JP2629319B2 - Film thickness monitor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a film thickness monitor provided in a vacuum evaporation apparatus or a sputtering apparatus.

2. Description of the Related Art As a conventional film thickness monitor, for example,
No. 17139.

FIG. 4 shows a block diagram and an optical system of this conventional film thickness monitor, wherein 1 is a monochromatic light source, 2 is an optical sensor, 3 is a lens for converting light from the light source 1 into substantially parallel light, Is a thin film being formed, 5 is a DC power supply for driving the light source 1, and 6 is a pen recorder for monitoring the output of the optical sensor. The surface to be measured 4 and the central axis of the lens 3 are set to be perpendicular to each other, and the light source 1 and the optical sensor 2 are in the same plane separated from the lens 3 by the focal length, and each of the light source 1 and the optical sensor 2 Are set so as to be point-symmetric with respect to the central axis of the lens 3.

In the conventional film thickness monitoring device configured as described above, the light becomes substantially parallel by the optical lens 3 from the light source 1 and is incident on the thin film 4. The incident light is reflected from the front surface and the back surface of the thin film 4 as substantially parallel light, condensed by the lens 3, and incident on the optical sensor 2.

The light reflected on the front and back surfaces of the thin film 4 interferes with each other due to the difference in their optical path lengths. Therefore, when the film thickness increases, the amount of reflected light, that is, the output of the optical sensor 2 becomes the fifth.
It changes to show peaks and valleys as shown.

The distance between the peak and the valley is determined by the refractive index of the thin film 4 and the wavelength of the light source 1. Therefore, the thickness of the thin film 4 can be determined by monitoring the output of the optical sensor 2 with the pen recorder 6 or the like.

Problems to be Solved by the Invention However, the above-described configuration has a problem that the film thickness cannot be accurately monitored. The reason is shown below. There is only one signal from the optical sensor 2 indicating the film thickness, and the number of peaks and valleys serving as accurate marks of the film thickness is small.
That is, the number of data for determining an accurate film thickness is small. In addition, since the level of the peaks and valleys changes with an increase in the film thickness due to the light absorption of the thin film, it is only possible to determine whether the film thickness corresponds to the peaks or the valleys only after exceeding them. It is difficult to control the film thickness.

Also, if the number of data is increased by using a plurality of the conventional film thickness monitors (the wavelengths of the light sources are different), the scale of the optical system becomes large, and it is difficult to install the optical system. Further, crosstalk occurs between the film thickness monitors due to the light reflected and scattered at each portion, and accurate data cannot be obtained.

Accordingly, it is an object of the present invention to provide a film thickness monitoring device that can be easily installed and can monitor the film thickness as accurately as possible by increasing the number of data when determining the film thickness.

Means for Solving the Problems The present invention provides a light source for irradiating an object to be measured with N (an integer of N ≧ 2) semiconductor lasers or light emitting diodes, each having a different emission wavelength. N oscillators each having a different oscillation frequency for supplying an oscillation frequency signal to the N light sources, a DC power supply for providing a DC signal to the N light sources, and an output of the DC power supply and the N oscillators. N adders that add and drive the N light emitting sources, an optical sensor that receives light reflected back from the device under test, a preamplifier that amplifies the output of the optical sensor, A film thickness monitor apparatus comprising: N lock-in amplifiers that synchronously detect an output of the preamplifier using signals of N oscillators as reference signals.

According to the present invention, with the above-described configuration, N light beams having different wavelengths are intensity-modulated at N different frequencies, and the outputs of the optical sensors are separated by N lock-in amplifiers. A signal indicating a film thickness without crosstalk is obtained. From these N signals, data N times as large as the data obtained by the conventional film thickness monitor can be obtained, so that a more accurate film thickness can be monitored.

Embodiment 1 FIG. 1 shows a block diagram and an optical system of a film thickness monitor according to a first embodiment of the present invention. FIG. 2 is a waveform diagram showing the waveform of each part shown in FIG. The time axis t in FIG. 2 represents a sufficiently small time. That is, it is assumed that the film thickness does not change in the period shown here. FIG. 3 is a waveform diagram showing the signals S3a and S3b of FIG. 2 which change with an increase in film thickness. This embodiment is for the case of N = 2.

