CN102096349B - Double-grating automatic alignment system for proximity nano lithography - Google Patents

Abstract

A double grating automatic alignment system for proximity nanometer lithography comprises a light path part, an image processing and circuit control part; the light path part comprises a laser light source, a lens group, a mask, a silicon chip, a mask grating, a silicon chip grating, a spectroscope, an objective lens and a CCD image detector; the laser forms uniform and collimated parallel light after passing through the lens group, passes through two silicon chips with close periods and overlapped with a certain gap and is subjected to diffraction for multiple times, two beams of diffraction light with the same order from the two gratings are subjected to interference superposition, Moire interference fringes with the period amplified relative to the original grating are formed on the surface of the silicon chip grating, and then the Moire interference fringes are imaged on a CCD image detector through an objective lens. The phase difference of the two groups of Moire interference fringes can be extracted by processing the image, so that the relative displacement between the mask and the silicon wafer is calculated, and then the silicon wafer is controlled by the circuit control part to move, so that the silicon wafer and the mask are completely aligned. The invention can realize real-time alignment, has high precision and can realize the automation of alignment.

Description

A dual-grating automatic alignment system for proximity nanolithography

technical field

The invention relates to an automatic alignment system in photolithography, in particular to a double grating automatic alignment system for proximity nano-lithography, and belongs to the technical field of micro-nano processing.

Background technique

With the research and development of highly integrated circuits and related devices, the feature size of integrated circuits (IC) is getting smaller and smaller, and the high-resolution micro-nano processing technology represented by photolithography has been greatly developed. Near-contact nanofabrication has become one of the next-generation mainstream technologies due to its simple operation and low cost, such as nanoimprinting, zone plate array imaging lithography, and X-ray lithography. With the improvement of photolithography resolution, mask silicon wafer alignment becomes one of the main factors affecting the accuracy of device feature size.

The current alignment methods can generally be based on geometric pattern marks, zone plates, and grating marks. Among them, the alignment method based on geometric pattern marking is to directly image the geometric pattern on the mask and the silicon wafer to the detector, and then extract the outline or center of the two geometric patterns through image processing, and calculate the relative coordinates of the two to achieve alignment. allow. Its operation and alignment marks are simple and easy to make, but the accuracy is relatively low, and it is mostly used for manual alignment in early low-resolution lithography. Alignment methods based on linear zone plates and diffraction grating marks both reflect the relative displacement of the mask silicon wafer with the light intensity signal, which can achieve high accuracy, but they cannot avoid the change of the mask silicon wafer gap and the symmetry of the mark , Photoresist coating, etching process and other factors affect the disturbance of the light intensity signal, and it needs to be processed by complex circuits, the cost is also high, and the degree of automation is low.

Contents of the invention

The technical problem to be solved in the present invention is: to overcome the deficiencies of the prior art, to provide a dual-grating automatic alignment system for proximity nanolithography, the system is not easily affected by the silicon wafer process, the alignment accuracy is high, and The operation is simple and easy, and the degree of automation is high.

The technical solution of the present invention: a double-grating automatic alignment system for proximity nanolithography, which consists of an optical path part, an image processing part and a circuit control part, wherein "

The optical path part includes: a laser light source, a lens group, a mask, a silicon wafer, a mask grating on the mask, a silicon wafer grating on the silicon wafer, a beam splitter, an objective lens, a CCD image detector and an image processing part and Circuit control part; the laser light source forms uniform collimated parallel light after passing through the lens group, and the parallel light passes through the silicon wafer grating on the silicon wafer and the mask grating on the mask after passing through the beam splitter. , and overlap with a certain gap, so that multiple diffractions occur, and a certain two beams of diffracted lights of the same level from two gratings interfere and superimpose, forming Moiré interference fringes whose period is magnified relative to the original grating on the surface of the silicon wafer grating , then pass through the beam splitter, and then pass through the objective lens to be imaged on the CCD image detector. The phase difference between the two groups of Moire interference fringes is extracted by processing the image processing part, and the relative displacement between the mask and the silicon wafer is calculated. Then control the movement of the silicon wafer through the circuit control part, so that the silicon wafer and the mask are completely aligned;

The image processing part includes five parts: image acquisition, image filtering, phase extraction, phase difference calculation and offset calculation, wherein the image acquisition is collected by a CCD and converted into a digital image, and then the upper and lower parts of the entire fringe image are separated, and then Carry out two-dimensional Fourier transform on the upper and lower parts of the image respectively, and by performing bandpass filtering in the frequency domain, the phases of the upper and lower parts of the image can be calculated respectively, and the phase difference of the upper and lower parts of the image can be obtained by calculating the phase difference Δφ, through Formula (1) calculates the offset between the mask and the silicon wafer;

$\mathrm{<span\; class="notranslate">\; \&Delta;x</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&Delta;\&phi;</span>}{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; P</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; |</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Wherein P ₁ and P ₂ are two groups of adjacent mask gratings (5) and mask grating (6) periods, Δφ is the phase difference obtained in image processing, and Δx is the silicon wafer (3) and mask ( 4) offset;

