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Interferometric thickness-measurement of non-transparent lithographically structured layers

IP.com Disclosure Number: IPCOM000020742D
Original Publication Date: 2003-Dec-11
Included in the Prior Art Database: 2003-Dec-11
Document File: 4 page(s) / 84K

Publishing Venue

IBM

Abstract

Disclosed is a method to measure the thickness of non-transparent structures on a lithographical structured wafer without need to remove the transparent photoresist layer.

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Page 1 of 4

Interferometric thickness-measurement of non-transparent lithographically structured layers

Main Idea of disclosure

1. Problem

Wafer-manufacturing often includes steps where different metal layers (NiFe, Cu, Au,...) are deposited on a lithographical structured wafer (step 1) e.g. by electroplating processes. In these processes the metal layers are grown up from a seed layer in between the photoresist-structures of thickness R (step 2). After the process the photoresist structures are removed (step 3).

    Today the thickness M of the grown metal layers is measured by profilometry after step 3 when the photoresist has been removed.

    The disclosed method measures the thickness of these non-transparent structures on a lithographical structured wafer.

The advantages are:
1. The method is able to measure while the photoresist is still on the wafer (after step 2).

     -> Therfore it is possible to deposit more than 1 metal layer within the same photoresist-structure.

(Necessary for bilayer- / multilayer-plating) -> feedback for process control is much faster than today

          (thickness values directly after step 2, today step 3 delays this information 30 min)

-> Rework is possible. Plate again if too thin / remove if too thick.
2. It is a non-contact method to measure the layer thickness
3. There are no moving parts during the measurement
4. The measurement itself is much faster than profilometry

2. Solution

    The disclosed method is based on white light interferometry: Light from a halogen-lightsource is guided through a fiber bundle, collimated by optics and directed on the wafer surface. The distance between Optics and Wafer is constant. Reflected light is collected by the same optics and sent to a commercial CCD-spectrometer. The spectra are processed by PC to get information about the layer thickness.

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    So far this is very similiar to commercially available white light interferometers. These commercially available tools measure the thickness R of transparent layers. The measured area is homogeneously covered with the transparent layer. Light is reflected from the resist top (A) and from the interface between resist and wafer-substrate (B). Because of a phase difference between the two different light waves the measured spectra are modulated. The modulation frequency contains information about the thickness R.

The (simplified) theoretical model for these signals is:

    I is the intensity in the modulated spectra and depends on the thickness of the layer R and the wavelength l.

    A is the amplitude of the signal and depends on the material properties. F is the phasedifference of the two light waves and depends on the thickness of the layer R and the wavelength l.

The disclosed method measure...