Device Fabrication

The key steps of fabricating devices for carbon-based nanoelectronics are:

Microfabrication: Photolithography, Metalization etc.

To learn more about microfabrication processes look at http://en.wikipedia.org/wiki/Photolithography. The most important source of information is always the photoresist manufacture's data sheet (see the T: drive, Group documents\Photoresist documentation). There are also some good textbooks in the OSU library that cover all aspects of microfabrication:

  • “Fundamentals of Microfabrication” 2nd Ed. by Madou (excellent level of detail)
  • “The science and engineering of microelectronic fabrication” 2nd Ed. by Campbell (good overview)
  • “Introduction to microelectronic fabrication” by Jaeger

For some philosophy on how to make a good recipe you might enjoy reading the zen of device making.

Handling clean chips

All experimental physicists working in this field have to learn best practices for handling chips.

Mask design

CAD software is used to design and export the patterns needed for photolithography. In some cases you will need to export .dxf files, in other case .gds files. Josh uses AuotoCAD, this is powerful, industry standard software which is free for academic users. Ethan used DesignCAD (2D version) when he worked in Delft. Matt and Landon like to use Layout Editor which is free, open source software.

Chrome masks are patterned using the direct write lithography system on campus. Chrome masks allow us to reach the 2 micron resolution limit of the contact aligner. There are two write lenses (labeled 10 mm and 2 mm), with a 2 µm or 0.5 µm spot size respectively. For the 2 µm spot size, a mask can be exposed at a rate of 1 square inch every 20 minutes regardless of the amount of exposed area. Using the 0.5 um spot size the write time is on the order of 1 square inch every 1.5 hrs. The write times are fairly insensitive to the complexity of the pattern data. However, large arrays of repetitive shapes can take up to 5 times longer to expose than normally expected.

There is an OSU Cleanroom wiki describing operating procedures for the DWL.

DWL Costs
Laser time$6/hour
Mask blank$12
developer/etchant/stripper $18

Before the DWL was installed on campus we had to send CAD drawings to a mask making company. Commerically produced chrome masks cost $400 - 800 each.

Image of a chrome mask made with the DWL 2 µm spot size. The pink is chrome, the grey is transparent. More mask images.

For less demanding resolution, for example 10-20 micron minimum feature size, a printed transparency can be used. These masks cost about $60 each and are purchased from a company in Bandon. Now we prefer to make chrome masks in the DWL.

Photolithography

We are using shared equipment in John Wager's lab to process our chips. The lab is in Owen Hall 4th floor West Wing. You can look through the big windows to see the impressive equipment. The lab is run by Chris Tasker, chris@eecs.oregonstate.edu. To use equipment in this lab you must be trained by someone in our group and then certified. Rick Presley is our main contact for certification, presley@engr.orst.edu.

The most important source of processing information is always the photoresist manufacture's data sheet (see the T: drive). For more technical advice you can also call from the photoresist company, MicroChem, (the company that distributes Shipley products). We have worked hard to get good/reliable recipes - some of the trials are documented.

Contact Aligner Walk Thru

Photoresist processing recipe

(updated 04-08-2010)

  • 5 min dehydration bake 190°C
  • spin LOR3B photoresist at 4000 rpm for 45 sec (bilayer process only)
    • Deposits ~250 nm of photoresist (according to LOR3B documentation)
    • ideal LOR3B thickness = 1.25*metal thickness
    • Skip this step for single layer process
  • 4 min softbake 190°C (bilayer process only)
    • Skip this step for single layer process
  • spin S1813 photoresist at 4000 rpm for 30 sec
  • 2 min softbake 115°C
  • 6 sec exposure
  • 45 sec develop
    • Developing solution: 4 parts DI H20, 1 part Shipley Mircoposit MF-351
    • Gently agitate substrate in developer bath. Afterward, gently spray rinse DI H2O followed by a quick dunk in a DI H20 bath with some agitation.
  • deposit 35nm metal
  • remove underlayer with mircoposit 1165 (located in Weniger 306 lab)
    • Put chips in 60°C remover for 30 min
    • Transfer to fresh 60°C remover for 30 min
  • rinse DI H20 then blow dry
    • Never rinse chip with acetone while LOR3B or it's residue is still on the chip! The LOR3B combines with acetone to form a sludge that can only be removed by scraping!

Other recipes

Cross contamination

  • Be extremely careful when using CD-26 & MF-351 in the same lab. One drop of MF-351 in a gallon of CD-26 ruins the whole gallon! This problem was so bad that Shipley built a separate facility just to keep these away from each other.