In FIG. 1, 1a and 1b denote emission wavelengths λ1 and λ, respectively.
2 (≠ λ1) light emitting diodes or semiconductor lasers and other light emitting sources, 2a and 2b have oscillation frequencies f1 and f2 (≠ f1), respectively
Oscillator, 3a and 3b are DC power supplies, 4a is DC power supply 3a and oscillator 2
adder for adding the output of a to drive the light emitting source 1a; 4b, an adder for adding the output of the DC power supply 3b and the oscillator 2b to drive the light emitting source 1b; 5 for an optical sensor; 6 for a preamplifier; 7a for the oscillator 2a
7b is a lock-in amplifier for synchronously detecting the output of the optical sensor 5 using the reference signal as a reference signal, 7b is a lock-in amplifier for synchronously detecting the output of the optical sensor 5 using the signal of the oscillator 2b as a reference signal, and 8 is a lock-in amplifier 7a, 7b The pen recorders 9a and 9b are lenses for converting the light of the light sources 1a and 1b into parallel light, 10 and 11 are half mirrors, and 12 is a thin film being formed.

The operation of the film thickness monitoring apparatus of the present embodiment configured as described above will be described below with reference to FIGS. Note that FIGS. 2 and 3 show that the light emitting sources 1a and 1b are λ
1 = 890 nm, λ2 = 660 nm light emitting diode, 5 mm square silicon photodiode as optical sensor 5, f1
= 4.5 KHz, f2 = 3 KHz, and the refractive index n = 4 of the thin film.

S1a and S1b are signals applied to the light-emitting sources 1a and 1b, respectively, and the output levels DCa and DCb of the respective DC power supplies have sine waves of amplitudes ACa and ACb and outputs f1 and f2 of the respective oscillators. This is a superimposed signal. The intensity of light emitted from the light emitting source is proportional to S1a and S1b.

Light emitted from the light emitting sources 1a and 1b and reflected by the thin film 12 enters the optical sensor 5 and becomes a signal obtained by superposing S1a and S1b as shown in S2. Here, α and β shown in the figure are constant coefficients. is there.

S2 is synchronously detected by lock-in amplifiers 7a and 7b. As a result, the amplitudes of the f1 and f2 components of S2 can be separated and extracted, resulting in S3a and S3b, respectively. S3a and S3b denote the intensity amplitudes of the light of wavelengths λ1 and λ2 reflected by the thin film 12, respectively.

Since S3a and S3b change as shown in FIG. 3 as the film thickness increases, the film thickness can be determined by monitoring these signals with the pen recorder 8 or the like.

According to the present embodiment configured as described above, the wavelength is λ
Intensity modulation is performed on the light of wavelengths 1 and 2 at frequencies f1 and f2, and the output of the optical sensor in which the reflected light signals of wavelengths λ1 and λ2 are mixed is separated by two lock-in amplifiers to indicate the film thickness. Two signals are obtained. This means that twice the number of data can be obtained than when a conventional film thickness monitor is used, and the film thickness can be monitored more accurately.

Further, in this embodiment, a signal in a very narrow band is extracted by performing synchronous detection with a lock-in amplifier. Therefore, the film thickness is hardly affected by various drifts, radiation noise, and the like, and the film thickness can be determined from a signal having a good S / N, so that the accuracy is high.

Effect of the Invention As described above, according to the present invention, the film thickness can be monitored more accurately. In addition, unlike the crystal resonator type film thickness meter, the film thickness of the surface to be measured itself is monitored. Therefore, the present invention has a great practical effect in controlling the film thickness when forming a thin film.

[Brief description of the drawings]

FIG. 1 is a block diagram of a film thickness monitor according to one embodiment of the present invention, FIG. 2 and FIG. 3 are operation waveform diagrams of the present embodiment,
FIG. 4 is a block diagram of a conventional film thickness monitor, and FIG. 5 is an operation waveform diagram of the conventional film thickness monitor. 1a, 1b: Light-emitting source, 2a, 2b: Oscillator, 3a, 3b: DC power supply, 4a, 4b: Adder, 5: Optical sensor, 6: Preamplifier, 7a, 7b: Lock-in amplifier , 8 ... pen recorder, 9a, 9b ... lens, 10,11 ... half mirror, 1
2 ... surface to be measured, 1 ... light emission source, 2 ... light sensor, 3
... Lens, 4 ... Measurement surface, 5 ... DC power supply, 6 ...
Pen recorder

Claims (1)

(57) [Claims] 1. A light source for irradiating an object to be measured with one of N (an integer of N ≧ 2) semiconductor lasers or light-emitting diodes, each having a different emission wavelength, and the N light sources. N oscillators each having a different oscillation frequency for supplying an oscillation frequency signal, a DC power supply for providing a DC signal to the N light sources, and adding the outputs of the DC power supply and the N oscillators, N adders for driving the light-emitting sources, an optical sensor for receiving light reflected back from the device under test, a preamplifier for amplifying the output of the optical sensor, and N oscillators A film thickness monitor device comprising: N lock-in amplifiers for synchronously detecting an output of the preamplifier using a signal as a reference signal.