Modern semiconductor manufacturing technology has advanced rapidly. Because of this, measurement using non-contact displacement and transparent coating thickness has become very critical. Applications including photo resist coating thickness monitoring, nanometer level wafer positioning and high precision wafer profiling require modern high-precision measurement instruments that were not available a decade ago.
Previoulsy these measurements were performed using fiber interferometry. There are several inherent limitations in incremental measurement which made accurate measurements difficult if not impossible. However, with advances in multispectral interferometry, applications that were considered tough or impossible can now be solved very easily.
Conventional Fiber Interferometers
The basic limitation of conventional interferometers is that they are not capable of determining the exact position of the target being measured. A schematic representation of a conventional fiber interferometer is shown in Figure 1
Figure 1. Traditional Fiber Interferometer
A single wavelength light source is focused on a target and reflected forming a superposition waveform which is monitored to measure the displacement. Any change in the target position will result in a change in the superposition waveform irrespective of the direction of the shift. This method of displacement measurement is inaccurate because the position may be determined but the direction is not clear.
In actual fact, this is not very significant since typically the setup will be able to determine the direction of movement. However, accurate determination of the target position is only possible when the start position is known and there has been no interruption in the measurement since the target left this known position. In case both these conditions are not fulfilled, accurate positioning of the measured target is not possible using a traditional interferometer.
Multispectral Fiber Interferometers
Figure 4. Keyence multispectral fiber interferometer
Another major drawback of a conventional interferometer is using a single wavelength laser light source. Interferometer technology has considerably advanced enabling the usage of a super luminescent diode (SLD) light source which provides enhanced functionalities.
Super Luminescent Diode Features
Conventionally, a single wavelength laser is commonly used as the light source. Using this technique, determination of the target displacement is limited to using the results of a single superposition waveform. A SLD can generate a light source with different wavelengths across a narrow band. Accordingly, it is not required to be restricted to the superposition waveform of a single wavelength of light but the entire band of the light source is used to generate multiple superposition waveforms.
The Advantage of Having Multiple Superposition Waveforms
Basically, each waveform could be used independently in a technique similar to a traditional interferometer. However, by using multiple waveforms the direction of motion is not only tracked in a way similar to a conventional interferometer but also the absolute position is determined by the relative amplitudes of the superposition waveforms.
Multispectral interferometers find applications in the following fields:
- Wafer Thickness Measurement
- Wafer Warpage Measurement
Wafer Thickness Measurement
Conventional interferometry technologies will not be able to perform a direct thickness check on a silicon wafer positioned between two fiber interferometers. This is mainly because no displacement is taking place during the measurement. The change in thickness and wafer uniformity can be measured but not the actual thickness.
Capacitive sensors require that the wafer should be grounded and a large measurement head be placed very close to the wafer surface. Hence using capacitive sensors is also not very feasible.
A multispectral interferometer enables determination of the absolute position for each of the sensor heads involved in the check. The data obtained is compared to directly generate absolute thickness data allowing a significant reduction in time. Additionally, a multispectral interferometer is mounted with a significant standoff that ensures an additional layer of protection against incidental wafer damage caused by wafer contact with the measurement device during abnormal part changeover.
Wafer Warpage Measurement
A conventional interferometer cannot directly measure warpage by positioning multiple measurement instruments at various points along the wafer surface. Either the target or the sensor had to be moved in order to measure warpage which may cause unnecessary damage and increases the tact time and setup costs.
These restrictions are overcome by a multispectral interferometer and simultaneous absolute measurements on multiple points of a target surface are possible without translating the sensors.
Multispectral interferometryovercomes most of the restrictions met by conventional interferometers in semiconductor manufacturing applications. The absolute nature of multispectral interferometry enables precise nanometer level measurements without a relative reference or a home position. This ability is critical in applications in high precision manufacturing where relative displacements cannot be used and providing a known reference is near to impossible.
With the rapid development of factory automation and enhanced focus on inspection and R&D tools, KEYENCE, as the leading supplier of sensors, measurement devices and microscopes, is developing and manufacturing innovative and reliable products that meet customer requirements in every manufacturing industry.
This information has been sourced, reviewed and adapted from materials provided by KEYENCE.
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