White-light Interferometry

Figure 1. White-light interferometer.

Features

White-light interferometry is a method for non-contact, non-destructive three-dimensional measurement of a sample surface using a high-intensity light source with a wide field of view, high vertical resolution, and a large dynamic range.

Application Examples

• Roughness of polished and deposited film surfaces such as semiconductor wafers

  1. Observation of defects and abnormal areas

  2. Observation of fabricated structures and devices such as patterned holes, trench bevels, and micro electro-mechanical systems (MEMS)

• Metal surfaces

  1. Ball bearings, soldering bonding

  2. Scratches, fractured surfaces, and die shapes of gear, cutters, and steel plates

  3. Roughness plated and friction surfaces

• Other materials

  1. Cross-sectional analysis

  2. Shapes of glass, optical and electrical components

Operational principle and equipment configuration

Figure 2. Surface analysis at high vertical resolution (left) using an interferometer, and medium lateral resolution (right) using a charge-coupled device (CCD) camera (>640 nm depending on the objective lens)

Data examples

Figure 3. Observation of a ZnO film surface.

Figure 4. Example analysis of Si/SiGe laminated sample surfaces (cross-hatch pattern).

Description of labelling and parameters:

1. Surface roughness analysis (filled plot):

   Region of interest (ROI) within FOV: \(0.71 \times 0.53\ \text{mm}\) (“Size X, Size Y”)

   Equidistant height curves indicated by colour

   Surface roughness in terms of \(P_{\text{v}}\) , \(R_{\text{a}}\), and \(Z_{RMS}\) (see Eqs. (1)-(3) below)

2. Arbitrary cross-sectional analysis (profile plot):
   Cross section indicated by the white line in (1)

   Horizontal axis=distance; Vertical axis=height

   \(P_{\text{v}}\), \(R_{\text{a}}\), and \(Z_{RMS}\) along the white line

3. Bird’s eye view of the region in (1)

4. Intensity distribution plot

   Shows intensity distribution of light scattered off the sample surface

   Optical microscopy does not yield surface roughness

5. Name and serial number of sample/file

	\[R_{a} = \frac{1}{N}\sum_{i}^{N}\left| Z_{i} - Z_{\text{cp}} \right|\]	(1)
	\[P_{\text{V}} = h_{\text{max}} - h_{\text{min}}\]	(2)
	\[d_{\text{RMS}} = \sqrt{\frac{1}{N}\sum_{i}^{N}\left( Z_{i} - Z_{\text{cp}} \right)^{2}}\]	(3)

where \(R_{a}\) is the average height difference from the centre, \(P_{\text{V}}\) the maximum height difference, \(d_{\text{RMS}}\) the root mean square of the height difference from the centre, \(Z_{i}\) the height of data point \(i\), \(Z_{\text{cp}}\) the height of the centre, and \(N\) the number of data points.

Data delivery format

• PDF file

Measurement specifications

Property	Value	Unit	Notes
Maximum sample size	\(127 \times 152 \times 104\)	\(w \times d \times h\) mm
Stage load carrying capacity	1.4	kg
Measurable area	\(0.05 \times 0.07\ \sim\) \(5 \times 7\)	\[\text{m}\text{m}^{2}\]	\(xy\) direction
Measurable height	<150	\[\mu\text{m}\]	\(z\) direction
Horizontal resolution	0.64 - 4.72	\[\mu\text{m}\]	\(xy\) direction. Depends on objective lens magnification
Vertical resolution	0.1	nm	High resolution

Items for enquiries

• Purpose and scope of the analysis

• Sample information:

  1. Quantity, availability of pre-analysis samples, sample cuttability

  2. Structure, shape, material, layer structure, film thickness, expected roughness

  3. Desired ROI

  4. Care instructions

• Delivery date:

  1. Desired delivery dates of preliminary and final results

  2. Handling instructions

• Other relevant information

Caution

Measurement can be difficult in the following cases:

• Weak reflection at the sample surface (laminated structure of transparent and/or high-reflectance films, see Figure 5). In this case, only information at the interface of the film with high reflectivity is detected, not any unevenness. Measurement could be possible if the sample is coated. Contact MST first.

Figure 5. Sample with high reflectance

• Sample surface reflectivity of 4% or less, such as a diffusively reflecting surface or a surface with a steep step (see Figure 6). In this case, the height cannot be measured because reflected light is not detected at the surface of the sample.

Figure 6. Sample with diffuse reflectance and/or scattering from a structure.