NANOELECTRONICS FABRICATION FACILITY (NFF), HKUST

NANOSYSTEM FABRICATION FACILITY (NFF), HKUST

Copyright © 1.2016 by Hong Kong University of Science & Technology. All rights reserved.

Standard Operating Manual ___________________________________________________________

Xenon Difluoride (XeF2) Etching System



Contents

1. Picture and Location

2. Process Capabilities

2.1 Cleanliness Standard

2.2 Possible Etching Materials

2.3 Process Specification

2.3.1 What the XeF2 Etching System Can Do

2.3.2 What the XeF2 Etching System Cannot Do

2.4 XeF2 Etching System Process Useful Information

3. Useful Information to Work in NFF

3.1 Emergency Responses and Communications

3.2 Become a Qualified XeF2 Etching System User

4. Operation Safety and Rules

4.1 Operation Safety

4.1.1 General Safety

4.1.2 Equipment Safety

4.2 Operation Rules

5. System Operation

5.1 XeF2 Etching System Descriptions

5.2 Initial Status Check

5.3 Initial System Check

6. Introduction to XeF2 Operation Control

6.1 Auto Mode Operation

6.2 Program Management

6.3 Manual Mode Operation

6.3.1 Chamber Evacuation

6.3.2 Chamber Vent

6.3.3 Chamber Lift Up/Down

6.4 Override/Setpoint

6.5 Sample Load/Unload

6.6 XeF2 Lecture Bottle Operation

7. Process Recording during Process

8. Clean Up

9. Check Out

https://snf.stanford.edu/SNF/equipment/metalization-sputtering/evaporation/innotec-es26c-e-beam-evaporator/#section-9



XeF2 Etching System

1. Picture and Location

Fig.1 XeF2 etching system

The XeF2 etching system is located at NFF Phase II cleanroom, Room 2240.

2. Process Capabilities

2.1 Cleanliness Standard

XeF2 etching system is classified as CLEAN equipment.



2.2 Possible Etching Materials

Silicon substrate prepared in CLEAN cleanliness standard.

2.3 Process Specification

2.3.1 What the XeF2 Etching System Can Do

One pieces of 4 inches full silicon substrate.

Small samples must be place on an oxide wafer in CLEAN cleanliness standard.

2.3.2 What the XeF2 Etching System Cannot Do

Substrate that generate particles during etching process.

Substrate that contains harmful substances.

Small samples are tiny and can be pump away.

2.4 XeF2 Etching System Process Useful Information

The XeF2 etching rate is strongly depending on the etching area.

Typical etch rate for less than 10% exposed area is around 6000A/cycle.

Typical etch rate for more than 90% exposed area is around 600A/cycle.

Typical etching uniformity is less than 20%

Oxide is the typical masking material used.

However, XeF2 has an extremely high selectivity to oxide. Therefore, user must pay

special attention to the pre-treatment of exposed silicon surface. i.e. HF solution dip

to remove the native oxide on the silicon surface.

Since the usage of this system is low, please pump out all the XeF2 gas remain in the

expansion chamber in order to make sure the gas use is suitable for etching.

At least 10-20 cycles dummy run is required to remove the gas in the gas cylinder.

An oxide dummy wafer must be used for holding small samples.

3. Useful Information to Work in NFF

3.1 Emergency Responses and Communications

In case of emergency issues, please contact NFF staffs,

Preason Lee – Deputy Safety office (x7900),

https://snf.stanford.edu/SNF/equipment/metalization-sputtering/evaporation/innotec-es26c-e-beam-evaporator/#section-9



CK Wong – senior technician (x7226)

In case of technical help, please contact NFF staffs,

CK Wong – senior technician (x7896)

Casper Chung –technician (x7896)

Brial Kwok – technician (x7896)

3.2 Become a Qualified XeF2 Etching System User

Please follow the procedures below to become a qualified user of the etching system:

Read all materials provided on the NFF website of the system.

Request XeF2 etching system operation training and examination online.

4. Operation Safety and Rules

4.1 Operation Safety

4.1.1 General Safety

XeF2 etching system user must familiar themselves with the following general safety

issues:

Location of emergency exits and assembly points

Procedures for obtaining first aid assistance must be known.

Various alarm sounds and emergency call procedures must be known.

4.1.2 Equipment Safety

XeF2 etching system user must be aware themselves of the following equipment safety

issues:

XeF2 is sensitive to the moisture; it forms hydrofluoric acid fumes when exposed to

air. A serious health hazard will be caused while inhalation or skin contact of the

fumes. In order to avoid the hazard from using the XeF2 etching system, user must be

aware of the following issues:

- To open the XeF2 cylinder, hold the cylinder with one hand and open the shutoff

valve gently with another hand.

