JPH0474914A - Tracking-type optical interference surface shape measuring apparatus - Google Patents

Abstract

PURPOSE:To measure the shape of the surface of an object with high resolution by forming a heterodyne interference means in two stages, and detecting the change of a focal point as the phase change with high resolution by one of the heterodyne means. CONSTITUTION:The light generated from a linearly polarizing laser is converted to a double frequency laser wherein a P wave frequency is orthogonal to an S wave frequency by a first heterodyne interference means, and divided into two, then introduced into a second and a third heterodyne interference means. A part of the laser is applied to an object to be measured. The reflecting light is, after experimenting Dopper shift or phase change corresponding to the roughness of the surface, interfered with a reference light on a second sensor. Accordingly, the phase difference corresponding to the amount of roughness is detected. A feedback means moves an objective lens up and down in accordance with the phase difference to control the distance between the object and objective lens constant. The third heterodyne interference means calculates the moving amount of the objective lens, thereby to obtain the amount of the shift DELTAz in a (z) direction with high accuracy.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a surface shape measuring device, and more specifically, a device that measures a surface shape using optical heterodyne interference between two types of laser beams having different frequencies. It is related to.

[Conventional technology] Conventionally, as a device for measuring surface shapes such as surface roughness,
A focus servo device that irradiates a focus detection beam onto a minute area on the surface of the object to be measured, detects defocus based on the intensity of the reflected light, and uses the detection signal to maintain a constant distance between the surface of the object and the objective lens. There is an apparatus which further measures the movement of the objective lens subjected to the focus servo using a hetero gain interference means to determine the overall shape of the surface of the object to be measured.

For example, an optical measuring device disclosed in Japanese Patent Application Laid-open No. 105408/1983 is one example. To briefly explain using FIG. 6, the Zeeman laser 10, which is an orthogonal two-frequency laser,
The measurement light with a frequency f and the reference light with a frequency f2 emitted from 0 are split into two beams by a half mirror 103 to measure the X coordinate and two coordinates. The beam for measuring two coordinates is further split into two by a polarizing beam splitter 104 into a measurement light S wave and a reference light P wave. The measurement light S wave is further reflected by the polarizing beam splitter 105 and passed through the λ/4 plate 10.
6 becomes circularly polarized light, and the focus detection device 120
The light enters the cap eye, which is made up of a lens 107 and a mirror 108, and is reflected. The reflected light follows the opposite optical path and passes through the λ/4 plate 106 again, and this time it becomes P
It is converted into a wave and returns to the polarizing beam splitter 105. However, due to the imperfection of the polarizing beam splitter 105, a part of the P wave is reflected and combined and enters the polarizing beam splitter 104, which is a wave filter. At this time, since the focus detection device 120 receives focus servo from the drive device 111, the reflected light from the mirror 108 undergoes a Doppler shift of Δf. Therefore, the optical frequency of the measurement light entering the polarizing beam splitter 104 is f, 10Δf.

On the other hand, the P wave, which is the reference light that has passed through the polarizing beam splitter 104, also passes through the λ/4 plate 113 and becomes circularly polarized light.
It reaches 16. The reference target reflected by the high-precision mirror 116 follows the opposite optical path, is combined and interfered with the previous measurement light by the polarizing beam splitter 104, and is f, -f2+△f by the sensor 109.
beat signal is detected. Note that since the high-precision mirror 116 is moved together with the object to be measured by the drive source 117, the reflected light is strictly Doppler shifted as before, but in this example, the high-precision mirror 116 is used. Therefore, it is ignored.

The signal detected by the sensor 109 is the reference signal fx f
* is compared to find Δf. And the well-known basic Doppler formula △f=2/λ・y m1v8=△Z/
The displacement amount Δ2 is calculated from Δt using the following formula.

△2=λ/2・(△f)・dt =λ/4π・f(δφ/δt)・dt =λ/4π・(φ0−φ.) However, φ0 is the measured phase φ. is the initial phase.

[Problem to be solved by the invention] However, this optical measuring device is
As shown in Publication No. 5408, the focusing means uses two sensors whose pinhole positions are slightly shifted, and the focus error amount is monitored by the output difference from the sensors, so the resolution is 0. It had the disadvantage that it remained at around 2μ and could not meet the specifications required in the semiconductor device and optical component fields, such as high resolution and wide measurable range.

In addition, Japanese Patent Application Laid-Open No. 61-105408 discloses that lens 1
07. A method is also described in which the cap eye consisting of the mirror 108 is removed and the surface shape of the object to be measured is directly measured. However, since the reference beam moves on the high precision mirror 116, the surface roughness and movement of the high precision mirror 116 are stage 11
It is impossible to avoid the effects of vibration and undulation of 8.
Highly accurate measurements could not be expected.

