How to Find Thickness Range Limits for Thin Film Measurement Solutions

For this study, the MProbe from Semiconsoft is used to measure the thickness range limit of a thin film. The equipment operates on the spectroscope reflectance measurement principle, in which the phase (j) of the light propagating in the material varies as a function of the wavelength (l), refractive index of the material (n) and the path length (d).

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The Measurement Process

The light reflected from the measurement sample is a blend of each of the beams that were reflected from the various material interfaces, each of these having a distinct phase (Fig. 1). Measurement is carried out by converting the variations in phase to differences in intensity as a function of the wavelength.

Light reflectance from the filmstack. Beam 1 is reflected from the top interface, Beam 2 is propagated though the layer and reflected back from the layer/substrate interface.(multiple reflection in the layer are not shown for clarity).

Figure 1. Light reflectance from the filmstack. Beam 1 is reflected from the top interface, Beam 2 is propagated though the layer and reflected back from the layer/substrate interface.(multiple reflection in the layer are not shown for clarity).

Simulated reflectance spectrum (visible range) for 100 μm SiO2 film (on Si substrate).

Figure 2. Simulated reflectance spectrum (visible range) for 100 μm SiO2 film (on Si substrate).

In the instance of thick films (Fig. 2), the subsequent reflectance spectrum indicates a distinct interference pattern. This enables determination of the thickness of the layer through measurement of the space between the fringes. In order to achieve this for the pattern indicated in Fig.2, there must be an adequately high wavelength resolution to allow the measurement system to resolve the peaks.

MProbe Vis System

The MProbe Vis system boasts <2 nm full-width half-maximum (FWHM), and, where a perfect interface exists between the film and substrate (roughness <1 nm), can measure 100 µm SiO2. If ideal conditions are not met, the amplitude of the fringes will fall and it will become almost impossible to achieve peak resolution. This places the detection limit of the system at approximately 100 µm, however, for an accurate measurement on actual samples, this falls to ~ 75 µm.

Between the maximum thickness and detection limit, measurement accuracy worsens. The use of a higher refractive index material results in a correlative reduction in maximum measurable thickness. For example, where R.I. is twice as high as SiO2 /quartz, the maximum measurable thickness will also be reduced by two times.

Measured reflectance (400-950 nm) of the Silicon rubber film (~ 50 μm thick) with thin Parylene layer. Interference fringes are clearly resolved at longer wavelength (600 nm +).

Figure 3. Measured reflectance (400-950 nm) of the Silicon rubber film (~ 50 μm thick) with thin Parylene layer. Interference fringes are clearly resolved at longer wavelength (600 nm +).

Fragment of Fig. 3 plot showing “pixel resolution” i.e. wavelength distance between individual pixels of the CCD (measurement points) for MProbe Vis system.

Figure 4. Fragment of Fig. 3 plot showing “pixel resolution” i.e. wavelength distance between individual pixels of the CCD (measurement points) for MProbe Vis system.

Analysis of the measurement data from Fig. 1 (limited wavelength range 600-950 nm is used). The peak position indicates the layer thickness.

Figure 5. Analysis of the measurement data from Fig. 1 (limited wavelength range 600-950 nm is used). The peak position indicates the layer thickness.

Measurement data using MProbeHR (wavelength resolution <0.35 nm). In 700-1000 nm wavelength range.

Figure 6. Measurement data using MProbeHR (wavelength resolution <0.35 nm). In 700-1000 nm wavelength range.

The impact of wavelength resolutions on the amplitude of the interference fringe can be established by comparing the measurement data of the high-resolution MProbeHR (Fig. 6) and MProbe Vis (Fig. 3): ~ 0.7% (VisHR) and ~ 0.4% (Vis). It is evident that there has been no impact on the precision of the measurement, and signals as low as 0.01% can still be determined with clarity.

Table I. Measurement thickness: maximum limits.
Thicknesses determined for material with Refractive index ~ 1.5.

System Wavelength Resolution, nm Pixel resolution, nm Maximum thickness, µm Detection limit, µm
MProbe Vis <2.0 0.2 75 µm 100 µm
MProbe Vis-E <1.0 0.2 150 µm 200 µm
MProbe UVVis <2.0 0.22 75 µm 100 µm
MProbe VisHR <0.35 0.2 420 µm 550 µm
MProbeNIR <3.5 1.6 150 µm 250 µm
MProbe VisHRX <0.1 0.035 1200 µm 1900 µm
MProbe NIRHR <0.3 0.1 1800 µm 2000 µm

The minimum thickness limit is governed by the system’s sensitivity. Equation 1 illustrates how sensitivity is increased through a reduction in wavelength.

