips. If one of the previous conditions occurs, immediately stop operating the ICP and place a "machine down" sign on the system. contact MiRC technical staff immediately. Also, if your wafer is not moved from the chamber to the loadlock, contact a MiRC staff member.
    4. Unload your sample from the loadlock. You will not be able to process your sample unless the staff modifiy the system.
    5. Click Utilities, Loadlock, Pump to pump the loadlock.
    6. Log out of the ICP at the access controller.

VI. Creating and Editing Recipes

Because the ICP has an automatic loadlock, each recipe requires two components: (1) batch file and (2) process file. The batch file controls the loadlock, manages the loading/unloading of the wafer, and calls the process file. The process file contains the steps to be executed once the wafer is loaded into the chamber. Generally the two files share the same name but have a different extension (e.g. bosch.bch, bosch.prc)

Because the ICP is a very complex machine, no users should modify a process file without consulting the MiRC staff or a trainer !!!

Batch file

To create or edit batch, select Process, Batch, New or Edit. To add a step, click on the button for the type of step you want to add at the top of the screen. To delete a step, select it and click Delete. Each batch file consists of at least five steps: pump loadloack, load, process file, unload, and an end step. After changes are made, Save the batch.

Note: When a batch file is modified, only the copy on the disk is changed. If the batch is already loaded, it will need to be reloaded for the changes to be reflected.

  • Pump Loadlock- This will be the first step in the batch. It is automatically included
  • Load Chamber- To add a load chamber step, clik on the button with the arrow pointing to R or L depending on which chamber you want to use. R stands for right and L stands for left. A process step will be added after the load step automatically. You will be prompted for a recipe to run, so make sure that you have created/modified your recipe accordingly prior to doing this.
  • Process- A process step runs a recipe you have already written. Add a process file by pressing the appropriate process button for the chamber you are using.
  • Unload The unload step moves the wafer / sample back into the loadlock. Press the button with the arrow pointing to the lock to add an unload step.
  • End/Vent There are two types of end steps: vent and end. A vent step will vent the loadlock automatically and an end step will not. The last step in a btach must be an end or vent step.

Process File

In general, a process file has four segments: initial step, gas stabilization step, process steps, and an end step. The initial step evacuates the chamber, removing the air inside, and brings the chamber to the process temperature. The gas stabilization step fills the chamber with the process gasses before RF power is applied. Because the RF power is off, no processing will actually take place during this step. The process steps perform the processing steps with the RF power turned on. The end step pumps out the process gases before the wafer is unloaded.

Note: Process files for the Bosch process contain multiple process steps (deposition, etch) and also contain a loop.

To edit a process file from within the Batch Editor, double click on the process filename. Or from the Process menu, choose Chamber, Edit to edit an existing recipe. A list of the recipe steps will be displayed on the right side of the screen. To edit a step, double click on it. After changes are made, Save the process.

Note: When a process file is modified, only the copy on the disk is changed. If the process is already loaded, it will need to be reloaded for the changes to be reflected.

  • Initial Step- All recipes start with an initial step. This step will evacuate the chamber and bring the chamber to the desired operating temperature. Set the pressure to 1.0 x 10-2 Torr and set the time to 30 seconds. This will cause the system to evacuate as much air as possible from the chamber before starting the process. Do not attempt to use pressure setpoints below 1.0 x 10-2 torr. The chamber will be evactuated as much as possible regardless of the setpoint. Set the temperature to your desired process temperature. Note that the temperature is measured in degrees Celsius. You must enter a name for your process in the Name box. The name may be up to 8 characters long, but MUST be a DOS-compatible file name -- that is, no non-alphanumeric symbols besides - and _ You must describe your recipe in the description box. The first few words will be displayed by the filename when you are loading your recipe.
  • Process Step- The process step dialog box has five major areas: time, temperature, pressure, gas flow, and power.
    • Time- Controls the time of the step. Use "Fixed Time" to set the time. Do not use the "Variable Time" option, since it does not work on this system; if chosen, it simply does the same thing as the "Fixed Time" option. If you want to change the run time for your process, it must always be done here.
    • Temperature- Set the temperature to your desired process temperature. The temperature should be the same for all process steps.
    • Pressure- Set the pressure to the desired process pressure. Note that the Bosch process has different pressures for each step.
    • Gas Flow- Set the flow rate for each gas here. The flow rates are given in sccm (standard cm3/min). DO NOT exceed 90% of the rated capacity for any MFC
    • Power- Set the RF power supplies to the desired power.
  • End Step All recipes have an end step that evacuates the chamber. Set the pressure to 1.0 x 10-2 torr and set the time to 30 seconds.

VII. Troubleshooting

Q:The menu option I want is grayed out. Why?

