JP2545948B2 - Etching equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] The present invention relates to an etching apparatus for detecting that an etching reaches a desired depth by using an optical means. The purpose is to provide a means for accurately monitoring the etching depth as it progresses and automatically stopping the etching. A light source that generates coherent light and a chamber that can transmit the coherent light inside and that holds an object to be etched. A photodetector that receives reflected light that has interfered with the surface of the object to be etched, a waveform analysis unit that frequency-analyzes the signal obtained by photoelectric conversion with the photodetector, and a frequency distribution of the interference wave obtained by the waveform analysis unit, And an etching depth calculating means for calculating the etching depth.

[Industrial applications]

The present invention relates to an etching apparatus that detects when an etching reaches a desired depth by using optical means.

In recent years, along with the demand for higher reliability of semiconductor devices, there is an absolute demand for accurate etching to a desired depth.

Conventionally, a method of monitoring the change in emission spectrum when exposing the wafer surface by etching away the etching film previously provided on the semiconductor wafer surface, such as a CVD-SiO ₂ film, or a thin film on the etching film Optical interferometry has been adopted to monitor optical interference.

However, there is a strong demand for control etching for etching and removing the semiconductor wafer itself to a desired depth, and even in such a case, an apparatus capable of automatically stopping etching is desired.

[Conventional technology]

 The conventional optical interference method will be described below.

In the optical interference method, coherent light is projected onto the film to be etched formed on the surface of the semiconductor wafer, and the reflected light r _A from the surface of the film to be etched and the reflected light r _B from the interface of the lower semiconductor wafer.
The change in the reflected light intensity due to the interference effect between and is detected, and when the change in the reflected light intensity becomes constant, it is judged as the etching end point, and it directly contacts the surface to be measured. It has the feature that it can be measured without any need.

[Problems to be Solved by the Invention]

However, with this method, when the surface to be etched cannot produce the thin film interference effect, that is, when there is no underlying reflection surface as in the case of groove etching with a certain depth in the Si (silicon) substrate, the etching depth is monitored. Will not be possible. Therefore, the conventional method has a drawback that it cannot be used for end point detection in etching removal of the semiconductor wafer itself.

The present invention has been made in view of the above-mentioned drawbacks of the prior art, and means for accurately monitoring the etching depth simultaneously with the etching operation and automatically stopping the etching even when the semiconductor wafer itself is etched. The purpose is to provide.

[Means for solving the problem]

In the present invention, as a means for solving such a problem,
A light source that generates coherent light, a chamber that can transmit the coherent light inside, and that holds an object to be etched, a photodetector that receives reflected light that interferes with the surface of the object to be etched, and photoelectric conversion by the photodetector. It is configured to have a waveform analysis means for frequency-analyzing the signal and an etching depth calculation means for calculating the etching depth based on the frequency distribution of the interference wave obtained by the waveform analysis means.

[Action]

Hereinafter, the principle and operation of the present invention will be described with reference to FIG. FIG. 4 illustrates how reflected light is captured by the device of the present invention.

In the present invention, as shown in FIG. 4 (a), the reflected light r _{1 from} the mask film surface, the reflected light r ₂ from the lower surface of the mask film, and the reflected light r ₃ from the semiconductor wafer surface are mutually reflected. Read the interference. As the etching progresses, as shown in FIG. 4B, the surface to be etched, that is, the wafer surface is removed, and at the same time, the mask film surface is generally removed at a slower rate than the surface to be etched. Therefore, the reflected light r ₁ , r ₃
Optical path length (strictly speaking, the optical path length of r ₂ also considering the refractive index of the mask film) changes as the etching progresses. Therefore, as the etching time elapses, the three interference states change, and the reflected light intensity information obtained by combining them is obtained from the photodetector. The signal waveform from the photodetector shows the result of combining the state changes of the interference between the three reflected lights in this way.
By separating the waves, the etch rate of the etched surface and the etch rate of the mask surface can be calculated. By thus monitoring the etching rate of the surface to be etched in real time, the etching depth can be monitored based on the elapsed time information, and the etching can be stopped at the desired depth.

〔Example〕

 (A) Description of one embodiment See FIG.

