EXPERIMENT 00

Measuring Nanometers and Micrometers with Color

(Instructor Guide)

PART 1: INSTRUCTOR’S GUIDE & PREPARATION

(Confidential: For Teaching Assistants and Faculty Use Only)

1. Experiment Overview

In this experiment, students characterize the relationship between spin speed and film thickness for a polymer film using Absorbance Spectroscopy.

2. Solution Preparation (Step-by-Step)

Complete this process 24 hours before the lab to ensure the polymer is fully dissolved.

Step A: Prepare the 12% PVA Stock (The Base)

• Chemicals: Use High Molecular Weight PVA (85,000–124,000 g/mol). Low MW PVA will result in films that are too thin.
• Recipe: 12% w/w Solution (e.g., Mix 12 g PVA into 88 g Deionized Water).
• Procedure:
  1. Heat the water to ≈ 80°C on a stir plate.
  2. Slowly sprinkle the PVA powder into the vortex to avoid clumping.
  3. Cover and stir for 2–3 hours until the solution is perfectly clear and viscous (like honey).

Step B: Make the "Master Dye Stock" (The Color)
Dissolve the following amount of dye powder in 100 mL of water. Store in a labeled bottle.

Dye	Mass of Powder	Volume Water	Resulting Conc.
Red #40	2.0 grams	100 mL	0.040 M
Blue #1	0.6 grams (600 mg)	100 mL	0.0076 M

Step C: Make the Student Solution (The Mix)
Mix the Master Dye Stock with your 12% PVA solution in a 1:9 ratio.

Example for a 50 mL batch (Enough for 5 groups):

• Pour 45 mL of 12% PVA into a beaker.
• Add 5 mL of Master Dye Stock.
• Stir gently until uniform (avoid bubbles).

3. Dye Reference Data (Extinction Coefficients)

Use these constants if students are calculating thickness from scratch. Note: These values apply to pure analytical grade dyes.

Option 1: FD&C Red #40 (Allura Red) - Recommended

Wavelength (λmax)	504 nm
Molar Mass (MW)	496.42 g/mol
Molar Extinction Coeff. (ε)	25,900 M-1cm-1

Option 2: FD&C Blue #1 (Brilliant Blue) - High Sensitivity

Wavelength (λmax)	630 nm
Molar Mass (MW)	792.85 g/mol
Molar Extinction Coeff. (ε)	138,000 M-1cm-1

4. Cheat Sheet: Recommended Lab Data (Scenario A)

Use this table to grade student accuracy. Based on 12% w/w PVA (High MW) and Red #40.

Spin Speed (RPM)	Inverse Sq Root (1/√RPM)	Theoretical Thickness (μm)	Approx. Absorbance*
1000	0.0316	17.1	1.25
2000	0.0224	12.1	0.88
3000	0.0183	9.9	0.72
4000	0.0158	8.5	0.62
5000	0.0141	7.6	0.55

*Absorbance values assume doping targets A=1.0 at 2200 RPM.

5. Purchasing Guide (Materials)

• Spin Coater: Instras Scientific SCK-300 Series.
• Polyvinyl Alcohol (PVA): High Molecular Weight (85k-124k). Sigma-Aldrich 89-98k MW.
• Dyes (Pure): Allura Red AC (Sigma).
• Plastic Slides: 25mm x 75mm Vinyl or Polycarbonate. Amazon Plastic Slides.

6. Determining the "Magic Number" (C_film)

The concentration of dye in the solid film is higher than in the liquid because the water evaporates.

Method A: The Theoretical Calculation (1:9 Mix Ratio)
Use this calculation if you followed the 1:9 mixing protocol exactly.

Method B: The "Assumed Standard" (Calibration Based)
Spin a reference slide at 2500 RPM. Assume thickness is 10 microns. Back-calculate:
C_film = A_ref / (ε × 0.001)

7. Equipment & Safety Notes

• EYE PROTECTION IS MANDATORY: Ensure all students wear ANSI-approved safety goggles.
• Mounting Plastic: Use 2cm x 2cm double-sided tape. Press firmly.
• Cutting: Ensure heavy-duty scissors are available.

8. Literature & References

• Meyerhofer, D. "Characteristics of resist films produced by spinning." J. Appl. Phys. 49.7 (1978). [DOI]
• Lee, U. G., et al. "A Modified Equation for Thickness..." Symmetry 11.9 (2019). [DOI]
• Hall, D. B., et al. "Spin coating of thin and ultrathin polymer films." Polym. Eng. Sci. 38.12 (1998). [DOI]

EXPERIMENT 00

Student Lab Manual

Name: ______________________________    Date: _______________

1. Objective & Applications

In this experiment, you will fabricate thin polymer films using a spin-coating technique and determine their thickness using quantitative spectroscopy.

Real-World Applications:
Spin coating is a standard industrial process used for creating uniform thin films on flat substrates. Key applications include:

• Microchip Fabrication: Applying photoresist layers ($~1 \mu m$) onto silicon wafers for lithography.
• Biosensors: Creating functional polymer coatings for glucose or DNA sensing.
• Microfluidics: Fabricating microfluidic devices and lab-on-a-chip systems using soft lithography.

