CN100547344C - Sinusoidal phase modulation interferometer for real-time measurement of surface topography - Google Patents

Abstract

A sinusoidal phase modulation interferometer for real-time measurement of surface topography includes a semiconductor laser, a first lens group and a beam splitter are sequentially arranged coaxially along the direction of travel of the outgoing light beam of the semiconductor laser, a measured object is arranged in the direction of travel of the transmitted light beam of the beam splitter, a reference plate is placed in the direction of travel of the reflected light beam of the beam splitter; a second lens and a detection element are sequentially placed in the direction of travel of the light beam reflected by the reference plate, and the invention is characterized in that: the output end of the detection element is connected to the first input end of the signal processing unit, the output end of the signal processing unit is connected to the computer, a DC power supply and a signal source are connected to the semiconductor laser through a driver, and the second output end of the signal source is connected to the second input end of the signal processing unit. The invention has the advantages of convenient operation, large measurement range and high precision.

Description

The sinusoidal phase modulation interferometer of real-time surface measurements pattern

Technical field

The present invention relates to the real-time measurement of surface topography, particularly a kind of sinusoidal phase modulation interferometer of real-time surface measurements pattern.

Background technology

Because the temperature stability of semiconductor laser (being designated hereinafter simply as LD) wavelength is solved preferably, the semiconductor laser interference instrument is is researched and developed widely.LD except that volume is little, power-saving, price low, an outstanding advantage is that wavelength-modulated is easy.This makes the light heterodyne technology that can improve measuring accuracy can realize by the injection current of direct modulation LD simply in the semiconductor laser interference instrument.By the modulation injection current, be easy to realize the phase modulation (PM) of interference signal, thereby realize the measurement of parameter degree of precision such as displacement, distance, face shape.Such as the people such as T.Suzuki of Japanese Niigata University a kind of sinusoidal phase modulation semiconductor laser interference instrument that is used for the surface measurements pattern (technology [1] Takamasa Suzuki has formerly been proposed, Osami Sasaki, JinsakuKaneda, Takeo Maruyama, " Real time two-dimensional surface profilemeasurement in a sinusoidal phase modulating laser diode interferometer; " Opt.Eng., 1994,33 (8), 2754-2759).The modulating current of this interferometer is

I _m(t)＝acos(ω _ct+θ)。(1)

The interference signal that photodetector CCD detects can be expressed as

S(x，y，t)＝S _dc+S ₀cos[-zcos(ω _ct+θ)+α(x，y)]， (2)

S _DcBe the DC component of interference signal, the direct current I of it and drive current ₀Relevant.S ₀Be the AC compounent amplitude of interference signal, it and current modulated I _m(t) relevant.A is the amplitude of modulating current, ω _cBe the angular frequency of modulating current, θ is the first phase of modulating current.Z is the sinusoidal phase modulation degree of depth, and (x y) is phase place to be measured to α.

Formerly in the technology [1], in the one-period of modulation signal, photodetector CCD is to four P of interference signal integration _i(its expression formula is as follows for x, y) (i=1～4)

$P_i(x,y)={\∫}_{(T/4)(i-1)}^{(T/4)i}S(x,y,t)\mathrm{dt},---\left(3\right)$

X wherein and y represent the position coordinates on testee surface.The sinusoidal phase modulation signal processing system is carried out plus and minus calculation to each pixel and is got:

P _c(x，y)＝-P ₄+P ₁-P ₂+P ₃＝A _ccosα(x，y)， (4)

P _s(x，y)＝-P ₄+P ₁+P ₂-P ₃＝A _ssinα(x，y)， (5)

Coefficient wherein:

$A_c=(8/\mathrm{\π})\underset{n=1}{\overset{\∞}{\mathrm{\Σ}}}[J_{2n}\left(z\right)/2n][1-{(-1)}^n]\sin \left(2\mathrm{n\θ}\right),$

$A_s=(8/\mathrm{\π})\underset{n=1}{\overset{\∞}{\mathrm{\Σ}}}[J_{2n-1}\left(z\right)/2(n-1)]{(-1)}^n\sin \left[(2n-1)\mathrm{\θ}\right].$

J _n(z) be n rank Bessel functions.

