Skip to content. | Skip to navigation

Sections
Personal tools

EVG 101 Spray Coater (evgspraycoat)

The EVG 101 spray coater allows pieces up to 6" wafers to be spray coated with photoresists and potentially other types of polymers or atomizable materials.
Contents

 

Tool Location

Spraycoat locationMachine image

 

Background

 

Introduction

There are increasing cases in which it becomes necessary for resist film deposition techniques to meet the requirements for coating high aspect ratio features[1], wafer-level via interconnections (shape and geometry)[2], highly integrated device structures[3], and in general the 3D microstructures found in MEMS, NEMS, and micro-systems industries[4]. The EVG101 Spray Coater offers some main process differences from spin-coating, listed below, for those requirements (also see section on standard run recipes for specs):

  • low to zero spin speed needed
  • little to no edge bead formation
  • no striations, streaks, or pinholes for extreme topographies
  • a relatively high coating uniformity
  • uses relatively little resist material
  • wide range of diluted resist mixtures can be used/easily changed

 

Physical Description

Hardware (w/ location):

 

  • 2 ultrasonic spray nozzles (right):  Vortex nozzle: planar coats, ~60° - 90° spray angle; Accumist nozzle: conformal coats, ~10° spray angle
  • adjustable alignment paddle (back)
  • main chuck: for 3 & 4 inch wafers (center), with plastic chamber ring
  • syringe pump for resist (right): ~15 – 20mL to fill the line
  • pressure regulators (back top) & flow valve (back bottom): BSR, EBR, bowl washer
  • syringe holder (left)

Software:

  • recipes are independent
  • drag and drop features or to copy existing recipes and modify upon them
  • 3 levels of operation: operation (run recipes), engineer (modify recipes), staff (switch nozzles)
  • main tabs (bottom of screen): “Recipes” - tables to create or modify recipes, “Jobs”-  GUI to perform actions (i.e. run a recipe, clean)

 

Prohibited Actions *IMPORTANT*

This is a short, non-comprehensive list of critical actions you must avoid doing with the tool. Do follow other cautions learned from the tool documentation or during training that are not part of this list.

 

One strike and you're off the tool (No exceptions)

1) Never let resist dry in the line. If you need to step away from the tool with resist in the line for more than 30 minutes, you must clean the line with acetone before you go. If you are done spraying, you have 30 minutes to clean out the line with acetone.

2) Never power down the computer.

 

Three strikes and you're off the tool

1) Never leave the tool without wiping down the chuck and bowl with acetone.

2) Never close the software. Refer to the Troubleshooting section to restart the software if it freezes up.

3) Never carry coated, un-baked wafers outside the machine without using a closed container.

 

 

Science of Spray Coating

 Resist Mixtures[4]

For a proper droplet size distribution from the nozzle, the solution being dispensed should have a viscosity of less than 20cSt. Common photoresists can be diluted with resist compatible solvents with same/similar chemistry as mean resist solvent, no chemical reaction with the resist or during the exposure, and a high vapor pressure. For example, a standard mixture used includes a combination of AZ/SPR, MEK/MBIK, and PGMEA.

There is a trade-off between resist surface smoothness and flow characteristics. Best results across severe topography are obtained with poor resist flow after deposition. This, however, leads to a certain roughness of

the resist surface. The surface of a several micron thick photoresist does not influence plating and hence is of no vital interest. In general, the resist mixture for conformal coats should be drier than for planar coats (see “Specs on Standard Runs”).

 

Coating Features[3]

As a liquid on non-planar surfaces, the resist tends to pull back from sharp topography edges with most coating techniques. Two forces are responsible for the effect (Fig. 1). On the one hand the gravity reduces the film thickness on the edge, since the resist tends to flow down the slope. On the other hand the surface tension of the resist pulls the resist back from the corner and leads to a bead formation.

In a corner we see the opposite effect. The liquid resist flows down the slope and material fills the corner. The formation of resist in the corner is defined by the surface tension of the used material, where the resist tries to form an ideal surface, which is as small as possible.

