Resistance Temperature detector (RTD) project

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Resistance Temperature detector (RTD) project

• Dr. Hisham E. Hegab
• College of Engineering Science
• Louisiana Tech University
• Ruston, LA
• hhegab_at_latech.edu

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RTD Sensor Overview

• Students
• Design geometry
• Create mask pattern using CAD package
• Perform photolithography
• Inspect resulting device
• Test and calibrate their senor

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Project Logistics/Organization

• Organization
• Class size up to 40 students
• Each student designs an RTD pattern creates
  their own corresponding mask pattern
• Optical lithography performed in teams of 4
  students but yields up to 6 RTDs per team
• Calibration performed in teams of 2 students
• Focused on having as much hands-on as possible

Living with the Lab Workshop July 14th, 2009
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Project Overview/RTD Design

• Introduction to temperature its measurement
• Look at different types of sensors
• Advantages/disadvantages
• Focus on RTDs
• Types (film vs. wire coil)
• Materials used
• Relevant fundamentals
• Brief overview of design

• Nickel film resistor design
• Use glass substrate
• 200 nm Ni film on 2-3 nm Cr
• pattern using optical photolithography
• Use RCTime circuit to sense resistance change of
  sensor

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1st Homework Assignment

• Students asked to design the geometry required to
  produce a 100 W RTD using a 200 nm thick nickel
  film.
• Also required to create a spreadsheet predicting
  its resistance vs. temperature (determine its
  sensitivity)
• Also assigned to create a mask pattern for
  fabricating their design
• Masks are fabricated using a high resolution
  printer (imagesetter) performed by teaching
  assistant

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Microfabrication Lecture 2nd class period

• Fabrication Steps
• Prepare the substrate for spinning
• Spin the photoresist
• Soft bake
• Apply a photomask and expose to UV light
• Develop the photoresist
• Rinse and dry
• Hard bake
• Etch
• Remove photoresist
• Dice individual RTDs
• Inspect resistor pattern
• Connect leads
• Seal sensor

• Provides background information on processing
  steps involved in photolithography
• Students learn about chemicals involved (e.g.,
  photoresists, developer, etchants) and safety
  considerations
• Students perform all steps in fabrication except
  Ni etching (nitric acid) and dicing RTDs (glass
  cutting)

Living with the Lab Workshop July 14th, 2009
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Review RC Time Circuit

• Students use RC time circuit to measure
  resistance of detector
• Have already been exposed to the basics of this
  measurement circuit in previous ENGR 120 course
• Provide brief review
• Now asked to look at what values of R1 and C
  would make their sensor perform the best

Living with the Lab Workshop July 14th, 2009
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Fabrication Class Period 3 4

• Provide students with step by step set of
  PowerPoint slides
• Slides contain explicit instructions for
  equipment as well as relevant safety warnings
• Lab room adjacent to classroom equipped with
  spinner, UV exposure station, hot plates
  developing station

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RTD Photolithography Fabrication
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Before beginning the fabrication process put on
latex gloves and a pair of safety glasses. The
gloves are used primarily to help keep the
substrate you will be handling clean. Handle the
substrate using tweezers and/or holding it by its
edges so that you do not get dust or dirt on the
surface. Dirt can cause imperfections in the
transfer of the RTD pattern while doing
photolithography. We will be using
Safety Precautions
Before You Begin!

• photoresist, MF-319 developer, and de-ionized
  (DI) water during the fabrication. While none of
  these chemicals are extremely hazardous, you
  should wear safety goggles to protect your eyes
  at all times. The developer is flammable so do
  not place it near the hot plate or any other
  source of heat.

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Your instructor and/or a lab assistant should
provide you with a substrate packaged in a
plastic container. The substrate is a 50 mil
thick microscope glass slide with a 3-5 nm layer
of chromium and a 200 nm layer of nickel
deposited on it. We will create 6 RTD patterns on
one 2" x 3" slide and then separate the
individual RTDs after they have been patterned
and etched. The substrate is already clean so it
is ready to start the fabrication process.
Obtain Substrate
Nickel
Chromium
Glass
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• Place Substrate in Spinner
• Lift the lid of the spinner and using tweezers
  place the substrate onto the chuck of the spinner.

Align Substrate Center the substrate on the
chuck by placing your fingers on both sides of
the substrate.
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• Mask Alignment
• After the soft bake, place the substrate inside
  the UV exposure station. Then place your mask
  pattern on top of the substrate. Be careful to
  align the mask pattern to the edges of your
  substrate.

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• Exposure
• Place a glass plate over your mask/substrate. The
  plate applies pressure to the mask so that it
  sits firmly on the substrate during the exposure
  process. Place the back edge of the glass plate
  down inside the UV station and lower the front
  edge into place so that you do not disturb the
  alignment of the mask. Close the chamber door
  and press the enter button to expose the
  mask/substrate for 120 seconds.

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• Develop the Photoresist
• Remove the substrate from the UV exposure station
  and take it to the developer tray. There are two
  small baths in the developer tray. One contains
  MF-319 developer and the other contains DI water.
  Holding the substrate with tweezers gently wash
  the substrate in the developer bath. Make sure to
  completely submerge it in the developer so that
  it will develop the substrate evenly. Take it out
  every 5 seconds to see if the developer has
  washed the photoresist away (rainbow or
  multi-colored areas that were exposed to UV light
  should disappear). As soon as it appears to be
  fully developed, dip the substrate into the DI
  water bath to stop the development process.

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Inspection Final Fab Class Period 5

• Perform optical inspection
• Check continuity room temperature resistance
• Solder lead wires
• Seal RTD

Living with the Lab Workshop July 14th, 2009
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Calibration/Testing Class Period 6

• Outside of class (HW) students design/build RC
  Time circuit to measure resistance of their RTD
  affects sensitivity
• Calibration performed using water bath stations
  provided by temperature controlled hot plates
• Perform regression analysis as homework

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Equipment Facilities

• Dedicated Facilities Equipment
• Automated dispensing spinner (6K)
• Timer controlled UV oven (3K)
• Hot plates (1K/each, need 3-4/lab)
• Shop microscopes (150/each, 1 per 4 students)
• Mask printer (8K or can be outsourced
  25/class)
• Clean air hood (3K)
• Clean, dry air
• Fume hood for etching (15-25K)

• Consumables
• Chemicals, glass slides, lead wires, sealing
  epoxy (3-5/student)
• Other Implementation Options
• Masks could be printed with high resolution laser
  printer
• Could limit photolithography to demonstrations
• Use available temperature sensor from parallax