The surface topography of testee is

$r(x,y)=\frac{\mathrm{\λ}}{4\mathrm{\π}}\mathrm{\α}(x,y)=\frac{\mathrm{\λ}}{4\mathrm{\π}}\arctan (P_s/P_c).---\left(6\right)$

(x is a surface topography value to be measured y) to r in the formula, and λ is a Wavelength of Laser.

Formerly the measuring accuracy of technology [1] is 14nm, and measurement range will be proofreaied and correct during use less than λ/4, and operation is difficulty, and reason has three: one, and the light intensity of semiconductor laser is modulated, makes S _DcAnd S _oChange in time, the phase value that this will influence in (6) formula causes measuring error; The 2nd, the wave length shift of semiconductor laser to influence surface topography measured value r (x, y); The 3rd, this method is to obtain the object surfaces pattern under given conditions, can't accurately measure z=2.45, θ=56 ° at present, and it also introduces measuring error.

Summary of the invention

The objective of the invention is to overcome existing shortcoming in the above-mentioned technology formerly [1], a kind of sinusoidal phase modulation interferometer of real-time surface measurements pattern is provided.The light intensity fluctuation that solution directly modulated laser wavelength causes is the little and low problem of precision with measurement range, and instrument requires to reach easy to operate, and the expansion measurement range improves measuring accuracy.

Technical solution of the present invention is as follows:

A kind of sinusoidal phase modulation interferometer of real-time surface measurements pattern, comprise the semiconductor laser instrument, along the outgoing beam working direction of this semiconductor laser with optical axis set gradually first lens combination, beam splitter, on the working direction of the transmitted light beam of this beam splitter, testee is set, on the working direction of the folded light beam of beam splitter, is placed with reference plate; Be placed with second lens and detecting element successively on the reference plate beam reflected working direction, be characterized in: the output terminal of described detecting element is connected with the first input end of signal processing unit, the output terminal of signal processing unit is connected with computing machine, direct supply is connected with semiconductor laser by driver with signal source, second output terminal of this signal source is connected with second input end of described signal processing unit, and this signal source is injected a sinusoidal current signal simultaneously to semiconductor laser and described signal processing unit.

Described beam splitter is meant that the Amici prism of the beam splitter that incident light can be divided into two-beam by the beam intensity ratio that approached 1: 1 or one side are coated with the parallel flat of analysing the light film.

Described reference plate is one facing to the parallel flat that is coated with anti-reflection film on the surface of beam splitter one side.Described detecting element is one dimension or 2 D photoelectric detector.

Described signal processing unit comprises real-time phase detection circuit, real-time phase discriminator and synchronizing circuit.

The inner structure of described real-time phase detection circuit comprises first amplifier, second amplifier, counting circuit, first low-pass filter and second low-pass filter.

The inner structure of described real-time phase discriminator comprises the 3rd amplifier, the 4th amplifier, and division circuit is separated circuitry phase, phase compensating circuit, described phase compensating circuit is made of single-chip microcomputer.

Advantage of the present invention:

1), obtain the surface topography of testee, finishes collection, processing and the demonstration of signal in the time of can making the total system high-precision real by real-time phase detection circuit and real-time phase discriminator.

2), adopt semiconductor laser, it is little to make total system have a volume, modulation is simple, the modulation accuracy advantages of higher.

3), compare with technology [1] formerly, measuring accuracy of the present invention is higher.Formerly in the technology [1], during the wavelength of directly modulated laser, the output intensity of semiconductor laser can change in time, has influenced the intensity of interference signal, causes coefficient A _cWith A _sCan not equate always, cause the phase place of measurement that error is arranged.Therefore light intensity changes meeting introducing measuring error.The present invention contains real-time phase discriminator, and this error has been avoided in the influence that changes of light intensity that utilized this circuit for eliminating, has improved the measuring accuracy of interferometer.

4), the present invention expands measurement range to one more than the wavelength by using phase compensating method.

5), formerly technology [1] uses look-up table to obtain the object being measured phase place, calculates the surface topography of object being measured again, measuring speed is slow.The present invention adopts real-time phase discriminator, can directly obtain the surface topography of object being measured by circuit, and measuring speed is fast.

6), the present invention does not need to proofread and correct z and θ, simple and convenient user operation.

Description of drawings

Fig. 1 is the structural representation of the laser interferometer of the real-time surface measurements pattern of the present invention.

