CN101876538A - Method for measuring clearance in proximity nanometer lithography - Google Patents

Abstract

The invention discloses a method for measuring clearance in proximity nanometer lithography and mainly aims at the controlling of the clearance of a masking silicon wafer in nanometer manufacturing technology, such as nano-imprint, wave zone plate array imaging and the like. The basic process of the method can be simply explained in the picture 1 that: incident plane waves pass through a silicon wafer gratings and a mask grating and diffract for multiple times, wherein the two periods of the gratings are approximate and the two gratings are overlapped with a certain gap; two beams of lateral diffracted light from the two gratings interfere with each other and are superposed, and form Moire interference fringes of which the period is amplified relative to the conventional gratings on the surfaces of the silicon wafer grating; and the fringes are imaged on a CCD image detector through an objective lens with certain multiplying power. The change of the gap between the two gratings causes the change of optical path difference of the two beams and then causes the change of the movement or phases of the interference fringes so that the aim of measuring the gap is fulfilled; the more approximate the period is, the higher the sensitivity of the measurement is; and, at the same time, because the periods of the two gratings are approximate, the included angle between the lateral diffracted light is very small, the frequency of the interference fringes is very low (namely, the period is long), and the requirement on the numerical aperture of the objective lens is low. With the development of microfabrication technology, the machining accuracy of the grafting is higher and the method has great significance for gap measurement in proximity nanometer lithography and the related field.

Description

Gap measuring method in a kind of proximity nano-photoetching

Technical field

The present invention relates to a kind of gap measuring method that is used for the proximity nano-photoetching, belong to micro-nano processing, high-accuracy metering association area.

Background technology

As the basis of large scale integrated circuit industry manufacturing technology, along with the research and development of highly integrated circuit and related device, the IC characteristic dimension is more and more little always for optical lithography, and the photoetching technique of high resolution has obtained significant progress.In addition, the fast development of nanosecond science and technology and nano-device characteristic dimension constantly dwindle the development that has also promoted nanoimprinting technology, as nano impression, array of zone plates imaging and photo-etching.Correspondingly, along with the raising of resolving power and the employing of large-sized silicon wafers, the clearance control of mask silicon chip will become the severe challenge that the proximity photoetching faces with measurement, and the clearance measurement precision must increase substantially thereupon.In projection lithography system, the raising of resolving power impels the depth of focus of object lens to dwindle, and the perpendicular displacement measure of the change and the focusing technology of silicon chip proposed new requirement equally.

Traditional clearance measurement or the focusing test technology in the projection lithography mainly adopt methods such as slit projecting and interference strength measurement.The former mainly is provided with slit in light path, by the reflection of silicon chip bottom surface, with vertical unique translation that is converted into slit hot spot on the detector image planes of silicon chip, yet that this method is mainly used in is early stage than the low resolution photoetching, and the degree that precision improves is limited.Interference strength method principle and Michelson interferometer are similar, the perpendicular displacement of silicon chip is converted into a few bundle optical grating diffraction interference of light intensity size variation, be subjected to the influence of silicon chip surface technological processs such as photoresist easily, the defective of introducing additional optical distance, mark as the repeatedly reflection in the photoresist is introduced the big systematic error that asymmetry error etc. causes.

Summary of the invention

The technical issues that need to address of the present invention are: overcome the weakness of existing gap measuring method on principle, further improve the clearance measurement precision, overcome of the influence of silicon chip technologies such as photoresist simultaneously to clearance measurement, provide the gap measuring method in a kind of proximity nano-photoetching, to adapt to the nanoimprinting technology that resolving power improves constantly.

Technical solution of the present invention is: the gap measuring method in a kind of proximity nano-photoetching, and step is as follows:

(1) on the mask silicon chip, adopts mask grating that two cycles are close and silicon chip grating mark grating respectively as clearance measurement;

(2) utilize these two mark gratings of plane wave light source vertical incidence, and at described two mark grating face generation diffraction;

(3) at the same level the diffraction light that comes from two mark gratings respectively meets at mask mark grating surface and interferes, and produces the interference fringe that phase place can change with the gap;

(4) receive on the angle of diffraction direction of the corresponding order of diffraction time by object lens and have the interference fringe of respective cycle, and make it to image on the ccd image detector;

(5) for opaque silicon chip substrate, utilize reflection diffracting light to realize the interference fringe imaging at the mask face, for transparent substrates such as quartz, utilize transmission diffraction light to realize the interference fringe imaging at the mask face; For opaque silicon chip substrate, in order to realize out share the same light road and improve antijamming capability of incident, adopt the Littrow reflective devices, make incident light to become the oblique incidence of angle, Littrow with the silicon chip normal;

(6) at corresponding to interference fringe image isolabeling, that do not have different PHASE DISTRIBUTION, adopt Fast Fourier Transform (FFT) to carry out phase analysis, extract the phase change of image, calculate two gaps between the mark grating and change, promptly realized the gap measure of the change of mask silicon chip.

