CN111189398A

CN111189398A - Wafer film thickness measuring device based on white light interference method

Info

Publication number: CN111189398A
Application number: CN202010029938.1A
Authority: CN
Inventors: 郑永军; 狄韦宇; 陆艺
Original assignee: China Jiliang University
Current assignee: China Jiliang University
Priority date: 2020-01-13
Filing date: 2020-01-13
Publication date: 2020-05-22

Abstract

The invention discloses a wafer film thickness measuring device based on a white light interferometry. The device mainly comprises a white light source, a spectrometer, a collimator, an optical fiber transmission unit and a PC. Based on the white light interferometry, the nondestructive measurement of the wafer film thickness can be realized, the time is short, the equipment is small and exquisite, the operation is simple, the precision is high, and the method is suitable for laboratory detection.

Description

Wafer film thickness measuring device based on white light interference method

Technical Field

The invention relates to the field of optical precision measurement and signal processing, in particular to a wafer film thickness measuring device based on a white light interference method.

Background

The wafer is critical to semiconductor devices and film thickness is one of the important parameters that affect the physical properties of the wafer. Generally, the film thickness is measured by ellipsometry, probe method, optical method, etc., and the ellipsometry apparatus is expensive, and the probe method may damage the wafer surface. Precision testing by using an optical principle has been one of the main methods in the technical field of metrology and testing, and in the field of optical measurement, a measurement system based on an interference principle has become one of the most accurate systems in physical quantity detection. The light interferometry and testing is essentially based on the measurement of the wavelength of light, and modern interferometry and measurement techniques have been able to achieve a measurement accuracy of a few percent of a wavelength, and interferometry is more characterized by higher sensitivity (or resolution) and accuracy. Most of the interference tests are non-contact, and surface damage and additional errors cannot be brought to a tested piece; the measuring object is wide and is not limited to metal or nonmetal; multiple parameters may be detected, such as: length, width, diameter, surface roughness, area, angle, etc.

Interferometric measurements can be expressed as: the white light interference spectroscopy mainly uses the interference principle and the spectral splitting principle of light, and uses the interference light intensity of the light at different wavelengths to solve. The light that the light source was emergent divides into two bundles through beam splitting prism, and one of them bundle incides the reference mirror, and another bundle incides the survey sample surface, and two bundles of lights all take place the reflection and incide to beam splitting prism, and these two bundles of lights can take place to interfere this moment. The interference light is collected by the spectrometer to obtain a white light spectrum interference signal, data is processed by a computer, the change of the result is displayed, and finally, the thickness value or the change quantity is read out.

How to establish a set of wafer film thickness measuring device based on white light interferometry is significant for wafer film thickness measurement, and equipment price, space size and operation difficulty are all influencing factors.

Disclosure of Invention

The invention aims to provide a wafer film thickness measuring device based on a white light interference method, aiming at the defects of the prior art. The device comprises a white light source, a microscope, a beam splitter, an interference objective lens, an optical fiber transmission unit, a collimator, a spectrometer, a USB transmission line and a computer. The spectrometer mainly comprises six parts which are respectively: the detector comprises a fiber inlet, a collimating mirror, a grating, a focusing mirror, an area detector and a detector with an OFLV filter.

The specific steps of the measurement are as follows: the white light source emits white light, the white light is vertically incident to the surface of the wafer through the optical fiber probe, an interference spectrum formed by the interference of reflected light on the upper surface and the lower surface of the sample film is received by the reflective optical fiber probe and then transmitted to the spectrometer through the optical fiber, the spectrometer continuously records reflected signals, and measured data are transmitted to the computer through the USB cable.

The invention has the beneficial effects that: the nondestructive measurement of the thickness of the wafer film can be realized, the time is fast, the equipment is small and exquisite, the operation is simple, the precision is high, and the device is suitable for laboratory detection.

Drawings

FIG. 1 is a schematic diagram of the apparatus of the present invention;

wherein: 1. the device comprises a white light source, 2, a light source collimating mirror, 3, a beam splitter, 4, a collimator, 5, a Michelson interference objective, 6, a wafer to be tested, 7, a fiber coupling lens, 8, a fiber, 9, a spectrometer, 10, a USB transmission line, 11 and a computer.

FIG. 2 is a diagram of the internal structure of the spectrometer;

FIG. 3 is a graph of light intensity obtained from light source testing;

FIG. 4 is a graph of collected light intensity versus wavelength.

Detailed Description

The invention will be further explained with reference to the drawings.

As shown in figure 1. White light emitted by the light source is expanded and collimated by the collimating lens to form parallel light, the parallel light is emitted into the Michelson interference objective lens by the beam splitter, the white light is contracted and collimated by the collimating lens and then vertically irradiated onto a wafer to be measured, reflected light is mutually interfered, the interference light intensity enters the optical fiber coupling unit after passing through the collimating lens, and the interference part is completed. Interference signals transmitted by the optical fibers enter the spectrometer, the computer collects spectrum signals from the spectrometer at regular time to acquire information such as light intensity, reflectivity and the like, the computer performs signal processing on the information, high-frequency noise information is filtered, then normalization processing is performed on the spectrum information, and the thickness of the wafer film is calculated by using a wavelength value corresponding to a peak value.