Fig. 2 is a signal processing flow synoptic diagram of the present invention.

Embodiment

The invention will be further described below in conjunction with drawings and Examples.

See also Fig. 1 earlier, Fig. 1 is the structural representation of the laser interferometer of the real-time surface measurements pattern of the present invention.As seen from the figure, the sinusoidal phase modulation interferometer of the real-time surface measurements pattern of the present invention, comprise semiconductor laser instrument 1, along the outgoing beam working direction of this semiconductor laser 1 with optical axis set gradually first lens combination 2, beam splitter 3, on the working direction of the transmitted light beam t1 of this beam splitter 3, testee 5 is set, be placed with reference plate 4 on the working direction of the folded light beam f1 of beam splitter 3: be placed with second lens 6 and detecting element 7 successively on the working direction of light beam f1 after by reference plate 4 reflections, the output terminal of described detecting element 7 is connected with the first input end 8a of signal processing unit 11, the output terminal of signal processing unit 11 is connected with computing machine 12, the driver 13 that is connected with semiconductor laser 1 is connected with signal source 15 with direct supply 14 respectively, second output terminal of this signal source 15 is connected with the second input end 8b of described signal processing unit 11, and this signal source 15 is injected a sinusoidal current signal simultaneously to semiconductor laser 1 and described signal processing unit 11.

Described beam splitter 3 is meant the beam splitter that incident light can be divided into two-beam by the beam intensity ratio that approached 1: 1.Be coated with the parallel flat of analysing the light film etc. as Amici prism or one side.

Described signal processing unit 11 comprises real-time phase detection circuit 8, real-time phase discriminator 9 and synchronizing circuit 10.The inner structure of signal processing unit 11 as shown in Figure 2.

The inner structure of described real-time phase detection circuit 8 comprises amplifier 801, amplifier 802, counting circuit 803, low-pass filter 804, low-pass filter 805.

The inner structure of described real-time phase discriminator 9 comprises amplifier 901, amplifier 902, and division circuit 903 is separated circuitry phase 904, phase compensating circuit 905.

Described driver 13 is semiconductor laser modulators, the injection current of its control semiconductor laser.

Described signal source 15 is to inject the signal generator of modulation signals to semiconductor laser modulator 13, makes the wavelength of semiconductor laser 1 by sinusoidal variations, thereby obtains the interference signal of sinusoidal phase modulation.

The course of work of the present invention is: the DC current i of direct supply 14 ₀Drive described semiconductor laser 1 with the sinusoidal current Δ i (t) of signal source 15 by semiconductor laser modulator (LM), sinusoidal voltage modulation signal and current modulated are respectively

V(t)＝Acosω _ct， (7)

Δi(t)＝acosω _ct， (8)

a＝K _LMA， (9)

Wherein A represents amplitude, ω _cThe expression angular frequency, K _LMConversion coefficient for the semiconductor laser modulator.

Semiconductor laser 1 output wavelength and light intensity are respectively

λ(t)＝λ ₀+Δλ(t)＝λ ₀+β ₁acosω _ct， (10)

g(t)＝g ₀+Δg(t)＝β ₂[Δi(t)+acosω _ct]， (11)

λ wherein ₀Be the centre wavelength of semiconductor laser 1, β ₁With β ₂Be respectively the wavelength-modulated coefficient and the intensity modulation coefficient of semiconductor laser 1.Consider the influence that light intensity changes, then the interference signal that receives of detecting element 7 is

s(x，y，t)＝s ₁(x，y，t)+s ₀(x，y，t)cos[z(x，y)cosω _ct+α ₀+α _r(x，y)]，(12)

X in the formula, y are the position coordinates on testee 5 surfaces, s ₁Be interference signal DC component, s ₀Amplitude for the signal communication component.Z is the sinusoidal phase modulation degree of depth, and λ is an optical maser wavelength, and they are subjected to light intensity to change (g (t)) influence, and its expression formula is:

s ₁(x，y，t)＝g(t-τ ₀)+g(t-τ _r)， (13)