The circuit control part includes offset comparison judgment, reading silicon chip position, judgment of moving direction and motor drive part; first obtains the offset of the image processing part, and then compares the absolute value of the offset with a certain setting If it is less than the threshold, it will exit, indicating that the mask silicon wafer has been completely aligned. If it is greater than the threshold, it indicates that the mask silicon wafer is not aligned. First determine the position of the silicon wafer and the mask, and then determine the silicon wafer The direction of movement, and finally the silicon wafer is driven by the motor, so that the silicon wafer moves the distance of the offset;

The feedback process is imaging through the optical part, collecting images through the CCD, obtaining the offset through the image processing part, and making judgments through the circuit control part, and then driving the motor to move the silicon wafer, and then through optical imaging, and collecting images in this cycle until the offset Exit if it is less than the set threshold, and realize full automation.

The two groups of adjacent mask gratings (5) and mask gratings (6) are respectively composed of two gratings whose periods are P ₁ and P ₂ , and P ₂ and P ₁ .

The threshold setting of the control part of the circuit should be on the order of nanometers, ranging from 1nm to 10nm.

The beneficial effect of the present invention compared with prior art is:

(1) The present invention directly aligns the mask and the silicon wafer according to the spatial phase characteristics, which can avoid the impact of silicon wafer process factors such as photoresist that affect the light intensity on the alignment, and has better process adaptability and anti-interference ability , to achieve high precision.

(2) The present invention forms feedback control through the optical part, image processing, and circuit control part, that is, the relative displacement of the mask silicon wafer is expressed in the phase change of the fringe through the principle of double-grating spatial phase interference imaging, and the fringe is obtained through the CCD imaging system Then the phase is obtained by the image processing phase extraction method and then the offset is calculated, and then the alignment control system is fed back through the control algorithm to complete the alignment of the entire silicon wafer and the mask. The invention is not easily affected by the silicon chip technology, has high alignment precision, is easy to operate, and has a high degree of automation.

(3) The present invention can be directly aligned through the designed double-grating mark diffraction imaging and simple image processing, and has the advantages of low cost and high production efficiency.

Description of drawings

Fig. 1 is a schematic diagram of the optical path structure of the present invention;

Figure 2a is a schematic diagram of the layout of grating marks on a mask; Figure 2b is a schematic diagram of the layout of grating marks on a silicon wafer;

Figure 3a is a schematic diagram of the misalignment of the mask grating and the silicon wafer grating; Figure 3b is a schematic diagram of the complete alignment of the mask grating and the silicon wafer grating;

Fig. 4 is the realization block diagram of image processing part of the present invention;

Fig. 5 is the realization block diagram of circuit control part of the present invention;

Fig. 6 is a feedback flowchart of the whole system of the present invention.

Detailed ways

As shown in Figure 1, the optical path of the present invention is composed of a laser light source 1, a lens group 2, a mask 3, a silicon wafer 4, a mask grating 5, a silicon wafer grating 6, a beam splitter 7, an objective lens 8, and a CCD image detector 9 . The laser light source 1 forms uniform collimated parallel light after passing through the lens group 2, and passes through the silicon wafer grating 6 on the silicon wafer 4 and the mask grating 5 on the mask 3. The periods of the two gratings are close, and the gap size It overlaps from 100nm to 200μm, so that multiple diffractions occur, and a certain two beams of diffracted light of the same level from two gratings interfere and superimpose, forming Moiré interference fringes whose period is enlarged relative to the original grating on the surface of the silicon wafer grating. Then pass through the beam splitter 7, and then pass through the objective lens 8 with a magnification of 8× to be imaged on the CCD image detector 9. When the mask 3 and the silicon wafer 4 are in the aligned state, the phase distributions of the two groups of fringes are consistent and the frequencies are equal; when there is a relative displacement between the mask 3 and the silicon wafer 4, the phase distributions of the two groups of fringes change and do not change. Agree again.

As shown in Figures 2a and 2b, the two groups of gratings on the mask 3 and the silicon wafer 4 adopt the layout shown in Figure 2, and the mask grating 5 and the silicon wafer grating 6 have periods P ₁ and P ₂ , P ₂ respectively. Two gratings with P ₁ are formed up and down, wherein in the embodiment of the present invention, P ₁ =10.0 μm and P ₂ =11 μm. When the mask 3 is reflected by the surface of the silicon wafer 4, the +1 order diffracted light of the grating 5 meets on the surface of the marking grating 6 on the right, that is, two sets of interference fringes are generated. Figure 3 shows two sets of interference fringes simulated according to the marks shown in Figure 2. When there is a certain relative displacement between the silicon wafer and the mask, the fringe distribution is shown in Figure 3a. It is easy to be distinguished; when the relative displacement between the silicon wafer and the mask is eliminated, the frequencies of the two groups of stripes are completely equal, such as 3b, at this time, the alignment of the mask 3 and the silicon wafer 4 is completed and an ideal state is achieved.