Participate in the angle of diffraction θ of the two bundles diffraction light at the same level of interference fringe imaging in the described step (3) ₁, θ ₂Respectively by

P ₁?sin?θ ₁＝nλ (1)

P ₂?sin?θ ₂＝nλ (2)

Decision, wherein n is that the order of diffraction is inferior, 1≤n≤P1/ λ, λ are incident wavelength, P ₁, P ₂The cycle of representing mask grating and silicon chip grating respectively.

The numerical aperture NA of object lens satisfies NA＞sin (θ in the described step (4) ₁-θ ₂)/2, wherein θ ₁, θ ₂Be the angle of diffraction of two bundles diffraction light at the same level.

Littrow reflective devices incident angle equates with certain order diffraction angle of mask grating or silicon chip grating in the described step (5), for the silicon chip grating+the n order diffraction, this angle is

θ ₂＝arcsin(nλ/2P ₂) (4)

Wherein, θ ₂Be incident angle, i.e. Littrow incident lower silicon slice grating+n order diffraction angle, P ₂The expression silicon chip grating cycle, 1≤n≤P ₁/ λ, λ are incident wavelength.

Principle of the present invention:

(1) be different from traditional More and measure, the grating of employing cycle difference (being more or less the same) is as basic token on the mask silicon chip.

(2) being different from the interference strength size variation monitoring gap that utilizes two bundle diffraction lights in the tradition changes, utilize the same order of diffraction of close grating of cycle time generation interference fringe image, the gap is changed the phase change that is converted into interference fringe, and silicon chip technological processs such as avoidable photoresist cause diffraction light intensity to change the system error factor that causes.

(3) based on the measuring method of interference fringe image, with the gap subtle change-promptly the optical path difference between the two beam interferometer light changes, obviously the moving or phase change of the striped that is converted into directly calculated the variation in mask silicon chip gap by phase analysis accurately.

(4), adopt the Littrow reflective devices to realize exit direction and the same light path of incident plane wave that certain two order of diffraction is inferior, but the scattered stray light of filtering substrate surface has improved antijamming capability at opaque substrates such as silicon chips.

(5) the Computer Image Processing method is incorporated in the measuring method based on interference image, can be by the noise in the special algorithm filtering stripe pattern and other frequency contents, extract phase change in the stripe pattern exactly, further improve the precision of clearance measurement.

The invention has the beneficial effects as follows:

(1) the present invention is based on More's imaging of grating, yet distinguishes to some extent with the following Moire effect of traditional large scale grating.In order to adapt to the clearance measurement requirement of nano-photoetching tens nanometers even nanometer scale precision, adopt low-light grid (cycle is a micron order) herein, only utilize wherein and two restraint at the same level the diffraction light participation More interference imaging that comes from two gratings, gap between two gratings changes the optical path difference variation that causes two bundle diffraction lights, thereby cause moving or the phase change of interference field of striped, the factor that can avoid silicon chip photoresist coating technology in traditional clearance measurement etc. to influence light intensity bring than mistake.

(2) approaching low-light grid of employing cycle, the fringe period that produces is exaggerated doubly a lot with respect to the former grating cycle, small gap variable quantity causes moving significantly of striped, object lens and the higher ccd image detector of pixel resolving power in conjunction with certain enlargement ratio, can realize high sensitivity detection, theoretical measuring accuracy can reach nanoscale.

(3) the high precision feature with interferometry combines with the convenience of Flame Image Process, to a certain extent, can utilize Flame Image Process and phase analysis algorithm to continue to improve measuring accuracy; Utilize the phase change monitoring gap of stripe pattern to change, can overcome the diffraction light intensity that is difficult in the classic method avoid and change the system error factor that brings; Simultaneously, notional result shows, the cycle of striped and Wavelength-independent, and the light source of clearance measurement system can be selected broadband light.

(4) in addition, the present invention not only can be applied to the clearance measurement of mask silicon chip in the nano-photoetching, can also be applied to the metering of mask silicon pad alignment and other nanometer, control association area.