An embodiment of the present invention will be described below. FIG. 1 is a device configuration diagram according to an embodiment of the present invention.

In this embodiment, an example applied to RIE (reactive ion etching) is shown, and the etching target 9 is made of Si.
The surface of a (silicon) wafer is coated with CVD-SiO ₂ as a mask, and etching is performed from the surface of the CVD-SiO ₂ .

In Fig. 1, laser light is used as the type of coherent light 50 in this etching device because of its excellent coherence, because of its stability, etching depth to be measured, and easy availability of the device. , Long wavelength, visible region laser light was selected. A He-Ne (helium-neon) laser oscillator was used as the light source 1. This output is 2mW. The laser beam 50 generated by the light source 1 is projected by opening the shutter 2 at the start of measurement, and the beam expander 8 expands the laser beam diameter to 8 mm while maintaining coherency.

The laser light 50 is sent through the quartz window 17 into the chamber 18 that actually performs the RIE process, and the Rf power supply 19
It is projected on a silicon wafer 9 as a member to be etched arranged on the electrode 20 connected to the.

Cl (chlorine) gas is flown into the chamber 18 to maintain the pressure in the chamber 18 at 10 mTorr. The output of the Rf power source, which is a high frequency source, is 800W.

In this way, the reflected light on the mask surface of the wafer 9 and the reflected light on the mask lower layer and the etched surface cause mutual interference, and the reflected light 51 travels in the same path as the coherent light 50 in the opposite direction. The reflected light 51 passes through the λ / 4 polarizing plate 4 after receiving the interference.

After that, the reflected light 51 is reflected by the polarization beam splitter 3.
By changing the direction of the reflected light 51 from the incident direction to the direction of 90 degrees,
It reaches the photodetector 10 without returning to the light source 1 side.

The reflected light 51 is converted into an electric signal by the photo detector 10, and the extracted waveform signal is amplified by the amplifier 11.

After that, the waveform signal is digitized by the A / D conversion circuit 12, the interference waveform is recorded by the recorder 13, and the frequency distribution is taken out by the waveform analysis means 141.

See FIG.

FIG. 5 shows an interference waveform on the etched surface obtained by the optical interference method.

The interference waveform obtained by the optical interferometry by actually observing for a certain period of time is shown in Fig. 5 (a). This waveform shows three interference waves (r ₁ and r _{2 in} Fig. 4) generated from each part of the observation surface. Interference wave, r ₂
And r ₃ and r ₃ and r ₁ ). That is, there are three waveforms of FIG. 5 (b), FIG. 5 (d), and FIG. 5 (e). Of these, the waveform of FIG. 5 (d) (light reflected from the lower surface of the mask film and the semiconductor wafer An interference wave generated from the reflected light from the surface) has a weak signal intensity and a long waveform period, and is usually difficult to separate from other waveforms. Analysis by Fourier transform may be performed as the waveform analysis means 141, but in that case, if the measurement period is short, it may be more difficult to separate the waveform as shown in FIG. 5 (c). In order to improve this, it is desirable to use the “maximum entropy method” in the waveform analysis means 141 as a frequency analysis method for separating this interference wave. By the way, this method is a method for calculating the frequency distribution of an observation phenomenon with high resolution in an extremely short measurement time, and recently, "waveform data processing for scientific calculation" (CQ publisher, April 30, 1986 first edition) (Issue) p.166-p.179.

As described above, after performing frequency analysis of the interference wave using the maximum entropy method, the following equation is transformed from the frequency value to obtain the etching rates e ₀ and e ₁ on the etched surface and the mask surface, respectively. be able to.

The frequency of the interference wave generated by the reflected light from the mask film surface and the reflected light from the etched surface of the semiconductor substrate is δ ₀ , the reflected light from the interface of the mask film and the semiconductor substrate, and the reflected light from the etched surface of the semiconductor substrate. If the frequency of the interference wave created by the reflected light is δ ₁ , Here, n ₀ is the refractive index of the mask film, d ₀₀ is the initial etching depth, λ ₀ is the wavelength of the incident light, and t is the etching time.

See FIG.

