Standard Operational Procedures for the Gaertner Ellispometer L117

Figure 1 Angled view of ellipsometer

Data Conversion

• Graphs

  The three film thickness and index graphs are for measurements taken at 70° incidence.
  
  

  		Graph 1		Graph 2		Graph 3
  Gives Film Thickness (d) of:		400Å to 1120Å		920Å to 1320Å		400Å to 1320Å
  Film Index (nf) of:		1.3 to 1.5		1.35 to 1.55		1.5 to 2.3
  P1 + P2 value of:		250° to 285°		250° to 320°		260° to 100°
  A2 - A1 value of:		60° to 150°		0° to 120°		10° to 150°
  ψ value of:		15° to 60°		30° to 90°		15° to 85°
  and Δ value of:		75° to 110°		40° to 110°		260° to 100°

  
  Graphs 4 and 5 give the thickness period (ω) for different film indices at 30°, 50° and 70° incidence. The thickness period (ω) is used for calculating the correct thickness of thicker films. For example, if Graph 1 gave a thickness of 1080Å (d) and 1.45 film index (n_f) and the film thickness was know to be approximately 4000Å, then using Graph 4 and film index of n_f = 1.45 at φ = 70° incidence the thickness period (ω) is 2870Å. This 2870Å would be added to 1080Å giving a film thickness of 3950Å. Other possible answers would be:
  
  (2 × 2870Å) + 1080Å = 6820Å
  (3 × 2870Å) + 1080Å = 9690Å
  (4 × 2870Å) + 1080Å = 12,560Å
  As can be seen from the above, the ellipsometer readings repeat themselves for thicker films. It is therefore necessary to know the thickness of the film to within approximately 2000Å so that the correct thickness can be selected from the possible thickness values.




• Tables

  The tables supplement the graphs by extending the region of coverage as well as being useful for measurements taken at 30° and 50°.

1. Turn on the instrument and warm up (approx. 15 min.).
2. Place the specimen on table and loosen table clamp screw (2) (see Figure 2)
3. Rotate knurled nut (3) at back underside of table (Figure 2). Raise or lower table to direct reflected laser beam into analyzer pinhole.


Figure 2 Components of the ellipsometer: Table clamp screw (2),
knurled nut (3) and specimen table of ellipsometer


4. Observe the extinction meter and change table height to get maximum reading meter. If meter reading exceeds 110, readjust gain control (4) to bring maximum reading below 100 (Figure 3). Tighten table clamp screw at maximum meter reading.


Figure 3 Backside of ellipsometer

1. With the specimen table set at the correct height adjust gain control until meter reads midway between 3/4 and full scale (150 to 200).
2. Rotate analyzer (right hand drum) slowly in the red numbered segment (0° to 90°) and set this drum to give lowest reading on extinction meter. NOTE: It is useful to rest both elbows on the table to minimize fatigue and improve setting accuracy.
3. Now rotate polarizer (left hand drum) slowly within the segment from 315° to 135° (red numbers) and set this drum to give a new and even lower meter reading. If meter reading exceeds 110, readjust gain control to bring maximum reading below 100. The numerical value on the meter is not important, only the settings of the polarizer and analyzer drums which give the least reading on extinction meter.
4. Go back to the analyzer and rotate slowly to drive the meter pointer to a new and lower position. The analyzer must be within the red numbered segment (0° to 90°), if it isn't, rotate the polarizer to a different setting within the red segment 315° to 135°. If the meter reading falls below 25 (1/8 full scale), use the gain control to bring the meter indicator reading to 50.
5. Go back to the polarizer and rotate slowly to drive the meter pointer to a still lower position. Work back and forth between analyzer and polarizer drums until the lowest possible meter reading is obtained. These drums settings correspond to true extinction.
6. Record the first analyzer A₁ and the first polarizer reading P₁ at extinction. You are now ready to obtain the second set of drum readings A₂ and P₂.
7. Add 90° to the first polarizer drum reading P₁, i.e. (P₁ + 90°). Rotate the polarizer drum to this sum.
8. From 180° subtract the first analyzer drum reading A₁, i.e.: (180° - A₁). Rotate the analyzer drum to this difference.
9. Slowly rotate polarizer drum to obtain lowest reading on the meter. It will not be necessary to rotate the drum more than a few degrees.
10. Slowly rotate analyzer drum to obtain a still lower meter reading. Adjust meter gain control to bring meter pointer around 50 if necessary.
11. Go back to polarizer drum and attempt to drive the meter pointer still lower. Set polarizer drum at lowest meter reading.
12. Now go to analyzer drum and attempt to drive the meter pointer to a lower reading. Set analyzer drum at lowest meter reading.
13. Work back and forth between analyzer and polarizer drums to obtain final lowest reading on the meter.
14. Record analyzer and polarizer readings (A₂ and P₂). The second set of values will vary slightly from the calculated (P₁ + 90°) and (180° - A₁) values but should not differ by more than 4° for accurate measurements.

1. Subtract the measured values of A₁ from A₂ and add P₂ to P₁. If (P₁ + P₂) ≥ 360°, subtract 360° from it to obtain the correct value of (P₁ + P₂).
2. Select the graph that has the values of (A₂ - A₁) and (P₁ + P₂) calculated in the previous step. On this graph locate the point of intersection of the (A₂ - A₁) and (P₁ + P₂) values. Read the values for film thickness (d) and index (n_f) at this point. When the point falls between the lines determine the correct value by proportion.
3. If the thickness determined from the graph is much lower than the expected thickness, proceed as follows: Refer to the graph of thickness periods (ω) and determine the period for the given film index at the proper angle of incidence. Add the original value of film thickness to a multiple of film period to obtain the sum which comes closer to the expected film thickness.

Example One

1^st Analyzer reading: A₁ = 57.6°

1^st Polarizer reading: P₁ = 93.7°

2^nd Analyzer reading: A₂ = 122.6°

2^nd Polarizer reading: P₂ = 183.5°

A₂ - A₁ = 122.6° - 57.6° = 65.0°

P₁ + P₂ = 93.7° + 183.5° = 277.2°

From Graph 2:

Thickness (d) = 1200Å

Refractive Index (n_f) = 1.465

Example Two

A₁ = 32.2°

P₁ = 327.6°

A₂ = 147.7°

P₂ = 57.5°

A₂ - A₁ = 147.7° - 32.2° = 115.5°

P₁ + P₂ = 57.5° + 327.6° = 385.1°

P₁ + P₂ ≥ 360°, \ P₁ + P₂ - 360° = 385.1° - 360° = 25.1°

From Graph 3:

Thickness (d) = 910Å

Refractive Index (n_f) = 2.15

Example Three

Using Example 1 the results are d = 1200Å and n_f = 1.465

If the expected thickness is about 4000Å then from Graph 4:

ω = 2820Å, hence d = 2820Å + 1200Å = 4020Å actual thickness.

Example Four

Using Example 2 the results are d = 910Å and n_f = 2.15

If the expected thickness is about 6000Å then from Graph 5:

ω = 1636Å, hence d = (3 × 1636Å) + 910Å = 5818Å actual thickness.

Calculate ψ using both analyzer readings and the relation:

Calculate Δ using both polarizer readings and the relation:


If (P₁ + P₂) ≥ 360°, subtract 360° from it before using it in the above.