The spray coating technique presents a successful reduction of this behavior, considering viscosity of resist solution and spray contact angle. It has also been demonstrated to do uniform planar coats and conformal coats across features such as 90° step corners, KOH-etched cavities, V-grooves, trenches, micro-lenses, and through-silicon-vias[1]. The deeper the step is the more difficult it is to guarantee same line width on the top and on the bottom of the structure. A continuity in line width has been demonstrated over a

Fig. 1. Schematic drawing for a liquid behavior

150µm step.

Fig. 1. Schematic drawing for a liquid behavior

Schematic of liquid in a corner activity

  Spray Coating Dynamics[4]

A key component to the spray coating technology is the ultrasonic spray nozzle, whose oscillation frequency is ~100kHz. This generates small resist droplets with a mean distribution of ~20mm in diameter (Fig. 2). The drops are accelerated with a CDA or Nitrogen stream. 

 Droplet diameter

The uniformity can be adjusted from the velocity profile of the spray nozzle over the substrate (Fig. 3). The nozzle passes over the slowly spinning wafer (e.g. ~30rpm) either from edge-to-edge or edge-to-center, and returns to the same point for the second, third passes and so on. For the planar coat, the Vortex nozzle has a wide spray angle for large-scale uniformity, and for the conformal coat, the Accumist nozzle has a narrow spray angle for coverage of severe topography (e.g. > 10mm and ~300mm vertical height/depth). Industry requirements set the usual range of resist coating thickness at ~3 - 10mm, and a standard spray coat run is ~5mm, adjustable with resist mixture, velocity profile, dispense rate, and number of passes/run. An electrical yield of 94.8% has been achieved with spray coating for optical wafer-level packaging applications, given that the shape of the via (a significant factor in the coating result) does not have overhangs on top and has a constant sidewall angle (this yield was demonstrated with a sampled 68° via sidewall angle)[2].

Nozzle and wafer motion

 

 Operational Manual

 

 TOOL ACQUAINTANCE

 The tool and its components are diagrammed below.

 Machine componentsSpecific machine components

START-UP

 

I) INITIAL CHECKS

       a)     Check availability on Badger and reserve a time

       b)    If machine is off, do not reboot.  Contact member on the “contact list”

       c)     Check if your desired nozzle (accumist or vortex) is placed on the machine.  If not, and you have not been trained to change nozzles, do not do so.  Contact member on the “contact list”

 

II)  PREPARING PHOTORESIST MIXTURE

       a)     A scale and dedicated clean beakers are stored in the storage chest beside the spray coater, in the drawer labeled “tools”

       b)    Mix your desired photoresist mixture.  It is recommended to do so by mass starting with solvents first.

       c)  Give mixture 24 hours to sit before use to insure uniform mixture.

 

TOOL SET-UP

 

I)  Ensure that the desired dispense nozzle is loaded on the machine.  If not, and if you are trained to do so, change the nozzle.

II)  Heated Chuck

Use of the heated chuck can help with resist coverage on sharp corners.  From a room-temperature start the chuck takes roughly 35-40 minutes to reach its maximum set point of 80C.

       a) Power on the controller for the chuck heater with the toggle switch on the top.

       b) Turn on the bath with the touch screen (lower left-hand corner)

       c) Select the set-point point on the touch screen and use the arrows to set the temperature set-point (MAXIMUM is 80C).

       d) Allow the heater proper time to reach the set-point.  Both the actual temperature and the set-point are displayed on the touch screen.

III)  Load syringe with photoresist

        a)    Available syringes are located in the storage chest beside the tool.  You may only use syringes located in the top two draws – these are either brand new or have only been used once.  DO NOT USE SYRINGES DEDICATED FOR ACETONE WASH.

        b)    If no syringes are available, contact member on the contact list.

        c)    Bring syringe to lithosolvent bench and pour desired amount of photoresist into beaker.

        d)    Draw photoresist into syringe. 30 ml or more is recommended, as the machine occasionally enters a fault mode with a low volume of source material.

        e)    Withdraw syringe from beaker and quickly invert.  Place red cap over the tip.  THE CAP WILL NOT STOP THE SYRINGE FROM LEAKING, SO KEEP SYRINGE IN INVERTED POSITION.

Inverted Syringe for loading

 

        f) Keeping syringe in inverted position and wrapping a cleanroom wipe over the tip, walk over the machine, and push-fit the resist line onto the tip of syringe.  Throughout this entire procedure, keep the syringe inverted.