Description of drawings

Fig. 1 is ultimate principle figure of the present invention;

Fig. 2 is reflective measurement basic framework figure;

Fig. 3 is reflective Littrow mounting frame diagram;

Fig. 4 is a kind of circular mark of practical application: Fig. 4 a represents to be positioned on the mask, the cycle is the mark grating of 10 μ m; Fig. 4 b represents to be positioned on the silicon chip, the cycle is the mark grating of 11 μ m;

Fig. 5 is the interference fringe of Fig. 4 mark correspondence: Fig. 5 a represents the interference fringe of initial reference position correspondence; Fig. 5 b represents that the gap changes and causes interference fringe after the phase change.

Embodiment

As depicted in figs. 1 and 2, the gap measuring method that is applied between mask and the transparent substrates of the present invention is:

At first, adopt grating 2 and the grating 3 mark grating as clearance measurement respectively on the mask silicon chip, the cycle of these two gratings is approaching, is respectively 11 μ m and 10 μ m; Diffraction successively takes place in the plane wave 1 of vertical incidence on silicon chip grating 2 and mask grating 3.At the same level the diffraction light (present embodiment is an example with+1 grade, and other grade is similar) that comes from two gratings meets on mask grating 3 surfaces and interferes, and produces the interference fringe that phase place can change with the gap.For two bundle+n diffraction lights, interference field intensity can be expressed as (might as well establish among the figure grating distributes along the x direction)

Wherein, I ₁=a _n ², I ₂=b _n ²Be two bundle+n diffraction light intensity,

Be initial phase, P=P ₁P ₂/ [n (P ₂-P ₁)] be the distribution cycles of two bundle+n diffraction light interference fringes along the grating face.Obviously, the cycle of striped is only time relevant with the grating cycle with the order of diffraction, have nothing to do with wavelength etc., and

Represent the interference field phase factor relevant with the gap, g represents the gap, and the gap value of mask silicon chip phase information direct and interference fringe interrelates thus.

Secondly, receive interference fringe image by object lens 4, inferior at the different orders of diffraction, must receive corresponding interference fringe in different directions.Because the grating cycle is very approaching, is respectively 10 μ m and 11 μ m, the differential seat angle Δ θ that is provided by formula (3) is very little.Therefore, the receiving angle of striped and corresponding diffraction angle ₁, θ ₂Approximately equal, and determine by following formula

$\mathrm{\θ}={\mathrm{\θ}}_1-\frac{\mathrm{\Δ\θ}}{2}={\mathrm{\θ}}_2+\frac{\mathrm{\Δ\θ}}{2}=\frac{{\mathrm{\θ}}_1+{\mathrm{\θ}}_2}{2}---\left(8\right)$

Wherein, angle of diffraction θ ₁, θ ₂Determine by formula (1) formula (2).In addition, the order of diffraction is inferior high more, and the angle of divergence of the relative mask silicon chip of receiving light path normal is big more, but need design grating, to carry the inferior diffracted intensity of respective stages.

Thus, striped receives by object lens 4, and makes it to image on the ccd image detector 5, and the phase changing capacity of stripe pattern calculates the gap variable quantity before and after must changing by calculated gap, also is Because the difference of time angle of diffraction at the same level is smaller, the numerical aperture NA of object lens 4 is smaller, only needs to satisfy NA＞sin Δ θ/2.

Wherein, at light tight silicon chip substrate, adopt a kind of reflective diffraction light to realize interference imaging, basic principle schematic as shown in Figure 2.Plane wave 1 incident mask grating 2 and silicon chip grating 3 is with close angle θ ₁, θ ₂Time diffraction light at the same level that returns interferes, and by object lens 4 interference fringe is imaged onto on the ccd detector 5 on the striped receive direction of Miao Shuing in front, and the phase change of calculating striped at last obtains the gap and changes, and is similar described in process and Fig. 1; Secondly,,, realize incident and the same light path of outgoing, adopt a kind of aforesaid Littrow to go into injection device in order further to improve antijamming capability at reflective elementary tactics shown in Figure 2.Make plane wave to become the Littrow angle direction incident of mask grating or silicon chip grating with mask silicon chip normal,, satisfy: 2P as Fig. 3 ₂Sin θ ₂=n λ, n are that the order of diffraction is inferior.Especially, employing silicon chip grating+1 order diffraction angle is as the Littrow angle, and its angle is

θ ₂＝arcsin(λ/2P ₂) (9)

Wherein, θ ₂Be the angle of diffraction under incident angle or this incident condition, P ₂Expression silicon chip grating.Its detailed process is: incident wave 1 through spectroscope 6 with angle θ ₂Incide on the silicon chip grating 3 after seeing through mask grating 2, time diffraction light at the same level that comes from two gratings almost returns along former road, is received by object lens 4 through spectroscope 6 backs once more, images in ccd detector 5 at last, and all the other processes and Fig. 1 are described similar.