- Close the XeF2 cylinder immediately after use.

- Do not attempt to change the XeF2 lecture bottle.



- The samples must be well baked before placing in the process chamber.

- Do not expose the process chamber to the atmosphere for long time to prevent

excess humidity.

- Do not skip any purge steps in the recipe.

There is an individual F2 gas detector attached to the system (Fig.2). In case there is

any XeF2 gas leakage, the detector will be alarmed LOCALLY, please leave the

laboratory with all the users immediately and inform NFF staff. The alarm 1 level is

1.0 ppm, and alarm 2 level is 2.0 ppm.

In emergency when using the equipment, please switch off the electrical socket

power button (Fig. 3) to interrupt the equipment power, and report to the NFF staffs

immediately. DO NOT attempt to resume the equipment on before the problem is

solved.

Be careful to the components of the equipment, which involves electric power,

mechanical injury and dangerous gas hazard.

Fig.2 XeF2 gas leakage detector



Fig.3 Electrical power socket

4.2 Operation Rules

Do not operate the equipment unless you are properly trained and authorize to

operate the equipment.

Reserve the equipment in NFF website in advance.

Reservation will be forfeited if user never shows up and check-in within half of the

session time and it may cause a $200 penalty.

Fill all the details of the log-sheet attached, i.e. date, name, project number, email,

project details, material …

Do not leave an on-going etching process unattended.

5. System Operation

5.1 XeF2 Etching System Description

The XeF2 etching system is a table top silicon etching machine. Because of the

particular characteristics of Xenon Difluoride gas, it shows nearly infinite selectivity to

silicon dioxide, and mainly used as an isotropic gaseous etchant for silicon, this

particularly an ideal solution for releasing MEMS devices. Therefore, the XeF2 silicon

etch can avoid many of the problems typically associated with wet etch processes.

The XeF2 etching system mainly consists of three components; etch chamber, expansion

chamber and the XeF2 lecture bottle, as shown in figure 4. The etching process is

https://en.wikipedia.org/wiki/Etching

https://en.wikipedia.org/wiki/Silicon



running in cycle steps instead of process time. Firstly, XeF2 gas from the lecture bottle

will be delivered into the expansion chamber until set pressure (0.3Torr). Secondly, the

gas will be delivered into the process etch chamber until designed pressure (2.5 or 3.0

Torr) or maximum 300 seconds. Finally, the etch process will be keep for designed step

time 30 seconds. Then, the process chamber will be evacuated to vacuum. At this point,

one cycle step is completed, and the entire process will be repeated for the remaining

cycles set in the recipe.

Fig.4 XeF2 Etcher Flow Diagram

5.2 Initial Status Check

Users are required to do the following checks before doing the sputter process:

Please check the equipment status from the shutdown notice board in the NFF

reservation website.

If the equipment has been reserved, then check the user name and project number

that displayed on the dedicated LCD screen are correct (Fig. 5).



Please check-in the reserved equipment on your own within half of the session time.

To do the check-in, please scan your NFF access card over the card reader attached.

After checked-in the equipment, the red LED on the card reader will be ON.

If you are failed to do the check-in, there is an interlock, and you cannot operate the

equipment normally.

Before operate the equipment, make sure you have read and ready to fill the details

of the log-sheets attached.

Fig.5 Reservation and Equipment check-in status LCD

5.3 Initial System Check

Check the readout of F2 gas detector is 0.0 ppm.

Visual check any gas leakage from the equipment.

Check any abnormal sound coming from the equipment.

Check mechanical pump is turned on.

Check the PLC controller is logged in the Main screen, as shown in Figure 6.



Fig.6 Main screen

6. Introduction to XeF2 Operation Control

6.1 Auto Mode Operation

To access the Auto Mode, please click on the Auto mode button in the main screen (Fig.

6). In Auto mode, recipe details can be edited and auto operation can be performed.

Details of the Auto Mode recipe file is shown in figure 7. In the recipe, step 1 to step 8

are the pre-purge steps. Then, step 9 to step 12 are the etch cycle steps. Finally, step 13

to step 18 are the post-purge steps.

