Temperature Sensor Project Overview

What is temperature? How do we measure it?

Thermometer – thermal expansion of a fluid – Galileo 1592 Thermocouple – Seebeck effect Electrical resistance changes with temperature – metal RTDs, thermistors Integrated circuit sensors – semiconductor diodes that are temperature sensitive Infrared Radiation – IR thermometry Bimetallic thermal expansion detection Change of state temperature detection - Liquid Crystals

Pass around samples

Comparing Temperature Sensors

Resistance Temperature Detectors (RTDs)

Resistivity of metals is a function of temperature Positive change in resistance with positive change in temperature

Resistivity, versus Resistance

A

LR

R = resistance (Ohms)

= resistivity (Ohm-m) L = length of material (m)

A = cross-sectional area of material (m2)

Resistance Temperature Coefficient,

121

12

TTR

RR

or ( )

where R2 & R1 are resistances of the material at T2 & T1, respectively. Notice that the expected resistance change of the RTD due to a temperature change can be easily computed.

RTD is linear over narrow ranges of temperatures, for wider temperature ranges use

)1()( 2bTaTRTR o

Where,

Ro = Resistance at 0 ˚C a, b = material constants which depend on purity of the material.

RTD Materials From Holman, J.P., Experimental Methods for Engineers, 6th Ed.

Example Problem Consider a 100 Ohm RTD that is made from nickel wire. How long is the nickel wire if its diameter is 0.1 mm, 0.05 mm, or 0.01 mm? RTD Geometries

Film Design Wire Coil Design

From Omega, Temperature Reference From Nicholas & White, Traceable Temperatures

Fabrication of Film RTD 1. Fabricate a simple nickel film RTD 2. Use a glass slide as the substrate 3. 200 nanometer film of nickel 4. Use process called photolithography to pattern our nickel

resistor

Ultraviolet Photolithography

Project Steps

1. Design RTD element 2. Create mask pattern for photolithography – Use SolidEdge drafting tool 3. Fabricate RTD element using photolithography 4. Create measurement circuit using RCTime 5. Test and calibrate RTD sensor

RTD Element Design Considerations

Design Parameters Nominal Resistance of 100 Ohms at room temperature (20oC) Nickel film is approximately 0.2 micrometers thick Lead wire attachment pads should be 5 mm x 5 mm and preferably positioned as far apart as possible

Design Guidelines Use a minimum line width of 200 micrometers Spacing between lines should be at least 200 micrometers Entire foot print of RTD element (resistor & pads) should fit inside 2.5 cm x 2.5 cm square

1. Design your RTD element (i.e., determine its length and width) such that it has a nominal resistance of 100 Ohms

at 20oC. Assume the RTD uses a nickel film that is 0.2 micrometers thick and has a resistivity of 1.2 x 10-7m. Follow the design guidelines mentioned above in selecting the width and length of your resistor. Create a spreadsheet that predicts the resistance of your temperature sensor for a temperature range from 0oC to 100oC in 5oC increments assuming that the RTD performs perfectly linear. Note: Present your solution using engineering format, and either manually cut and tape your Excel spreadsheet onto your homework or paste it in electronically. 2. Create your mask design in Solid Edge based on the design of your RTD element from problem 1 using the drafting portion of the Solid Edge program. Follow the design guidelines in preparing your mask on Solid Edge. Your drawing should be placed in one of the slots on the template shown below; students at a table in class (or the group of 3 or 4 students that is completing the fishtank project together) should decide which student will be #1, #2, #3 and #4 and put their drawing in the correct slot on the template. Fill up ALL of the slots on the template even if your group has less than four members since a SINGLE mask will be printed for each group and since you may need an extra RTD as backup. Please also leave the “sample” RTD drawings on the template in place just in case you need them. Print out a copy of your drawing to submit with your homework (either the template with just your drawing or the template with all of the student’s drawings in your group will do). Before coming to class, the students in the group should combine all of their drawings onto a single template and name the file: “tableX instructor” where “X” is the table number and “instructor” is the last name of your instructor. For example, students at table 5 in Dr. Hall’s class would name the file “table5 hall”. Of course, the file extension will be .dft since this is a Solid Edge drafting file. These files will be collected on a USB thumb drive at the beginning of class, and a mask will be printed for each group for sensor fabrication.

Additional Information: Why is the resistivity of the nickel film different from the value of nickel in the table? Is the film mixed with another element? The resistivity is of the nickel film is most likely from the grain structure that forms during its deposition. It is probably has more imperfections than a typical pure sample of bulk nickel. It is fairly common for thin films of materials to have properties that are not quite the same as a bulk sample of the same material. The value given in the homework is what we are typically getting for the nickel films that are on the glass substrates the students will be using. Should the calculations for the RTD account for the difference in the geometry in the lead wire attachments?

We have generally neglected these but I think it would be valuable for the students to investigate it on their own. For their RTDs, I suspect it is negligible and they will end up calibrating their RTDs with the lead wires they attach. Lead wire resistance is an issue when using two-wire RTDs. What is often done to solve this issue is to use a 4-wire arrangement where 2 wires deliver the current thru the RTD and 2 other wires are used solely to measure the RTDs voltage drop and the resistance is calculated from V=IR. This eliminates the effects of voltage drops due to resistance in the lead wires. More details on photoresist and photolithography. A photoresist is a light sensitive material that's properties change when it is exposed to light (UV light in our case). So what happens in our project is we use a mask pattern of the RTD to protect areas of the photoresist from being exposed to UV light. After the exposure, we use a developer to wash away the photoresist in the areas that were exposed. The unexposed areas don't wash away in the developer. At this point, there are a couple different options in photolithography. One can deposit another material or etch away material in the areas that are not protected by the photoresist. We will be etching away the nickel that is not protected by the photoresist.