$s_0(x,y,t)=2\sqrt{g(t-{\mathrm{\τ}}_0)g(t-{\mathrm{\τ}}_r)}.---\left(14\right)$

τ in the formula ₀=l ₀/ c, τ _r=l _r/ c, l ₀And l _rBe respectively the light path of object light and the light path of reference light, c is the light velocity.(12) α in the formula ₀Be the phase place of reference plate 4 interference signal when static, it interferes the initial light path difference 2D of arms by two ₀Decision:

${\mathrm{\α}}_0=\frac{4\mathrm{\π}}{\mathrm{\λ}}{D}_0.---\left(15\right)$

α _r(it is calculated by following formula for x, the y) phase change of the interference signal that causes for testee 5 surface topographies:

α _r(x，y)＝(4π/λ ₀)r(x，y)， (16)

Wherein: (x y) is the pattern value of testee 5 surperficial any points to r.The DC component of (12) formula being launched and ignoring wherein can get:

s(x，y，t)＝s ₀{cosα(x，y)[J ₀(z)-2J ₂(z)cos2ω _ct+…]

-sinα(x，y)[2J ₁(z)cosω _ct-2J ₃(z)cos3ω _ct+…]}，(17)

α in the formula (x, y)=α ₀+ α _r(x, y), J _n(z) be n rank Bessel functions.

The synchronizing signal that described signal source 15 is sent is through the synchronous output end mouth 10a of synchronizing circuit 10 output.Under the control of synchronizing signal, interference signal after first amplifier 801 amplifies, is imported the first input end mouth 803a of counting circuit 803 from the first input end mouth 8a input of first amplifier 801 again; The modulation signal that described signal source 15 is sent after second amplifier 802 amplifies, is also imported the second input port 803b of counting circuit from the first input end mouth 8b input of second amplifier 802.Counting circuit 803 carries out after multiplication calculates, and after importing first low-pass filter 804 and second low-pass filter 805 respectively and carrying out low-pass filtering, obtains signal

P ₁(x，y，t)＝K ₁K ₂K _mK _L1s ₀AJ ₁(z)sinα(x，y)， (18)

P ₂(x，y，t)＝K ₁K ₂K _mK _L2s ₀AJ ₂(z)cosα(x，y)， (19)

K wherein ₁Be the gain of first amplifier 801, K ₂Be the gain of second amplifier 802, K _mBe the gain of counting circuit 803, K _L1Be the gain of first low-pass filter 804, K _L2It is the gain of second low-pass filter 805.

P ₁(x, y is after t) the input port 901a by the 3rd amplifier 901 imports the 3rd amplifier 901 and amplifies, through the first input end mouth 903a of division circuit 903 input division circuit 903.P ₂(x, y is after t) the input port 902a by the 4th amplifier 902 imports the 4th amplifier 902 and amplifies, through the second input port 903b of division circuit 903 input division circuit 903.Signal P ₁(x, y, t) and P ₂(x, y t) make division arithmetic in division circuit 903, obtain signal:

$P(x,y)=\frac{J_1\left(z\right)K_3}{J_2\left(z\right){\mathrm{<figure-callout\; id="4"\; label="K"\; filenames="C20071003726200141.png"\; state="\{\{state\}\}">K</figure-callout>}}_4}\tan \left[\mathrm{\&alpha;}(x,y)\right]=K\tan \left[\mathrm{\&alpha;}(x,y)\right],---\left(20\right)$

K wherein ₃Be gain, the K of the 3rd amplifier 901 ₄Be the gain of the 4th amplifier 902,

$K=\frac{J_1\left(z\right)K_3}{J_2\left(z\right){\mathrm{<figure-callout\; id="4"\; label="K"\; filenames="C20071003726200141.png"\; state="\{\{state\}\}">K</figure-callout>}}_4}.$

(x, y) signal is from separating the first input end mouth 904a input of circuitry phase 904, and separates and obtain phase place mutually by separating circuitry phase 904 for P

α(x，y)＝arctan[P(x，y)/K]。(21)

So the surface topography of testee 5 is:

r(x，y)＝α(x，y)λ ₀/(4π)。(22)

By (22) formula as can be known, the light intensity of surface topography and semiconductor laser 1 is irrelevant, illustrates that the present invention can eliminate the influence of light intensity variation to measurement result.