To run the auto process, press the IDLE and then the START button, then the process

will start to run from step 1 to step 20 with designed etch cycle loops. A PULSE button

can be used to hold the running process, and NEXT button can be used to skip the

current running step to the next. However, there is a LOOP button to access the loop set

point page (Fig.8). In the loop set point page, user can define the number of loop cycles

depends on the etch depth. The loop starts at step 9 and ends at step12, and will be



repeated until all loops run. To set the loops, click on the “number”, and then the digit

keypad will be pop-up. In the digit keypad, key-in the loop number digits, then press the

ENT key and ACCEPT button. However, the loop 0 is the initial loop to be run, i.e. 1

loop set equal to 2 loop cycles. In addition, the loop set point page is individual to the

program page, i.e. the loop set point set in program file 2 is applicable to the program

file 3.

After setting the number of loop, then click the BACK button back to the Auto mode

page.

Fig.7 Auto mode operation



Fig.8 Loop set point page

6.2 Program Management

To access the program management, please click on the Program Management button in

the main screen (Fig. 6). In Program Management, Recipe can be checked and edited as

shown in figure 9. In the recipe editor, the only available etching recipe is program File

2, and XeF2 user is not authorized to make any changes in the recipe files. For safety

reason, user is required to run program File 3 to cycles purge the chamber after XeF2

etching process before unloading the sample.



Fig.9 Program Management

6.3 Manual Mode Operation

In order to access the manual mode operation, please click on the MANUAL Mode

button in the main screen (Fig. 6). In Manual mode screen, chamber vent and evacuation

can be executed using the PV1 & PV2 buttons, chamber lift up/down can be executed

using Y7 button, as shown in figure 10.

However, XeF2 user is strictly prohibited to operate the PV3, PV4, PV5 & PV6 buttons,

otherwise there will be risk of hazardous XeF2 gas leakage, and could result in personal

injury. In addition, there are two pressure gauge readouts, one for chamber pressure (AI1)

and one for expansion chamber pressure (AI2).

Details of the manual operation will be described in the following sections.



Fig.10 MANUAL Mode screen

6.3.1 Chamber Evacuation

To evacuate the chamber, press PV1 “Manual On” button on the screen, then the

chamber will be evacuate. Once the “Manual On” button is pressed, the PV1 will

become red in color, otherwise it is green. The chamber pressure can then be read from

the display, “Chamber Pressure”. Typically, a 0.01Torr< base pressure can be reached

within a few minutes time.

6.3.2 Chamber Vent

To load/unload sample into the process chamber, press the PV1 “Manual Off” button to

stop the chamber evacuation. Then, press PV2 “Manual On” button, and the chamber

will be vent to atmosphere pressure. Finally, press the PV2 “Manual Off” button to turn

the vent valve off if the chamber has been vented.

6.3.3 Chamber Lift Up/Down

To lift up the chamber top cover, the chamber must be completely vented. To make sure

the chamber is completely vented, a minimum 5 minutes venting time is required. Once



the chamber is vented, Y7 “Manual On” button can be pressed to lift the chamber top

cover. Then, Y7 “Manual Off” button can be pressed again after samples load/unloading.

Once the cover down to its original position, the chamber can be evacuate again.

6.4 Override/Setpoint

XeF2 user is not allowed to access this page, please do not click on the button.

6.5 Sample Load/Unload

For safety reason, the chamber was built inside a plastic housing with full exhaust and

gas leakage detection. Therefore, please do not open the housing door immediately once

the chamber top cover lifted up. Wait 10 seconds for detecting any residue gas from the

process chamber. If there is any residue process gas detected, the detector will be

alarmed, then leave and inform NFF staff immediately. To open the housing door, push

the knob on the door latch as show in figure 11. Once the door open, then wafer can be

load/unload in the center of chamber. However, user must check the availability of the

o-ring attached to the gas inlet pipe (Fig.12). If the o-ring was gone, please inform

module technician, otherwise the process parameters will be change.

Fig.11 Chamber housing door



Fig.12 Process Chamber

6.6 XeF2 Lecture Bottle Operation

For safety reason, XeF2 lecture bottle is mounted inside the system. To open the lecture

bottle, firstly open the system side door (Fig.13a), then hold the lecture bottle with one

hand, and turn the bottle shutoff valve anti-clockwise slowly (Fig.13b).



Fig.13a System side door Fig.13b XeF2 lecture bottle

7. Process Recording during Process

Please be reminded you are required to fill all the details of the log sheets. If users

fail to do this, a punishment will be given.

Write down any problems or comments in the log sheets

8. Clean Up

Clean up the area of the equipment.

Close the shutoff valve of XeF2 cylinder.

Close housing door and system side door.

Pump down the chamber to 0.01Torr.

To avoid oil mist back streaming to the chamber, stop the pumping once 0.01Torr

base pressure achieved.

9. Check Out

Check out the equipment immediately after use.

NANOELECTRONICS FABRICATION FACILITY (NFF), HKUST

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