FIG. 6 shows an example of a waveform recorded by actual etching as a change in signal intensity (vertical axis) along time (horizontal axis). By performing waveform analysis on this, the etch rate could be obtained from the previous equation. As in the period v to w of the waveform in FIG. 6, since the actual signal waveform is an interference composite waveform of three waves, the interference waveforms may cancel each other for a certain period. The frequency analysis of the waveform becomes difficult by the usual Fourier transform analysis. However, if the maximum entropy method is used, such waveform analysis can be performed with high resolution.

As described above, the time when the desired etching depth is reached is detected by monitoring the etching rate in real time. At that point, etching is stopped.

The actual operation procedure is as follows:
Open and start projecting the coherent light 50. Next, the Rf power supply 19 is activated at point q in FIG. 6 to start etching. When it is determined that the etching is completed, the stop means 16 is instructed to close the shutter 2 at point r in FIG. 6, and the projection of the laser beam 50 is stopped. After that, the Rf power supply 19 is stopped at point s in FIG.

See FIG.

FIG. 2 is an explanatory view of the principle of data processing of the present invention. The state of waveform analysis of the apparatus will be described below.

The waveform analysis unit 141 calculates the frequency distribution of the interference wave obtained from the entire laser projection surface in a certain time, and this frequency distribution is transferred to the etching depth calculation unit 142, and the etching depth calculation unit 142 calculates the period and wave number. After paying attention to the property that the obtained wave number correlates with the etching depth, the etching rate is obtained and the etching depth is obtained. Further, the value of the etching depth thus obtained is transferred to the stopping means 16, and when it is judged that the etching depth is a desired depth, the etching and the monitoring of the etching are instructed to end.

Actually, as a result of measuring whether or not the etching depth is accurate by using the etching apparatus of the present invention after the completion of the etching operation by using the film thickness meter, it was as follows.

According to the example in FIG. 6, the depth actually etched. The depth measured by the etching apparatus of the present invention by the calculation of 4.59 μm.
... 4.46 μm In another example in which similar etching was performed, the actual etching depth was 6.66 μm. The depth measured by the etching apparatus of the present invention by calculation ...
... 6.32 μm These indicate that the depth measured by the etching apparatus of the present invention was extremely accurate.

Further, the waveform analysis means 141 used for this calculation in the present embodiment.
Is configured so that a so-called 16-bit personal computer can perform calculation by the maximum entropy method, and the calculation program is also very simple.

By the way, generally, the maximum entropy method is said to have a drawback that it takes time to process data. However, the time required for the calculation in this embodiment is about 10 seconds for processing the 10 pieces of collected data, and the monitoring in substantially real time is possible.

Display 15 other than recorder 13 for data output
Was placed.

The waveform analysis means 141, based on the result processed by itself,
An instruction signal for stopping the projection of the coherent light is transferred to the stop means 16, and the shutter 2 is closed. 20 seconds after that, a signal instructing to stop the etching is transferred to the stop means 16 and the Rf power supply 19 is stopped.

As described above, by expanding the optical diameter of the coherent light and separating the waveform in which a number of coherent waves are overlapped with each other, the state of the entire surface to be etched can be accurately measured simultaneously with etching. I was able to monitor.

(B) Description of Other Embodiments By the way, in the above embodiment, various modifications can be made in accordance with the gist of the present invention.

Another embodiment is shown in FIG. 3 as an apparatus configuration diagram similar to FIG. In the figure, the same reference numerals as in FIG. 1 indicate the same elements as in FIG. Only the part of the transmission section of the coherent light 50 that is different from the embodiment of FIG. 1 is used, and the parts used have the same specifications. An optical fiber 6 is used in the transmission section of the coherent light 50 different from that shown in FIG.

In this embodiment, by using the optical fiber 6,
Since the measuring device group and the Rf power source 19 are isolated from each other, it is possible to reduce the influence of noise generated on the Rf power source 19 side on the measuring device group.

In the example of FIG. 3, first, the optical diameter of the coherent light is set to about the core diameter of the optical fiber 6 through the lens 5. At this time, the optical fiber 6 has a length of about 5 m, and a multimode fiber is used.

At the end of the optical fiber 6, the coherent light 50 is expanded again by the lens 7 to the original coherent light diameter (about 3 mm).