Reflectance simulation for bare Si and 1 nm SiO2 on Si.in UV range. Two curves can be clearly distinguished in UV.

Figure 7. Reflectance simulation for bare Si and 1 nm SiO2 on Si.in UV range. Two curves can be clearly distinguished in UV.

Measured reflectance: Polymer monolayers on gold substrate Gold (red), 1.3 nm monolayer (Green), 2.6 nm (yellow)

Figure 8. Measured reflectance: Polymer monolayers on gold substrate
Gold (red), 1.3 nm monolayer (Green), 2.6 nm (yellow)

Measurement of 1.3 nm polymer on gold. Model vs. measured data

Figure 9. Measurement of 1.3 nm polymer on gold.
Model vs. measured data

Reflectance simulation for bare Si and 10 nm SiO2 on Si invisible range. Two curves can be clearly distinguished

Figure 10. Reflectance simulation for bare Si and 10 nm SiO2 In Si invisible range. Two curves can be clearly distinguished

In thin films, no distinctive interference pattern is detected, with the spectrum instead displaying a smooth curve that moves down with increasing thickness. In comparison to thick films, the measurement is affected far more by precise calibration and measurement conditions. As a result of this, the minimum thickness is determined at the point where a variation in thickness results in a ~ 1% change in reflectivity.

While it is possible to measure smaller changes at the 0.01% level, perfect interfaces and measurement conditions are necessary, and so it is marked as a detection limit: technically possible to measure, but problematic to achieve under real conditions. Accuracy levels decline between the minimum thickness and detection limit.

Table II. Measurement thickness: minimum limits.
Thicknesses determined for material with Refractive index ~ 1.5.

System Wavelength Range, nm Minimum thickness Detection limit, µm
MProbe Vis 400-1000 nm 10 nm 1 nm
MProbe Vis-E 400-1000 nm 10 nm 1 nm
MProbe UVVis 200-1000 nm 1 nm 0.1 nm
MProbe VisHR 700-1100 nm 1 µm 100 nm
MProbeNIR 900-1700 nm 50 nm 10 nm
MProbe HRX 800-870 nm 10 µm 1 µm
MProbe NIRHR 1500-1550 nm 10 µm 5 µm

Thin-Film Measurement Solutions

In the near-infrared range (wavelength >1,000 nm), silicon is transparent, but it is possible to measure it with the MProbe system, along with very thin SI, which is measurable in the visible range. Two MProbe systems are covering NIR range: MProbe NIR and MProbe NIRHR.

MProbe NIR HR is mainly used for thicker Si samples, while small spot measurement is used to lessen the effects of roughness and non-uniformity in thickness often encountered in thick Si samples.

Reflectance spectrum (simulation) of 500 μm Si (1500-1550 nm range with 0.3 nm resolution) corresponding to MProbe NIRHR

Figure 11. Reflectance spectrum (simulation) of 500 μm Si (1500-1550 nm range with 0.3 nm resolution) corresponding to MProbe NIRHR.

Data analysis results (data from Fig. 11).

Figure 12. Data analysis results (data from Fig. 11).

Data analysis results of 4 μm (MProbeNIRHR).

Figure 13. Data analysis results of 4 μm (MProbeNIRHR).

Reflectance spectrum (simulation) for 35 μm Si (MProbe NIR).

Figure 14. Reflectance spectrum (simulation) for 35 μm Si (MProbe NIR).

 Data analysis results for 35 μm Si -MProbe NIR (data from Fig. 14).

Figure 15. Data analysis results for 35 μm Si -MProbe NIR (data from Fig. 14).

Table III. Minimum thickness range on Si

System Wavelength Range, nm Minimum thickness Detection limit, µm
MProbeNIR 900-1700 nm 100 nm 50 nm
MProbe NIRHR 1500-1550 nm 4 µm 2 µm

Table IV. Maximum thickness range on Si

System Wavelength Range, nm Maximum thickness Detection limit, µm
MProbeNIR 900-1700 nm 30 µm 40 µm
MProbe NIRHR 1500-1550 nm 500 µm 800 µm

This information has been sourced, reviewed and adapted from materials provided by SemiconSoft.

For more information on this source, please visit SemiconSoft.

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