Syringe hookup

 

        g)     On the computer control’s screen, select “pumps” from bottom toolbar.  On the next screen, select the pump in the top left corner, and choose “change material”

        h)    Using the motor controls, hit “up” button until the syringe can fit easily within the holder.

         i)     It is now safe to invert the syringe around to its original downward orientation.  Slide the syringe into the holder.

  Loaded Syringe with plunger up

 

       j)     Using the motor controls, hit the “down” button until the plunger touches the top of the syringe.  Flip the red lever to the right, so that the plunger pushes on the top of the syringe.

Loaded Syringe with plunger down

 

        k)     Flow the photoresist in the photoresist line.  Tap the “down” button on the motor controller until resist has traveled the distance of the whole line and has entered the nozzle, and no air bubbles are left in the line. 

         l)    On the computer control screen, select “okay” to having finished changing the material.

 

Coat Wafer

I)  Select “recipe” from lower task selection on computer control, and either edit or make a new recipe. Refer to the "Making Recipes" section later in this documentation to set up your recipes.

II)  Select “jobs” from lower task selection on computer control, and select “start.”

III)  It will prompt you to put a wafer on the raised chuck. 

      a)     Slide wafer onto chuck, making sure the end it pressed against the aligner. 

      b)    The flat part of the wafer cannot be the part pressed against the aligned, or the wafer will not be centered.

Wafer alignment

 

IV)  Be sure that the door to the chamber is closed.  Select “okay” on the computer controller to run the recipe.

 

Post – Coating

I)  Wait 1 minute before opening door for fumes to dissipate.

II)  Open door, and take wafer off of chuck and place in wafer holder.

      a)     Do not tilt wafer while moving or storing it before baking.  The resist will run on the surface.

III)  If you want to coat another wafer, repeat from ( 4) Coat Wafer: step I )

IV)  If you are finished begin clean-up.

 

Clean-Up

I)  Remove all wafers from wafer holder to a hot plate to bake.  While moving the wafers, be sure to have them covered – using either the wafer carriers (stored in the storage chest) or your own.

II)  Removing syringe

     a)     On the computer control’s screen, select “pumps” from bottom toolbar.  On the next screen, select the pump in the top left corner, and choose “change material”

     b)    Using the motor controls, hit “up” button until the syringe can be easily removed from the holder.

     c)     Slide syringe out of holder and invert – keeping the tip pointed up. 

     d)    Pull the resist line out and place in holder, located under the syringe holder.

     e)     Place cap on syringe, and while inverted, carry over the lithosolvent bench.  Squeeze out remaining resist into disposal.

      f)      Fill up beaker with acetone.  Re-load syringe with acetone 3 times, and 1 time with IPA.  Finally, re-load the syringe with acetone, invert, and carry back over to machine.

III)  Acetone Wash

     a)     Referring to ( 3) Tool Set-up: steps g-j) place the acetone syringe in the machine. 

     b)    Keep pressing down button on motor controller until acetone has completely filled the line and is running out of the bottom of the nozzle.

     c)     Exercise the head by running a "Dummy Dispense." To do this:

          i) Change to the "Jobs" screen.

          ii) Left click on the arm you used (most likely the vertical bar just to the right of the circle in the center that looks like a bowl).

          iii) Left click "Dummy Dispense."

          iv) Choose the nozzle you coated with (most likely Accumist) and click "Ok."

          v) Set dispense volume, dispense rate, and cycles to: 500 uL, 10 uL/ , and 1.

          vi) Click "Dispense."

          vii) When prompted, set the N2 pressure to the maximum, 2000 dl/min.

          viii) Wait for the prompts in the terminal window to declare the dummy dispense is finished.

     d) Unmount the syringe.

     e) Bring the syringe to lithosolvent bench and discard the extra acetone in the carboy.

     f) Dry the syringe at lithosolvent bench by pumping air in and out of it gently.

     g) To dry the line, suck 30-40 mL of air into the dry syringe by hand, load the syringe in the machine, then push the air out through the line using the "down" button.

     h) Remove syringe, place syringe in zip-lock bag, and place in the drawer beneath the one you took it from (if it was brand new, place in 1-time used syringe drawer, if from 1-time used syringe drawer, place in acetone syringe drawer).

     i) Take acetone bottle and spray on a clean wipe.  Wipe the inside of the bowl and chuck until all resist has been cleared.  Dry with clean wipes.