The present invention has been made in order to solve the above-mentioned problems, and its purpose is to provide a second hetero gain interference means that monitors the distance between the objective lens and the measurement object using the phase difference of reflected light. , and a feedback means to keep the distance constant at all times, and by measuring the movement of the objective lens with a third heterodyne interference means, the measurable range is greatly expanded while having a high resolution of several nanometers. It is an object of the present invention to provide a tracking type light wave interference type surface shape measuring device that is not limited by the depth of focus of an objective lens.

[Means for Solving the Problems] In order to achieve this object, the tracking type optical interference surface shape measuring device of the present invention includes a laser device that emits a linearly polarized laser beam, and an orthogonal two-frequency laser beam that emits the laser beam. a first heterodyne interference means for converting into Feedback means for always keeping the distance between the objective lens and the object to be measured constant; and a third feedback means for measuring the displacement of the objective lens controlled by the feedback means.
The apparatus is comprised of a heterodyne interference means, and a scanning means for scanning the object to be measured in the x and y force directions.

Note that, in order to further improve measurement accuracy, a low-noise type heterodyne interference means that does not mix polarization components may be used as the first heterodyne interference means.

[Function] In the tracking type light wave interference surface shape measuring device of the present invention having the above configuration, the frequency f emitted from the linearly polarized laser that is the light source. The light has a P wave frequency f by the first heterodyne interference means. +fm and an orthogonal two-frequency laser having an S-wave frequency f0, a part of which is detected by the first sensor as a reference beat signal. The orthogonal two-frequency laser generated by the first hetero gain interference means is divided into two by the separation means and guided to the second and third hetero gain interference means. A part of the laser guided by the second heterodyne interference means is irradiated onto the object through the objective lens, and the reflected light is reflected by the object depending on the unevenness of its surface as the object is scanned in the XY force direction by the scanning means. The light then undergoes a Doppler shift or a phase change, enters the second heterodyne interference means again, is made to interfere with the reference light on the second sensor, and a phase difference corresponding to the amount of unevenness is detected. This phase difference information is sent to the feedback means, and the feedback means moves the objective lens up and down according to this phase difference to control the distance between the object to be measured and the objective lens to always be constant.

Further, the third heterodyne interference means measures the amount of movement of the objective lens, and calculates the amount of displacement △ in the Z direction according to the xy coordinates.
2 with high precision without being limited by the depth of focus of an objective lens. The measurable range is determined by the limit of the amount of feedback provided by the feedback means.

[Example] Hereinafter, a tracking type light wave interference surface shape measuring device embodying the present invention will be described with reference to the drawings.

As shown in FIG. 1, a linearly polarized laser 10 serving as a light source is S
Set to polarization, the beam first enters the first heterodyne interferometer 12 . As shown in Figure 2, this heterodyne interference device has a low-noise configuration, and the incident beam is transmitted through a non-polarizing beam splitter (hereinafter referred to as NPB).
The light is divided into transmitted light and reflected light at 14 (abbreviated as S). The transmitted light passes through a mirror 16 and enters a polarization beam splitter (hereinafter abbreviated as PBS) 18, which is an optical multiplexer. In addition, the reflected light is transmitted through an acousto-optic modulator (hereinafter abbreviated as AOM) 20
, 22, respectively 80. OM) (Z.

The original optical frequency f after undergoing frequency modulation of 80.1 MH2.

In contrast, it is converted to an optical frequency of f, +100 KHz. The converted light passes through the mirror 24 and has a wavelength of λ/2
The S wave enters the plate 26 and is converted into a P wave. At this time, λ/
Since it is not completely converted due to the imperfection of the second plate 26, some S-wave components remain as optical noise and enter the PB818, which is a multiplexer, as P+ΔS. However, P
Since the transmittance of BSI8 for S waves is 0.005%, this optical noise △S cannot be transmitted. Only P waves or PBS
18 will pass through. As a result, the S-wave frequency f is output from the PBS 18. , P wave frequency f.

A nearly perfect orthogonal dual frequency laser of +100 KH2 is emitted. However, in the orthogonal dual-frequency laser emitted from the Zeman laser in the prior art, components of about several percent of each other coexist, causing nonlinear measurement errors.

The orthogonal two-frequency laser emitted from the first heterogain interference device 12 enters the second and third heterodyne interference devices, and a part of it passes through the NPBS 28°30 and enters the polarizer 3.
2, the plane of polarization is aligned, and the sensor 34 detects a reference beat signal 8 expressed by the following equation.

■ , ~ 1 ten CO3[-2π f B t +
φ8゜However, fB: Reference frequency 100KH2k
Two wave numbers φ,. : Initial phase.