The light source adopts a xenon lamp light source, and the xenon lamp is selected as the light source, so that the xenon lamp light source has the following advantages: the xenon lamp is a continuous spectrum, the spectral distribution is almost irrelevant to the change of the input power of the lamp, and the spectral energy distribution is almost unchanged in the service life; the xenon lamp has good consistency of optical and electrical parameters, and the working state is slightly influenced by the change of external conditions; the xenon lamp has high electro-optic conversion efficiency, and can output high-energy parallel light and the like.

The microscope adopts an Axioplan2Imaging research-grade upright optical microscope produced by Zeiss company, and can provide a light path for eliminating stray light for an interference system. The microscope provides a six-hole converter, which can conveniently replace the objective lens.

The interference objective lens adopts a Michelson type fiber interference objective lens of Nikon company, and the magnification is 5X. The device comprises an objective lens, a spectroscope and a reference plate, wherein light emitted by a light source passes through the microscope and is divided into two parts by a beam splitter prism, one part is used as reference light to enter the reference mirror and be reflected, the other part is used as measuring light to enter the surface of a sample and be reflected, and the two reflected lights are reflected to the beam splitter prism and are interfered. Because the angle between the sample and the measured sample needs to be adjusted in the experiment so as to carry out measurement better, the 5XMichelson type interference objective lens can be adjusted through two knobs arranged on the interference objective lens, and the reference lens angle can be adjusted in a larger range through the knobs so as to be adjusted to an ideal angle.

The optical fiber is responsible for transmitting light in the test system and transmitting the interference signal of the microscope field of view to the micro spectrometer. The system selects the optical fiber as an advanced optical fiber component produced by ocean optics companies, the inner layer of the optical fiber connecting wire is a single wire steel ring wrapped by silicon resin, and the outer layer of the optical fiber connecting wire is a Nomex braided fabric, so that the stress is better reduced and an effective protection effect is achieved. The assembly ends are easy to handle metal rings-high precision SMA connectors. One end of the optical fiber is connected with the adapter, and the other end of the optical fiber is connected with the micro spectrometer so as to transmit the interference optical signal into the spectrometer.

The spectrometer is a QE PRO spectrometer produced by ocean optics and is a core component of a spectrum acquisition module. The internal structure of the device is shown in figure 2. Mainly comprises six parts which are respectively: the detector comprises a fiber inlet, a collimating mirror, a grating, a focusing mirror, an area detector and a detector with an OFLV filter. Light enters the spectrometer from the optical fiber, is projected onto the grating after passing through the filter and the collimator, white light is dispersed into a spectrum by the grating, and is projected onto the detector through the focusing lens, and then the detector transmits a light signal into the computer. The optical fiber connector fixes the input optical fiber on the spectrometer, so that the light from the input optical fiber can enter the optical platform; the filter confines the optical radiation to a predetermined wavelength region; the collimating lens focuses light entering the optical platform onto a grating of the spectrometer, so that collimation between a light path and the grating is ensured; the grating diffracts the light from the collimating lens and guides the diffracted light to the focusing lens; the focusing mirror receives the light reflected from the grating and focuses the light onto the detector; the detector converts the detected optical signal into a nm wavelength system; the area detector provides a quantum efficiency of 90% and pixels in vertical columns to extract light from the entire height of the spectrometer's slit image, significantly improving the signal-to-noise ratio.

Light source stability has a significant impact on testing, and unstable light sources can cause testing to fail, for which reason the relative stability of the light source is tested. As shown in FIG. 3, the light intensity of a certain wavelength varies with time, the wavelength is chosen to be 590nm, and the acquisition time is 50 ms. The light intensity contains not only noise introduced by the light source, but also noise introduced by the environment and the system. The light source noise can directly cause the instability of light intensity, the environmental noise is mainly generated by stray light, and the system noise is mainly random noise such as dark noise generated by a spectrometer. It can be seen from the figure that the light intensity changes steadily and slowly over time. The light source is required to have short-time stability, and the light intensity is basically stable in a short time as can be seen from fig. 3, so that the test requirement can be met. FIG. 4 is a graph of the wavelength and intensity of light collected. The above is believed that the device of the present invention can effectively measure spectral signals.

Claims

1. Wafer film thickness measuring device based on white light interferometry, its characterized in that: the device comprises a white light source, a microscope, a beam splitter, an interference objective lens, an optical fiber transmission unit, a collimator, a spectrometer, a USB transmission line and a computer; the spectrometer mainly comprises six parts which are respectively: the detector comprises an optical fiber inlet, a collimating mirror, a grating, a focusing mirror, a region detector and a detector with an OFLV filter;

white light emitted by a light source is expanded and collimated by a collimating lens to form parallel light, the parallel light is emitted into a Michelson interference objective lens by a beam splitter, the white light is contracted and collimated by the collimating lens and then vertically irradiated onto a wafer to be measured, reflected light is mutually interfered, and the interference light intensity enters an optical fiber coupling unit after passing through the collimating lens to complete an interference part;

interference signals transmitted by the optical fibers enter the spectrometer, the computer collects spectrum signals from the spectrometer at regular time to acquire information such as light intensity, reflectivity and the like, the computer performs signal processing on the information, high-frequency noise information is filtered, then normalization processing is performed on the spectrum information, and the thickness of the wafer film is calculated by using a wavelength value corresponding to a peak value.