From (21) formula as can be known, the phase place principal value interval is at [pi/2 ,+pi/2], so surfaceness r (x, maximum measurement range λ/4 y).In order to enlarge measurement range, with signal P ₁(x, y is t) from the first input end mouth 905a input phase compensating circuit 905 of phase compensating circuit 905; Will (x be y) through the second input port 905b input phase compensating circuit 905 of phase compensating circuit 905 from the phase place α that separates circuitry phase 904 output; With signal P ₂(x, y is t) from the 3rd input port 905c input phase compensating circuit 905 of phase compensating circuit 905.(x, value y) (symbol) saltus step judges that (its compensation method is as shown in table 1 for x, the y) phase compensation value of place quadrant thereby obtain phase place α with cos α by 905 couples of sin α of phase compensating circuit (x).

Table 1 phase compensation

Cos α (x, y) sin α (x, y) phase compensation

-→+ +π

+ +→- -π

-→+ -π

- +→- +π

According to the phase compensating method of table 1, we just can expand measurement range to one more than the wavelength.(x, measuring accuracy y) can reach 0.01rad to phase place α, if adopt central wavelength lambda ₀Be the semiconductor laser 1 of 785nm, then testee surface longitudinal Measurement Resolution is 0.62nm.

The device that present embodiment adopts as shown in Figure 1.The index of modulation of driver 13 is 1.56 * 10 ^-3Nm/mA.Direct supply 14 provides the DC current of 50mA for semiconductor laser 1.Signal source 15 is injected the sinusoidal phase modulation signal to semiconductor laser 1, changes the wavelength of semiconductor laser 1, the phase place in the interferometric modulator signal.

Semiconductor laser 1 recited above is that the employing wavelength is the semiconductor laser of 785nm.

Described beam splitter 3 is meant the Amici prism that incident light can be divided into two-beam by the beam intensity ratio that approached 1: 1.

Described reference plate 4 is level crossings of an electroplate.

Described testee 5 is wedges.

Described second lens 6 are that a focal length is the lens of 75mm.

Described detecting element 7 is two-dimensional CCD photodetectors.

Described signal processing unit 11 is made of with synchronizing circuit 10 real-time phase detection circuit 8, real-time phase discriminator 9.

Described real-time phase detection circuit 8 is made of first amplifier 801, second amplifier 802, counting circuit 803, low-pass filter 804 and 805.First amplifier 801 and second amplifier 802 are made of chip LM232, and counting circuit 803 is finished by chip AD532, and low-pass filter is made of chip LF356 and follower BC108.

Described real-time phase discriminator 9 by the 3rd amplifier 901, the 4th amplifier 902, division circuit 903, separate circuitry phase 904, phase compensating circuit 905 constitutes.The 3rd amplifier 901, the 4th amplifier 902 are finished by chip LM232, and divider 903 is finished by chip AD538.Separating circuitry phase 904 and phase compensating circuit 905 is finished by single-chip microcomputer.The described circuitry phase 904 of separating is that a model is the single-chip microcomputer of ADuc812 with phase compensating circuit 905.

Described driver 13 is semiconductor laser modulators, is used for converting the sinusoidal voltage modulation signal to the sinusoidal current modulation signal.

Described signal source 15 is signal generators, is used to produce the required sinusoidal voltage modulation signal of sinusoidal phase modulation.

The course of work of the present invention is: the light that sends as the semiconductor laser of semiconductor laser 1 shines on the Amici prism 3 after expanding bundle and collimation by object lens 2.Folded light beam f1 by Amici prism 3 shines on the silver-plated level crossing 4, and the transmitted light beam t1 that sees through Amici prism 3 shines on the wedge 5.The folded light beam of the folded light beam of silver-plated level crossing 4 and wedge 5 is interfered, and the interference signal of generation is imaged on the 2 D photoelectric detector C CD7 through lens 6.2 D photoelectric detector C CD7 with interference signal be converted to electric signal s (x, y, t),

s(x，y，t)＝s ₁(x，y，t)+s ₀(x，y，t)cos[z(x，y)cos(ω _ct+θ)+α(x，y，t)]，(23)

Wherein α (x, y, t)=α ₀+ α _r(x, y, t).(24)