A second heterodyne interferometer starts from the PBS 36 and includes a beam expander 38 . An objective lens 40, a measurement arm consisting of an outward path and a return path leading to the object to be measured 58, and a polarizer 4.
6. Non-polarized beam suburi ivy 48. cap eye 5
It is composed of a reference arm consisting of an outward path leading to zero and a backward path. In addition, the first and third hetero gain interference devices are
Starting from NPBS28, polarizer 70. Mirror 68. λ
The reference arm that passes through the /2 plate 66 and reaches the NPB862, and the N
PBS30. PBS36. Polarizer 46. NPBS48.
Cap Eye 50. NPBS62 after NPBS48
It consists of a measurement arm that extends to.

In the second heterodyne interferometer, PBS36
The wave and S wave are divided into two separately. The P wave, which is a transmitted wave, is expanded into parallel light by the beam expander 38, enters the objective lens 40, and is focused and irradiated onto the object to be measured. by the way,
When the object to be measured 58 is moved by the XY stage, the reflected light undergoes a phase change (2k・△Z2) according to the surface irregularities (△Z2), and follows the opposite optical path to the PBS 36,
The light passes through the NPBS 30 and the polarizer 42 and reaches the sensor 44 .

On the other hand, the light wave reflected by the PB 336 contains a small amount of P component (approximately 1%) in addition to the S component due to the nature of PBS. Therefore, it passes through the polarizer 46 and becomes a complete S wave. The S wave is partially reflected by the NPBS 48, enters a cap eye 50 fixed to the objective lens, and is reflected. This reflected light also follows the opposite optical path and passes through the NPBS48.
After passing through the polarizer 46, it is combined and interfered with the above-mentioned P wave on the PB836, and the NPBS30. Sensor 4 via polarizer 42
Enter 4. The signal strength detected by the sensor 44 is PB8
If the distance from 36 to cap eye 50 is 21, and the distance to the measurement object is 22, then ■, ~1+CO8[2k・
(Z, −Z,)−2π f,t +φ,. However, fB: Reference frequency 100KH2k: Wave number φ. : Initial phase.

Reference signal I obtained by the first heterogain interferometer
Comparing with Il~1+CO5[-2πfBt+φllo], and assuming that the phase of I with respect to ■ during focus is φ2B, φPB is: φ,=φBO−φp. 2k (Zl-22). This is due to the change in optical path length due to unevenness on the surface of the measurement object △z
2 is the movement ΔZ of the objective lens 40.

This indicates that you can cancel with . This φ, 8 is shown in FIG.

The feedback means is composed of a feedback circuit 72 as shown in FIG. 4 and an actuator such as a piezoelectric element 54, for example. As shown in FIG. 4, the feedback circuit 72 measures this φFi1 with high precision using a high-frequency quartz clock 92, an AND device 88, and a counter circuit 80, and calculates the phase φFi1 set at the time of focusing.
Compare FOCLI8 and subtracter 82, and then calculate the difference by D/
The A converter 84 converts it into an analog quantity, amplifies it, and sends it to the piezoelectric element 54. For example, depending on the amount of unevenness △Z2 of the surface shape, φ becomes △φ7. If only changes, the subtracter 8
The difference △φ from 2. A digital amount is outputted, and a corresponding analog amount is applied to the piezoelectric element 54. and,
The objective lens 40 is controlled to move by △Z, (=△Z2), and △Z2 is canceled by △Z. In other words, the feedback circuit 72 always outputs Δφ1. - Z-axis stage 5 with objective lens 40 fixed so that
6 is controlled to move, and the amount of unevenness on the surface of the object to be measured 58 is replaced by the movement of the objective lens 40.

Therefore, measurements can be made up to the limits of the feedback means without being limited by the depth of focus of the objective lens 40. The movement of the objective lens 40 is then measured with high precision by the third heterodyne interference device.

The measurement beam of the third heterodyne interferometer is taken from the reference beam of the second heterodyne interferometer. In other words, it is reflected by the cap eye 50 and the NPBS48
．． The beam that passes through the sensor 62 and reaches the sensor 64 is defined as a measurement beam. Since the objective lens 40 is moved by the feedback means, the measurement beam is emitted by k·Δ2 according to the amount of movement. Contains phase change information. On the other hand, the reference beam is taken from an orthogonal dual frequency laser transmitted through the NPBS 28. In other words, by applying a polarizer to the orthogonal dual-frequency laser, only the modulated P wave is extracted, which is then transmitted through the mirror 68 and the λ/2 plate 66 to become the S polarized wave, which is the same as the previous measurement beam, and is combined. The signals are combined and interfered on the NPBS 62, which is the receiver. The third sensor 64 generates a measurement signal ■ corresponding to the amount of movement of the objective lens 40. is detected. As before, the optical path of the reference light from the NPBS 28 to the mirror 68 to the sensor 64 is Z3.
, the optical path of the measurement light from NPBS 28 to NPBS 48 is Z4, the optical path from NPBS 48 to cap eye 50 is 21, the optical path from NPBS 48 to sensor 64 is Z,
Then, the interference intensity of the measurement signal detected by the sensor 64 is Io~1+CO8[k (Z, -Z, -Z, -2・
2. )−2πf, t+φDo] =1+CO8[k (ZCONST 2 ” Zl)
2πf, t+φDo] where φo0: initial phase k, two-wave number fB: reference beat frequency.