Photoresist removal

Matt has documented that hot PR remover leaves less PR residue than any other method we have tried.

Ebeam lithography

Matt and Ethan have used the ebeam system at CAMCOR (University of Oregon, 50 minute drive). A basic recipe is available.

In September 2011 the new ebeam lithography system was installed in the OSU Electron Microscopy Facility. Doug Kezsler's group, and Inpria are currently the expert users.

Cleaving & Dicing Wafers

The initial photolithography steps are done on 3“ wafers. To grow nanotubes, however, we have to cleave the Si [1 0 0] wafer or ST-cut quartz into smaller pieces that will fit in our furnace. Matt is the expert when it comes to cleaving.

The only way to get truly square quartz pieces is to use a wafer dicing saw. Pallavi Dhagat has used American Precision Dicing Inc. The cost is a minimum of $150.

Metal deposition

Thermal evaporation

After doing photolithography, a thin layer of metal (tens of nanometers thick) is deposited on a nanotube chip. The interface between nanotube and metal is critical for device operation (for example, we do not store the chip in a gelpak before evaporation. This will lead to hydrocarbon contamination).

Our workhorse deposition system is the thermal evaporator in Janet Tate's lab, 4th floor Weniger Hall. We have used this system for chrome, gold, aluminum, and iron. Please see Matt Leyden for training. Always follow the standard operating procedure in a step by step fashion - missing one step can ruin your sample and the equipment. Matt and Josh renovated this evaporator in summer 2008.

Evaporation boats are made Tungstun and cost $6-8 each (R. D. Mathis). The boats should be kept clean and reused when possible.

Background info about thermal evaporation

Thin film deposition is monitored by a quartz crystal microbalance (QCM). The operator enters the density of the metal, the z-ratio of the metal and the tooling factor of the evaporator. Density and z-ratio can be looked up on a table. The tooling factor is a geometric factor, basically the ratio of the distance between source and QCM and the distance between the source and sample. When the QCM readings do not agree with AFM characterization of film thickness, users typically adjust the tooling factor.

For thermal evapoartion, metal is usually held in a boat made out of Tungsten (W) because the melting point of W is 3422 °C. It is possible to break the boat by heating or cooling too fast. It is also bad for the sample if the temperature inside the evaporator gets too hot.

This evaporation table gives useful advice about which metals can be thermally evaporated. Similar information is on the Kurt Lesker website. For example,

  • Iron attacks W so thermal evaporation may not be possible.
  • Ti outgases when first heated.
  • Cr is available in rod form, so a boat is not necessary. Cr is a great sticking layer.
  • Pt and Au are both suitable for thermal evporation in a W boat, these metals do not stick well to an oxide surface.

Electron Beam Deposition

The clean room in Kelley has a new ebeam deposition system. Metals like iron & palladium which are difficult to evaporate thermally can be deposited with the ebeam system. Ask Josh Kevek for more information.

Deposition Alternative: Sputtering

Sputtering is an alternative to thermal evaporation that we have not yet explored. The deposition rate should be more stable. Milo Koretski has a beautiful new sputtering system set up in the chemical engineering department which he used for CNT catalyst deposition.

John Wager's group has an ebeam evaporator. E-beam evaporation is especially useful for metals that are hard to control or hard to melt, like iron and molybdenum.

Etching nanotubes

When making devices from aligned CNTs the unused tubes need to be etched. We currently use the O2 plasma in the Owen Hall clean room. This O2 plasma removes photoresist at a rate of about 100 nm/minute. The recipe from Rogers Nature Nanotech 2007 is 50 mTorr, 20 sccm O2, 30 W, 30 s.

More oxygen plasma details.

Other etching options

Vacuum Anneal

See here for the procedure to do this in our e-beam system. Annealing some kinds of nanotube devices in vacuum seems to decrease contact resistance, improve a leaky gate oxide, and remove surface contaminants. Starting in the ~0.01 mTorr range increase the temperature in increments of 100 C. When you get to 600 C, turn off the heater and allow the device to cool in vacuum to ~200 C before venting. So far, results suggest this improves resistances by ~80%. See the Phil Collin's paper for more information.

Cornell Nanofabrication Facility

User fees (March 2010)

  • One time fee for introductory training $420
  • Daily fee (covers chemical usage) $10
  • Mask making $100 for simple, $300 for complex
  • Photolithography on the stepper, $60 per hour
  • Metal evaporation, $60 per 2 hour session + $0.80 per nm of gold deposited
  • Reactive Ion Etching, $30 per hour, need about a 30 minute session to do 500 nm etch.

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