At last, at different marks, stripe pattern has different PHASE DISTRIBUTION, must adopt corresponding phase analysis algorithm.For general grating, first filtering, denoising obtain more clearly striped distribution plan, extract the phase value of each point again by frequency domain methods such as Fourier transform or wavelet transformations, directly calculate phase change then; For linear grating, can carry out Fast Fourier Transform (FFT) to the linear distributed stripes image after filtering, the denoising, directly extract the phase change that obtains image.

For above-mentioned arbitrary measurement mechanism, can design parameters such as the profile of mark grating, cycles, obtain having specific PHASE DISTRIBUTION, isoparametric interference fringe image of cycle.Among Fig. 4 circular mark and the corresponding interference fringe image that is adopted.Wherein, two of Fig. 4 (a), Fig. 4 (b) expressions lay respectively on the mask silicon chip, the cycle is respectively 10 μ m and 11 μ m, size are the circular grating marker of 660 μ m * 660 μ m.Fig. 5 (a), Fig. 5 (b) are the interference fringe that two marks+1 order diffraction forms, and size still is 660 μ m * 660 μ m; Fig. 4 (a) is the initial strip print image, the stripe pattern when Fig. 4 (b) causes phase change 2 π for the gap changes.In addition, show among Fig. 4 that fringe period is 110 μ m, consistent with notional result noted earlier.

The non-elaborated part of the present invention belongs to techniques well known.

Claims (4)

1. the gap measuring method in the proximity nano-photoetching is characterized in that step is as follows:

(1) on the mask silicon chip, adopts mask grating that two cycles are close and silicon chip grating mark grating respectively as clearance measurement;

(2) utilize these two mark gratings of plane wave light source vertical incidence, and at described two mark grating face generation diffraction;

(3) at the same level the diffraction light that comes from two mark gratings respectively meets at mask mark grating surface and interferes, and produces the interference fringe that phase place can change with the gap;

(4) by object lens with the corresponding order of diffraction time approximately equalised direction of angle of diffraction on receive and have the interference fringe of respective cycle, and make it to image on the ccd image detector;

(5) for opaque silicon chip substrate, utilize reflection diffracting light to realize the interference fringe imaging at the mask face, for transparent substrates such as quartz, utilize transmission diffraction light to realize the interference fringe imaging at the mask face; For opaque silicon chip substrate, in order to realize out share the same light road and improve antijamming capability of incident, adopt the Littrow reflective devices, make incident light to become the oblique incidence of angle, Littrow with the silicon chip normal;

(6) at corresponding to interference fringe image isolabeling, that do not have different PHASE DISTRIBUTION, adopt Fast Fourier Transform (FFT) to carry out phase analysis, extract the phase change of image, calculate two gaps between the mark grating and change, promptly realized the gap measure of the change of mask silicon chip.

2. the gap measuring method in a kind of proximity nano-photoetching according to claim 1 is characterized in that: the angle of diffraction θ that participates in the two bundles diffraction light at the same level of interference fringe imaging in the described step (3) ₁, θ ₂Respectively by

P ₁sinθ ₁＝nλ(1)

P ₂sinθ ₂＝nλ(2)

Decision, wherein n is that the order of diffraction is inferior, 1≤n≤P ₁/ λ, λ are incident wavelength, P ₁, P ₂The cycle of representing mask grating and silicon chip grating respectively.

3. the gap measuring method in a kind of proximity nano-photoetching according to claim 1 is characterized in that: the numerical aperture NA of object lens satisfies NA＞sin (θ in the described step (4) ₁-θ ₂)/2, wherein θ ₁, θ ₂Be the angle of diffraction of two bundles diffraction light at the same level.

4. the gap measuring method in a kind of proximity nano-photoetching according to claim 1, it is characterized in that: Littrow reflective devices incident angle equates with certain order diffraction angle of mask grating or silicon chip grating in the described step (5), for the silicon chip grating+the n order diffraction, this angle is

θ ₂＝arcsin(nλ/2P ₂) (4)

Wherein, θ ₂Be incident angle, i.e. Littrow incident lower silicon slice grating+n order diffraction angle, P ₂The expression silicon chip grating cycle, 1≤n≤P ₁/ λ, λ are incident wavelength.

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