 

How to Become a User

1.  "All Litho" is required before training on any of the lithography tools. Contact the trainers for the class schedule and training materials (which must be reviewed before the class).

2.  Read all material on the SNF website concerning this tool, including Background, Process Capabilities, Operating Procedures and Process Monitoring.

3.  Shadow a current user (check Badger for reserved slots and a list of currently trained users).

4.  Contact Mahnaz (mahnaz@) to take the written examination for the tool.  This is a short, closed notes/books/webs quiz.  Be sure to include the name of the user you shadowed.

5.  Upon passing the written examination, contact Mahnaz to schedule a qualifying exam.  During this exam you will demonstrate your knowledge of how to run the system on your own.  Upon passing your qualifying exam, you will be receive full user privileges to the machine.

General Recipe Trends

Some noted trends on adjusting parameters for recipes are listed below.  These will be useful for troubleshooting your process.  (Supplied by EVG.)

ISSUE Probable Cause(s) Probable Solution(s)

Pinholes 

  • Film too dry

  • Not enough material

  • Increase the ratio of low vapor pressure solvent to high vapor pressure solvent.

  • Increase the number of passes (decrease solids content) or low dispense rates more passes.

Pull Back 

  • Film too wet

  • Too high dispense rate 

  • Decrease the ratio of low vapor pressure solvent to high vapor pressure solvent.

  • Lower the dispense rate (need to adjust the recipe to achieve the same thickness).

Edge Pull Back

Film too wet 

  • Decrease the ratio of low vapor pressure solvent to high vapor pressure solvent.

  • Use the heated chuck.

 Bottom Topography
Not Covered
  • Nozzle pressure too high
  • Dispense rate too high
  • Topography too severe 
  • Decrease nozzle pressure
  • Lower dispense rate
  • Possibly beyond capabilities 
 Film roughness more macro than micro
  • Incorrect dilution
  • Nozzle pressure too low 
  • Play with dilution
  • Decrease nozzle pressure 
 Center to edge
thickness variation
  • Spray profile not correct
  • Nozzle off center 
  • Adjust profile as needed (slower for thicker; faster for thinner).
  • Adjust nozzle to ensure arc is over the center of the substrate. 
 Large resist droplets
  • Improper ultrasonic settings
  • Mixture too viscous 
  • Adjust ultrasonic power
  • Mix new dilution (more solvent to lower solids content) 
 Air bubbles
  • Improper ultrasonic settings
  • Improper mixture
  • Adjust ultrasonic power.
  • Make new mixture. 
 Extremely thin at center Material too fluid and is
being pushed away in center
  • Decrease the ratio of low vapor pressure solvent to high vapor pressure solvent.
  • Increase speed at center of the wafer.




[1] Cooper et al. “Conformal Photoresist Coatings for High Aspect Ratio Features” (2007).

[2] Shariff et al. “Via Interconnections for Wafer-Level Packaging” (2007).

[3] Fischer, Suss. “Spray Coating – A Solution for Resist Film Deposition Across Severe Topography” (2004).

[4] Weider et al. “Spray Coating for MEMS, NEMS, and Micro Systems” (2002).

 

Suggested Starting Recipes

Planar Coat  (uniformity = +-5% across surface of wafer)

Vortex nozzle:

• Recipe: MY1

•mixture:  7.5% SPR 220-7, 68% MEK, 24.5% PGMEA

• Coat Thickness: ~3um per 7 passes

• Dispense rate (DR): 20 uL/s

• Nozzle pressure (NP): 100 mbar

• Spin speed (SS): 30 rpm

• Suckback rate (SR): 10 uL/s

• Suckback volume (SV): 35 uL

• Velocity profile (VP): 1-100, 2-300, 3-600, 4-1100, 5-1700, 6-1100, etc.

• VP Indices: 9

 

Accumist nozzle:

 • Recipe: MY2

•mixture:  7.5% SPR 220-7, 68% MEK, 24.5% PGMEA

• Coat Thickness: ~8um per 6 passes

• Dispense rate (DR): 20 uL/s

• Nozzle pressure (NP): 100 mbar

• Spin speed (SS): 30 rpm

• Suckback rate (SR): 10 uL/s

• Suckback volume (SV): 35 uL

• Velocity profile (VP): 1-200, 2-300, 3-500, 4-700, 5-900, 6-700, etc.

