Thin Film Measurement Solutions - Thickness Range Limits for the MProbe

In this experiment, the thickness range limit of a thin film is measured using the MProbe from Semiconsoft. This instrument works on the spectroscope reflectance measurement principle, where 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).

(1)

The Measurement Process

The light that is reflected from the measurement sample is a combination of all the beams that were reflected from the numerous material interfaces – each of them having a different phase (Fig. 1). The measurement process involves converting the differences in phase to intensity variations 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)

The resulting reflectance spectrum shows, in the case of the thick films (Fig. 2), a distinct interference pattern. This allows the thickness of the layer to be determined by measuring the distance between the fringes. If this is to be done for the pattern on Fig. 2, then the wavelength resolution must be sufficiently high so that the measurement system can resolve the peaks.

MProbe Vis System

The MProbe Vis system has <2 nm full-width half-maximum (FWHM), and can measure 100 µm SiO2 assuming a perfect interface between the film and substrate (roughness < 1 nm), etc. If the conditions aren’t perfect the amplitude of the fringes will decrease and peak resolution will become near impossible. This makes the detection limit of the system around 100 µm; for a precise measurement on real samples this reduces to ~ 75 µm.

Measurement precision gets worse at the point between the maximum thickness and detection limit. Using a higher refractive index material causes a corresponding reduction in maximum measurable thickness. For example, if R.I. is twice as high as SiO2 /quartz, the maximum measurable thickness will be reduced two times as well.

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.

When the measurement data of the high-resolution MProbeHR (Fig. 6) and MProbe Vis (Fig. 3) is compared, the effect that the wavelength resolution has on the amplitude of the interference fringes can be determined: ~ 0.7% (VisHR) and ~ 0.4% (Vis). It is also clear that there hasn’t been an effect on measurement accuracy, and signals down to 0.1% can be clearly determined.

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

It is the sensitivity of the system that determines the minimum thickness limit. Equation 1 shows how a reduction in wavelength increases sensitivity.

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 on Si invisible range. Two curves can be clearly distinguished

Thin films do not have distinct interference pattern, the displayed spectrum is instead a smooth curve that shifts down with increasing thickness. There is a greater effect by accurate calibration and measurement conditions on the measurement compared to thick films. Due to this, the minimum thickness is determined at the point where a change in thickness causes a ~ 1% change in reflectivity.

Although smaller changes at the 0.1% level can be measured, they require perfect interfaces and measurement conditions, so it is defined as a detection limit (possible to measure but difficult to achieve in practice). The accuracy deteriorates 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

Silicon is transparent in the near-infrared range (wavelength >1000 nm), but it can be measured by the MProbe system (very thin Si can also be measured in the visible range). It requires the two MProbe configuration to cover the cover NIR range: MProbe NIR and MProbe NIRHR. MProbe NIR HR is primarily used for thicker Si samples and small spot measurement is used to reduce the effects of roughness and non-uniformity in thickness commonly seen 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.

Citations

Please use one of the following formats to cite this article in your essay, paper or report:

  • APA

    SemiconSoft. (2018, December 03). Thin Film Measurement Solutions - Thickness Range Limits for the MProbe. AZoM. Retrieved on August 15, 2020 from https://www.azom.com/article.aspx?ArticleID=16979.

  • MLA

    SemiconSoft. "Thin Film Measurement Solutions - Thickness Range Limits for the MProbe". AZoM. 15 August 2020. <https://www.azom.com/article.aspx?ArticleID=16979>.

  • Chicago

    SemiconSoft. "Thin Film Measurement Solutions - Thickness Range Limits for the MProbe". AZoM. https://www.azom.com/article.aspx?ArticleID=16979. (accessed August 15, 2020).

  • Harvard

    SemiconSoft. 2018. Thin Film Measurement Solutions - Thickness Range Limits for the MProbe. AZoM, viewed 15 August 2020, https://www.azom.com/article.aspx?ArticleID=16979.

Tell Us What You Think

Do you have a review, update or anything you would like to add to this article?

Leave your feedback
Submit