2. Theory

A. The Beer-Lambert Law

To measure the thickness of our films, we will use the principles of light absorption. The Beer-Lambert Law relates the attenuation of light to the properties of the material through which it is traveling:

[Image of Beer-Lambert Law diagram]

Where:

• A (Absorbance): The amount of light absorbed (unitless). This is what the spectrophotometer measures.
• ε (Extinction Coefficient): How strongly the dye molecule absorbs light at a specific wavelength (M^-1cm^-1). This is a constant property of the dye.
• l (Path Length): The distance the light travels through the material (cm). In a liquid experiment, this is the width of the cuvette. In this experiment, "l" is the Film Thickness.
• c (Concentration): The molarity of the dye in the material (M).

The Conceptual Flip:
In most chemistry labs, you use a fixed path length (cuvette width = 1 cm) to solve for an unknown concentration (c). In this materials science lab, we are doing the reverse. We know the concentration of dye in the solid plastic (c), and we will measure Absorbance (A) to solve for the unknown path length (l), which is our film thickness.

B. The Physics of Spin Coating

When a polymer solution is spun at high speeds, centrifugal force spreads the fluid while viscosity resists it. As the solvent (water) evaporates, the film solidifies. The relationship between final thickness (h) and spin speed (ω) is described by the Meyerhofer Model:

[Image of spin coating speed vs time graph]

We will linearize this by plotting Thickness vs. 1/√RPM. The slope of this line (k) characterizes our specific polymer system.

(Note: For ultra-thin films below 500nm, scientists often use Ellipsometry instead of Absorbance, but that equipment is significantly more expensive.)

3. Safety & Equipment

1. EYE PROTECTION (MANDATORY): ANSI-approved Safety Goggles are required by law. You may not operate the spin coater without them. This protects against chemical splash and potential high-speed projectile debris.
2. High Speed Rotation: The spinner reaches 5000 RPM. NEVER operate without the lid covered.
3. Projectile Hazard: Plastic slides are light. Ensure they are taped firmly to the chuck.
4. Chemicals: The dye will stain skin and clothing. Wear gloves.

4. Procedure

Part A: Slide Preparation

1. Obtain 5 plastic slides.
2. Clean: Gently wipe with a damp Kimwipe. Do not use Acetone (it melts plastic).
3. Label: Mark the top corner (the area you won't coat) with: 1500, 2500, 3500, 4500, UNK.

Part B: Spin Coating (The Calibration Series)

1. Mount: Place the slide labeled 1500 on the chuck. Use double-sided tape. Center it visually.
2. Deposition: Pipette ≈ 1.0 mL of the PVA solution onto the center of the slide. Avoid bubbles.
3. The Spin Cycle (Strict Timing):
   • Spread Step: Set speed to 500 RPM. Run for 10 seconds.
   • Spin Step: Immediately ramp to 1500 RPM. Run for 60 seconds.
   • Visual Check: It must be matte/dry. If wet/glossy, spin 30s more.
4. Dry: Remove slide and place on a paper towel. Wait 10 minutes.
5. Repeat: Repeat for 2500, 3500, and 4500 RPM slides.

Part C: The Unknown (Swap Protocol)

1. Receive Assignment: The instructor will assign your group a specific RPM. Write this down in your notebook, but do not write it on the slide.
2. Create Sample: Spin your UNK slide at this assigned speed using the standard procedure.
3. Label: Label the slide only with your Group Name/ID.
4. Hand In: Bring the dry slide to the instructor.
5. Swap: The instructor will trade your slide for a sample created by another group.
6. Goal: Your task is to determine the speed the other group used to create the slide you are now holding.

Part D: Cutting & Measuring

1. Cut: Using heavy scissors, cut a 1 cm strip from the very CENTER of the slide. Discard the edges (edge beads).
2. Blank: Insert a clean, uncoated strip of plastic into the colorimeter. Press "Calibrate" or "Blank".
3. Measure: Insert your coated samples (and the Swapped Unknown). Record Absorbance below.

5. Data Analysis

Constants (Provided by Instructor):

• Dye Extinction Coeff (ε): ____________________ M^-1cm^-1
• Concentration of Dye in Solid Film (C_film): ____________________ M

Slide ID	Speed (RPM)	1/√RPM	Absorbance (A)	Thickness (μm)
A	1500	0.0258
B	2500	0.0200
C	3500	0.0169
D	4500	0.0149
UNK (From Group: ____)	???	???

Calculations Reference

Use the equations below to complete your data table and solve for the unknown speed.

6. Appendix: Troubleshooting

Symptom	Diagnosis	Solution
The Comet	Dust particle causing a streak.	Clean slide better; check solution for dust.
Starry Night	Many small pinholes (Bubbles).	Pipetted too fast. Let solution de-gas.
Orange Peel	Bumpy texture.	Viscosity too high or speed too low.
Wet Center	Glossy center, dry edges.	Spin time too short. Increase to 90s.