• VP Indices: 9

 

Conformal Coat

• Recipe: MY3, Nozzle: Accumist

•mixture:  6.5% SPR 220-7, 68% MEK, 25.5% PGMEA

• Coat Thickness: ~5um

• Dispense rate (DR): 10 uL/s

• Nozzle pressure (NP): 100 mbar

• Spin speed (SS): 30 rpm

• Suckback rate (SR): 10 uL/s

• Suckback volume (SV): 35 uL

• Velocity profile (VP): 1-200, 2-300, 3-400, 4-800, 5-1800, 6-800, etc.

• VP Indices: 9, Passes: 7

• Heated Chuck = 800 C

 

 Pre-Coating Treatments

It has been found experimentally that it is better to treat a Si wafer with HMDS, whereas SiO2 wafers are better off without any HMDS treatment.  See the images below for evidence.

HMDS tests

 

Making Recipes

It is recommended that users copy working recipes as a starting point rather than build them from the ground up. This is because this is most efficient, but also because there are a few initial steps in each recipe that should not be omitted. You will need "Engineer" rather than "Operator" status for your login to view the "Recipes" tab.

 

Copying a bank of recipes into your own

To copy and save a "bank" (or "set") of recipes:

  1. Select File > Open.
  2. Choose a bank of recipes to open (Recommended: the most recent "SNF Defaults," "stanford spray," or a user name who's run recipes you will be emulating).
  3. Once the correct bank of recipes is open, choose Save As and input an appropriate name. Please do not overwrite "SNF Defaults."

 

Copying a single recipe

To copy a single recipe:

  1. Right click on a recipe and select the "Copy" command in the pop-up menu with a left click.
  2. Right click on the folder in which you would like to create a copy of the recipe and left click the "Paste" command. The folder is likely at the top of the list of recipes. The recipe will be named in the format "Copy of <old recipe name>" and you can right click the recipe and choose "Rename" command to give it an appropriate new name. It is also possible to use the "Copy" command, perform "File > Open" to open a new bank of recipes, then save the recipe you copied in a folder in this new bank of recipes.

 

Modifying a recipe

Adding steps to recipes is very intuitive. If the simple description that follows does not suffice, open the online documentation in the help menus at the tool for further details.

To add steps to a recipe, drag and drop the icons from the column of possible commands (nominally in the middle of the "Recipes" page) to their appropriate location on the step-by-step listing of the recipe (nominally on the right). Hover the mouse over each icon to learn what functions they perform. Alternatively, current steps in a recipe can be highlighted (left click or shift-left click for multiples), and right clicking will allow you to copy, paste, or delete these steps.

 Exotic Tool Uses

"Deep Lithography"

A method to spraycoat and pattern the bottom of large (millimeter-scale), deep (100's of microns) features using ASML was developed and the documentation is as follows.

Short report: link

All files (large .rar): link

 

 Conclusion

The EVG spray coater is an automated machine for dispensing photoresist on 4" silicon, glass, or quartz wafers or pieces.  The spin speed, pressure, volumetric ratio of photoresist to solvent, and chuck temperature can all be varied by the user.  For average results under different operating conditions, see “Average Results” below.  The EVG spray coater allows users to deposit conformal layers of photoresist or polymers on 3D structured wafers.  The major advantages a spray coater has over traditional spin coaters are:

1) Conformal coating
        - The ability to make a uniform topography coating over a patterned surface

 
2) Planar coating
        - The ability to make a planar topography coating over a patterned surface
3) Able to spray resist without spinning wafer
        - prevents damage on already released delicate MEMs structures 
 
4) Ability to coat for 3D structures using resist as a patternable sacrificial layer
 

Troubleshooting:

NEVER close the spray coating software. This may put the tool down for days, as it will require the visit of a technician from EVG.

 
Problem Suggested Resolution/Solution
Error:   DIS1:failed to move to position 3772! Restart the EVG items in the services start/stop menu.  1) Click on the services button on the bottom windows toolbar (it has two little gears).  2) scroll down to EVG item (there are five) and select the correct one.  3) Select restart from the "action" pull down menu at the top left of the services window.

 

Document Actions