Under synchronizing signal 10a control, (t) the first input end mouth 8a from first amplifier 801 imports first amplifier 801 to interference signal s for x, y, after first amplifier 801 amplifies, imports the first input end mouth 803a of counting circuit 803 again; Modulation signal V (t)=Acos ω _cT imports second amplifier 802 from the input port 8b of second amplifier 802, after second amplifier 802 amplifies, and the second input port 803b of input counting circuit 803.The two carries out multiplication calculating via counting circuit 803, and the result is input to respectively in first low-pass filter 804 and second low-pass filter 805 and carries out low-pass filtering, obtains signal P ₁(x, y, t), P ₂(x, y, t).With P ₁(x, y, t), P ₂(t) first amplifier 901 and second amplifier 902 of importing real-time phase discriminator 9 respectively amplifies for x, y, and by division circuit 903 as division arithmetic get signal P (x, y).(x, y) circuitry phase 904 is separated in input to division circuit 903, separates and obtains phase place α mutually (x, y), thereby the surface topography that obtains wedge is by separating circuitry phase 904 with signal P

r(x，y)＝α(x，y)λ ₀/(4π)。(25)

The wedge three-dimensional surface shape figure that records shows (only computed Presentation Function) by computing machine 12.The sinusoidal voltage modulation signal that signal generator 15 produces is input in the semiconductor laser modulator 13, converts the sinusoidal voltage modulation signal to current modulated, and current modulated is injected the wavelength of LD modulated laser the most at last.The DC current of injecting semiconductor laser 1 is 50mA.The centre wavelength of semiconductor laser is 785nm, and the modulation signal amplitude is 1.5V, and modulating frequency is 100Hz.With this understanding, the duplicate measurements precision that general experimental situation records the pattern of wedge is several nanometers, and measurement range surpasses a wavelength.

The output signal of entire measuring device and semiconductor laser intensity modulation are irrelevant, illustrate that real-time phase discriminator eliminated the influence of intensity modulation to measuring accuracy.

Claims (5)

1, a kind of sinusoidal phase modulation interferometer of real-time surface measurements pattern, comprise semiconductor laser instrument (1), along the outgoing beam working direction of this semiconductor laser (1) with optical axis set gradually first lens combination (2), beam splitter (3), on the working direction of the transmitted light beam (t1) of this beam splitter (3), testee (5) is set, on the working direction of the folded light beam (f1) of beam splitter (3), is placed with reference plate (4); Be placed with second lens (6) and detecting element (7) successively on the working direction after described folded light beam (f1) is reflected by reference plate (4), the output terminal of described detecting element (7) is connected with the first input end (8a) of signal processing unit (11), the output terminal of signal processing unit (11) is connected with computing machine (12), the driver (13) that is connected with semiconductor laser (1) is connected with signal source (15) with direct supply (14) respectively, second output terminal of this signal source (15) is connected with second input end (8b) of described signal processing unit (11), this signal source (15) is injected a sinusoidal current signal simultaneously to semiconductor laser (1) and described signal processing unit (11), described driver (13) is the semiconductor laser modulator, be used to control the injection current of semiconductor laser, it is characterized in that: described signal processing unit (11) comprises real-time phase detection circuit (8), real-time phase discriminator (9) and synchronizing circuit (10);

The inner structure of described real-time phase detection circuit (8) comprises first amplifier (801), second amplifier (802), counting circuit (803), first low-pass filter (804), second low-pass filter (805);

The inner structure of described real-time phase discriminator (9) comprises the 3rd amplifier (901), the 4th amplifier (902), and division circuit (903) is separated circuitry phase (904), phase compensating circuit (905), described phase compensating circuit (905) is made of single-chip microcomputer.

2, the sinusoidal phase modulation interferometer of real-time surface measurements pattern according to claim 1 is characterized in that described beam splitter (3) is meant the beam splitter that incident light can be divided into two-beam by the beam intensity ratio that approached 1: 1.

3, the sinusoidal phase modulation interferometer of real-time surface measurements pattern according to claim 2 is characterized in that described beam splitter is that Amici prism or one side are coated with the parallel flat of analysing the light film.

4, the sinusoidal phase modulation interferometer of real-time surface measurements pattern according to claim 1 is characterized in that described reference plate (4) is one facing to the parallel flat that is coated with anti-reflection film on the surface of beam splitter (3) one sides.

5, the sinusoidal phase modulation interferometer of real-time surface measurements pattern according to claim 1 is characterized in that described detecting element (7) is one dimension or 2 D photoelectric detector.

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