A: Some menu options are not available depending on which mode the system is in. Try switching between the ON, STANDBY, and READY modes with the buttons at the bottom of the screen.

Q:The alarm is going off. What do I do?

A: To silence the alarm, press the ALARM SILENCE button at the lower right corner of the screen. At the bottom center of the screen, there is a box labeled "Alarm" that will tell you why the alarm is going off. Unless you understand exactly what caused the alarm and you can solve the problem, contact MiRC staff immediately.

Q:The backside helium cooling parameters do not seem normal (i.e. high flow and low pressure). How do I fix this problem?

A: Abort the process, unload the wafer, and check to make sure the back of the wafer is flat and clean. If so, try again. If the problem persists, contact MiRC staff.

Q:A process parameter (e.g. RF reflected power, chamber pressure, etc.) does not seem normal. What do I do?

A: Abort the process and contact MiRC staff.

Q:My wafer broke in the chamber. What do I do?

A: Wafer fragments that remain in the chamber will severely damage the ICP. Put a "machine down" sign on the system and contact MiRC technical staff immediately.

VIII. Suggested Practices

Mask Materials

  • Left chamber: photoresist
  • Right chamber: photoresist, silicon dioxide, silicon nitride
  • !! STRONGLY RECOMMEND AZ4620 RESIST !!

Mounting Recipe

  • Use AZ4620, spin at 1500rpm for 30 seconds with 500rpm/sec ramp rate.
  • Place sample onto wet PR, cure for a minimum of 10 minutes at 105C on a hot plate at 105C.
  • Resulting PR thickness is ~13.5 microns.

Curing Recipe and Rules

  • All samples with photoresist MUST be cured sufficiently before processing in the ICP. Poorly cured photoresist leads to sputtering of photoresist into the chamber that adheres to the chamber walls, which in turn creates a polymer buildup that can fall onto and destroy your wafer. Improperly cured samples will also stick to the clamp in the left chamber, which results in them being stuck in the left chamber and can get broken very easily by the clamp.
  • Placing poorly cured photoresist into the system willl result in your loss of access to the system and/or suspension from the cleanroom!! Much longer times (e.g. 30 minute cure in 120C oven) are required for Shiply 18** resists.
  • A good rule of thumb is to postbake wafers with AZ4620 on a hot plate at 105C for a minimum of 10 minutes before processing. Much longer times (e.g. 30 min in convection ovens at 110C) is required for Shipley 18** resists. Also, other resists use different parameters -- consult with someone who has used this resist before putting it into the system to find out how to cure it properly.

IX. DO's and DON'Ts

DO

  • CURE YOUR SAMPLE PROPERLY - Good rule of thumb: At least 10 min at 100C (AZ series) and at least 25 min at 115C (Shipley)
  • always mount your wafer if you may come within 30 um of etching through the entire wafer.
  • make sure the back of your wafer is clean of PR and other contamination.
  • check the He cooling, plasma colors, and reflected power to ensure the machine is functioning properly.
  • make sure you pump down the machine after use.
  • ask for permission from the MiRC staff to etch polymers.
  • take notes of EVERY run so that problems may be identified as soon as possible.
  • report any cracks, chips, or broken wafers IMMEDIATELY to the staff

DO NOT

  • use metal as a mask or etchstop.
  • use SU8 in the machine.
  • Use polymers other than photoresist in the right chamber.
  • Use polymers other than photoresist in the left chamber that have not been approved by Gary.
  • Etch through a wafer without mounting it to another wafer.
  • Use any substrate other than a 4" wafer.
  • Stare into the windows of the etching chamber.
  • Put uncured or incompletely cured photoresist into the machine.
  • DO NOT do more than one thing at once with the software - IT WILL CRASH. (e.g. do not vent the chamber or run a process and at the same time edit a recipe).

X. Check-off requirements

A basic understanding of how the machine works.

  • How does the ICP etch a material?
  • How is the ICP different than an RIE?
  • Why there are 2 power sources (RF1 and RF2)?
  • What is the Bosch process?
  • What is the He cooling chuck?
  • How are wafers held in place in each chamber?
  • What is the difference between a batch and process file?
  • Be able to explain the steps of the batch screen.
  • Be able to explain the steps of the process screens.

What can and cannot be etched in each chamber?

What is the largest and smallest size wafer that can be used?

Photoresist and Mounting of samples

  • Why does photoresist need to be fully cured before ICP etching?
  • When is it necessary to mount a wafer?
  • What is a good rule of thumb for curing/mounting a sample?

Be able to load and run a sample and explain what needs to be observed during the start of a run:

  • He Cooling
  • Plasma Color
  • Reflected Power
  • Identify Gases

Be able to abort a process correctly.

Explain how the machine should be left when finished.