Similarly, ■ for reference signal ■8. If the phase of is φDB, then φDB”φ80−φoo+2・k−Z.

k”　ZCONST becomes.

This means that when the distance Z to the objective lens 40 changes, I
This shows that the phase of ID with respect to a changes, and Z
, changes by △Z, taking into account the round trip of the optical path, 2.
The phase changes (Δφon) by k·ΔZ.

In other words, △Z is △Z+=λ/4π・△φ. B = λ/4π・ (φDBZ−φDBO) However, φDBO
: represents the phase before Zl changes φosz: represents the phase after Z+ changes.

By measuring this phase change Δφ including 2π or more, it becomes possible to measure λ/2 or more.

When a measurement object having a step shape of, for example, about 30 nm was actually measured using the measuring device configured as described above, measurement results (data) as shown in FIG. 5 were obtained. In addition, a conventional measuring device, for example, a lens 67 and a mirror 6
When the above-mentioned object is measured using a measuring device from which the cap eye consisting of 8 is removed, measurement results (data) as shown in FIG. 7 are obtained. Therefore, when comparing and examining the above two measurement results, it is found that in the conventional measuring device, optical noise caused by the Zeeman laser 61, surface roughness of the high precision mirror 76,
It can be seen that highly accurate measurement is not possible due to vibrations of the X stage 78 and the like.

As described above, by configuring the heterogain interference device in two stages, the surface shape of the object to be measured can be measured with high resolution without being limited by the depth of focus of the objective lens.

[Effects of the Invention] As is clear from the detailed description above, according to the present invention, the heterodyne interference means has a two-stage configuration, and one of the heterogain interference means detects a focus change in the form of a phase change with high resolution, and lnm by piezoelectric element etc.
The focus servo is applied to the objective lens with an accuracy of It has the advantage of being able to measure

[Brief explanation of drawings]

1 to 5 show embodiments embodying the present invention. FIG. 1 is a configuration diagram of a tracking type light wave interference surface shape measuring device, and FIG. 2 is a configuration diagram of a low noise type heterodyne interference device. 3 is a diagram showing φFB, which is the relative phase of the reference signal and the focus signal, FIG. 4 is a block diagram showing the configuration of the feedback circuit, and FIG. 5 is a diagram showing fine shape measurement data according to the present invention. FIG. 6 is an optical system diagram of a conventional heterodyne interference surface shape measuring device, and FIG. 7 is a diagram showing fine shape measurement data by the conventional method. In the figure, 12 is an orthogonal two-frequency converter, 14 is a non-polarizing beam splitter, 16.24 is a mirror, 18°36 is a polarizing beam splitter, 20.22 is an acousto-optic modulator, 26.6
6 is a λ/2 plate, 28.62 is a non-polarizing beam splitter,
30.48 is a non-polarizing beam splitter, 32,46.4
2 is a polarizer, 34, 44, 64 is an optical sensor, 36 is a polarizing beam splitter, 38 is a beam expander, 40 is an objective lens, 50 is a cap eye, 54 is a piezoelectric element,
56 is a Z-axis stage, 58 is an object to be measured, 60 is an XY stage, 72 is a feedback circuit, 80 is a counter, 8
2 is a subtracter, 84 is a D/A converter, 86 is an amplifier, 88
is an AND device, 90 is an inverter, and 92 is a high frequency quartz clock.

Claims (1)

[Claims] 1. A laser device that emits a linearly polarized laser beam; a first heterodyne interference means that converts the laser beam into an orthogonal two-frequency laser; and an objective lens and an object lens using the orthogonal two-frequency laser. a second heterodyne interference means for monitoring changes in the distance to the object to be measured using a phase difference amount of the reflected light; a feedback means for always keeping the distance between the objective lens and the object to be constant based on the amount of phase difference; and control by the feedback means. a third heterodyne interference means for measuring the amount of displacement of the objective lens by using the orthogonal dual frequency laser; and a scanning means for scanning the object to be measured in the x and y directions. Interferometric surface profile measurement device.