As shown in Figure 4, the image processing part of this implementation example is composed of five parts. First, the image is collected by the CCD image detector and converted into a digital image, and the upper and lower parts of the stripes in the digital image are separated, and then the two parts of the image are respectively Carry out two-dimensional Fourier transform to enter the frequency domain, filter out the noise and extract the effective frequency part through band-pass filtering, and calculate the phases of the upper and lower parts of the stripes, so that the phase difference between the upper and lower parts can be obtained, and finally through the formula (1 ) to calculate the offset between the silicon wafer and the mask.

As shown in Figure 5, the electronic control part of this embodiment first obtains the offset Δx between the mask and the silicon wafer from the image processing part. Then determine the absolute value of the offset and compare it with a certain set threshold t. In this implementation example, the threshold value is selected to be 5nm. If it is less than the threshold value, it will exit. If it is greater than the threshold value, first determine the position of the silicon wafer and the mask, and then determine The movement direction of the silicon wafer, and finally the silicon wafer is driven by the motor, so that the silicon wafer moves the distance of the offset.

As shown in Figure 6, in the entire feedback process of this implementation example, the optical path part is replaced by a silicon grating and a mask grating. The two gratings generate interference fringes and obtain a fringe image through the CCD, and then calculate the mask silicon through the image processing part. The offset is sent to the circuit control part for comparison. If it is not aligned, the feedback is driven by the motor to move the silicon wafer. This completes an alignment process and enters the second round of imaging until the offset is less than the set threshold. , the whole process is fully automated.

Parts not described in detail in the present invention belong to the well-known technology in the art.

Claims (3)

1. one kind is used for it is characterized in that being made up of light path part, image processing section and circuit control section near formula nano-photoetching double grating Automatic Alignment System, wherein:

Said light path part comprises: LASER Light Source (1), lens combination (2), mask (3), silicon chip (4), be positioned at mask grating (5) on the mask, be positioned at silicon chip grating (6), spectroscope (7), object lens (8), ccd image detector (9) and image processing section (10) and circuit control section (11) on the silicon chip; Form the directional light of even collimation after LASER Light Source (1) the process lens combination (2), through silicon chip grating (6) on the silicon chip (4) and the mask grating (5) on the mask (3), the cycle of these two gratings is approaching through spectroscope (7) back for this directional light; And it is overlapping with certain interval; Repeatedly diffraction takes place thus, and certain the two bundle diffraction light at the same level that comes from two gratings interferes stack, forms the Moire fringe that the cycle is exaggerated with respect to former grating on the surface of silicon chip grating; See through spectroscope (7) again; Pass through object lens (8) then and image on the ccd image detector (9),, calculate the relative displacement between mask and the silicon chip through image processing section (10) being handled the phase differential that extracts two groups of Moire fringes; Move through circuit control section (11) control silicon chip (4) again, silicon chip (4) is aimed at mask (3) fully;

Said image processing section comprises that IMAQ, image filtering, phase extraction, phase difference calculating and side-play amount calculate five parts and form; Wherein IMAQ is to gather and convert to digital picture through CCD; Then separately with whole stripe pattern top and the bottom; Respectively the top and the bottom image is carried out two-dimensional Fourier transform again,, can calculate the phase place of top and the bottom image respectively through carrying out bandpass filtering at frequency domain; Two-part phase place asks difference can get phase difference φ about in the image, calculates the side-play amount of mask and silicon chip through formula (1);

$\mathrm{\&Delta;x}=\frac{\mathrm{\&Delta;\&phi;}{P}_1{P}_2}{2\mathrm{\&pi;}|P_1+P_2|}---\left(1\right)$

P wherein ₁With P ₂Be two groups of adjacent mask gratings (5) and silicon chip grating (6) cycle, Δ φ is the phase differential that obtains in the Flame Image Process, and Δ x is the silicon chip (3) asked and the side-play amount of mask (4);

Said circuit control section comprises side-play amount contrast judgement, reads the silicon chip position, judges moving direction and motor-driven part; At first obtain the side-play amount of image processing section, absolute value and a certain preset threshold to said side-play amount compares again, if less than threshold value then withdraw from; Show that the mask silicon chip aims at fully,, then show the misalignment of mask silicon chip if greater than threshold value; Confirm the position of silicon chip and mask earlier; Confirm the silicon chip moving direction again, through the motor-driven silicon chip, make silicon chip move the distance of side-play amount at last;

Feedback procedure promptly through the opticator imaging, through the CCD images acquired, draws side-play amount through image processing section; The circuit control section is judged; Drive motor moves silicon chip afterwards, passes through optical imagery again, and images acquired so circulates; Withdraw from less than preset threshold up to side-play amount, realized full automation.

2. according to claim 1 being used for is characterized in that near formula nano-photoetching double grating Automatic Alignment System: said two groups of adjacent mask gratings (5) and silicon chip grating (6) are P by the cycle respectively ₁With P ₂, P ₂With P ₁Two gratings constitute up and down.

3. according to claim 1 being used near formula nano-photoetching double grating Automatic Alignment System, it is characterized in that: the threshold value setting of said circuit control section should be nanometer scale, and scope arrives 10nm at 1nm.

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