By connecting the optical system device and the photodetector 10 with an optical fiber and changing the device configuration of the present embodiment so as to isolate the two, the noise applied to the measured signal can be further reduced.

In addition, as an example, the present etching apparatus has been described as an apparatus for subjecting a semiconductor to fine processing by using a silicon wafer as a member to be etched, but in comparison with the case of performing fine film processing in other industrial fields. However, it can be operated sufficiently.

Alternatively, in the present embodiment, a He—Ne (helium-neon) laser oscillator is used as the light source 1, but if a semiconductor laser is used, the light source 1 has the features of small size and light weight.

On the other hand, if an LED (light emitting diode) is used as a light source, the coherence is reduced, but it is effective as a light source with less noise.

Furthermore, although the shutter 2 is opened and closed in order to start and stop the projection of the coherent light, it may be possible to instruct the light source 1 directly from the stop means 16.

Further, if the calculation procedure in the waveform analysis means 141 is implemented by hardware, higher speed data processing can be expected.

Further, in the present embodiment, the maximum entropy method is used as the calculation method for converting the waveform signal into the frequency distribution. However, when the waveform is simple, Fourier transform may be used, or another effective calculation method may be used. Good.

However, the calculation method to be adopted here must be selected on condition that the separation of the interference wave can be performed successfully.

〔The invention's effect〕

As described above, according to the present invention, the etching state can be monitored even when a control etch with no underlying layer is required as in the case of etching the semiconductor wafer itself, and the etching removal is performed to a desired depth. Then, the etching can be stopped.

[Brief description of drawings]

FIG. 1 is a block diagram of an apparatus according to an embodiment of the present invention, FIG. 2 is an explanatory view of a principle of data processing according to an embodiment of the present invention, and FIG. 3 is another embodiment of the present invention. FIG. 4 is an explanatory diagram of an interference wave on the surface of the object to be etched (enlarged cross-sectional view of an essential part), and FIG. 5 is interference on the surface to be etched obtained by “optical interferometry”. Waveform, FIG. 6 is another interference waveform measured by the device of the present invention (an example in which the waveform disappears due to interference). In FIGS. 1 and 3, 1 ... Light source 2 ... Shutter 3 ... Polarizing beam splitter 4 ... Polarizing plate (λ / 4 polarizing plate) 5 ... Lens 6 ... Optical fiber 7 ... Lens 8 ... … Beam expander 9 …… Subject to be etched (wafer) 10 …… Photo detector 11 …… Amplifier 12 …… A / D conversion circuit 13 …… Recorder 141 …… Waveform analysis means 142 …… Etching depth calculation means 15 …… Display 16 …… Stopping means 17 …… Window 18 …… Chamber 19 …… Rf power source 20 …… Electrode 50 …… Coherent light (laser light) 51 …… Reflected light. In FIG. 2, a ... Process of converting interference wave into light → electric signal b ... Process of amplifying signal c ... Process of converting to digital signal d ... Process of obtaining frequency distribution e ... Calculation of period・ Calculation of wave number f …… Calculation of etch rate ・ Calculation of etching depth g …… Judgment of whether the desired depth has been reached h …… Etching work and etching monitor stop. In Fig. 4, r1 is the reflected wave on the mask film surface, r2 is the reflected wave on the lower surface of the mask film, r3 is the reflected wave on the etched surface of the semiconductor wafer. In FIG. 6, p ... Coherent light projection start point q ... Etching start point r ... Etching end point s ... Coherent light projection end point v ... Waveform erasing start point w ... Waveform erasing end point is there.

Claims (1)

(57) [Claims] 1. A light source (1) for generating coherent light (50), a chamber (18) capable of transmitting the coherent light (50) therein, and containing an object to be etched (9), and an object to be etched. (9) A photodetector (10) that receives reflected light (51) that interferes on the surface, and a signal obtained by photoelectrically converting the photodetector (10),
A waveform analysis means (141) for performing frequency analysis using the maximum entropy method, and an etching depth calculation means (142) for calculating an etching depth based on the frequency distribution of the interference wave obtained by the waveform analysis means (141). An etching apparatus having.