DMD digital lithography exposure system and its alignment detection method

Abstract

The application discloses a DMD digital lithography exposure system and a detection method thereof, and the system comprises a light source, an illumination system, a DMD, a projection system, a rear lens and a digital camera which are sequentially arranged, and the digital camera is installed on the back focal plane of the rear lens; the light emitted by the light source is irradiated on the DMD through the illumination system, the DMD is conjugated with the image plane to form an image, and the pattern of the DMD is imaged on the image plane through the projection system. The application can check the exposure system adjustment by observing the energy distribution, shape and size of the spectrum plane, and the spectrum plane of the exposure system is imaged on the focal plane of the rear lens and observed through the digital camera by reasonably selecting the light aperture and focal length of the rear lens according to the numerical aperture and focal length of the exposure system; when the focal length of the exposure system is unknown and only part of the spectrum plane can be observed, the pattern can be spliced by moving the digital camera, the structure is simple, the design is convenient, the spectrum plane can reflect more exposure details, and the matching problem of the illumination system and the projection system in the exposure system adjustment process can be better reflected.

Description

Technical Field

[0001] This invention relates to a DMD lithography exposure system and its assembly and inspection method, belonging to the field of semiconductor integrated circuit or printed circuit manufacturing technology. Background Technology

[0002] Photolithography is the core of semiconductor device fabrication. As the density of processed devices continues to increase, the linewidth of photolithography is constantly decreasing, while the cost of photolithography masks is gradually rising. Therefore, seeking low-cost or maskless photolithography technologies has become a research hotspot. Maskless digital lithography based on digital micromirror devices (DMDs) uses DMDs instead of traditional masks. This eliminates the need to change the mask when changing exposure patterns, reducing photolithography costs, decreasing changeover time, and improving the flexibility of photolithography technology. It has gained widespread application. The exposure system is a key subsystem of the lithography machine, and its resolution directly determines the minimum linewidth of the exposure. In a DMD digital lithography system, the exposure system includes a light source, an illumination system, the DMD, and a projection system.

[0003] To ensure high exposure quality in the DMD digital engraving exposure system, the relative positional accuracy of the illumination and projection systems is required to be very high. The positional tolerances for the spacing, eccentricity, and tilt of the lenses in the illumination and projection systems are also very strict. Even slight changes in their relative positions can have a significant impact on the exposure quality.

[0004] Currently, the assembly and adjustment of DMD digital exposure machines in the industry involves precise assembly and adjustment of either the illumination system or the projection system. The accuracy of the assembly and adjustment of these two systems directly affects the resolution of the exposure system. However, the matching between the illumination and projection (including DMD) systems also affects the resolution of the exposure system. Typically, this matching is achieved through simple mechanical positioning, followed by observation of the actual exposure pattern. This method not only wastes a significant amount of test plates and testing time but also fails to provide a quantitative evaluation of the matching degree between the illumination and projection systems, making it difficult to consistently achieve optimal exposure performance. This patent provides a method that uses a rear-mounted optical system to observe the spectral plane of the exposure system and determines the matching degree between the illumination and projection systems by observing the spectral plane. This method, along with other assembly and adjustment detection methods, ensures the exposure quality of the exposure system. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a DMD digital lithography exposure system and its assembly and adjustment detection method. The method uses the observation of the exposure system's spectral plane as a criterion for judging the quality of the system's assembly and adjustment. The progress of the exposure system's assembly and adjustment is characterized by observing the energy distribution, shape, and size of the spectral plane. A 4F system is constructed using a rear lens system to observe the exposure system's spectral plane without damaging the system.

[0006] The first objective of this invention is to provide a DMD digital lithography exposure system, comprising a light source, an illumination system, a DMD, a projection system, a rear lens, and a digital camera arranged sequentially. The digital camera is mounted on the rear focal plane of the rear lens and is movable on the rear focal plane. Light emitted from the light source is uniformly collimated and shaped by the illumination system to illuminate the object surface DMD. The DMD and the image plane are conjugate to form an image. The pattern of the DMD is imaged onto the image plane by the projection system. The rear lens is positioned behind the image plane.

[0007] In one embodiment of the present invention, the projection system is a dual telecentric system.

[0008] In one embodiment of the present invention, the aperture plane of the projection system is also the spectral plane of the DMD digital lithography exposure system, and the light intensity distribution on the spectral plane is the spectral image of the DMD digital lithography exposure system.

[0009] In one embodiment of the present invention, the aperture D of the rear lens, the image-side aperture angle u' of the projection system, the exposure field of view y', and the focal length f of the rear lens satisfy the following relationship: D>2(f*tanu'+y').

[0010] The second objective of this invention is to provide a method for assembly and inspection of a DMD lithography exposure system, wherein the method utilizes the aforementioned DMD digital lithography exposure system and includes the following steps:

[0011] Step 1: Install the lighting system and projection system using the initially designed mechanical positioning, then install the position of the rear lens according to the 4F optical principle, and install the digital camera on the rear focal plane of the rear lens;

[0012] Step 2: The digital camera records the spectral image of the exposure system, recording the size and clarity of the spectrum, and extracts the energy distribution of the spectrum through digital grayscale.

[0013] Step 3: When coordinating the rear lens with the projection system, to solve the matching problem between the lighting system and the projection system, keep the projection system unchanged and change the lighting mode and position of the lighting system; observe whether the spectrum image is rotationally symmetrical based on the center of the aperture. This indicates whether the optical axis of the lighting system matches the optical axis of the projection system. If not, adjust the tilt angle of the lighting system or the DMD.

[0014] Step 4: If the spectrum is blurry, it means that the distance between the lighting system and the projection system in the direction of light propagation is not appropriate. Change the distance between the lighting system and the DMD and observe the clarity of the spectrum.

[0015] Step 5: Extract grayscale values ​​from the spectrum using a digital camera, ensuring that the light intensity energy at each point in the spectrum is rotationally symmetrical about the center of the aperture. If the grayscale values ​​are not symmetrical about the center, check the cleanliness of the lighting system and the lens, and adjust the tilt angle of the light source and DMD.

[0016] Step 6: Repeat steps 2 to 5 above until the spectral image is clear and rotates symmetrical about the center of the aperture. The adjustment is then complete.

[0017] In one embodiment of the present invention, in the fifth step, grayscale extraction of the spectrum is performed by a digital camera. Grayscale extraction does not require uniform light intensity energy at all points on the spectrum.

[0018] In one embodiment of the present invention, in the fifth step, the spectrum is extracted in grayscale using a digital camera. It is required that the light intensity energy at the spectrum is rotationally symmetrical about the center of the aperture, and the spectral ellipticity is equal to 1. The spectral ellipticity is used to characterize the different distributions of light intensity energy in the longitudinal and transverse directions of the spectrum.

[0019] In one embodiment of the present invention, in the fifth step, grayscale extraction of the spectrum is performed using a digital camera, requiring that the light intensity energy at the spectrum be rotationally symmetric about the center of the aperture, and that the spectral polarity Ux = Uy; wherein, with the center of the aperture as the origin, the aperture plane is divided into four quadrants; the spectral light intensity distribution is divided along the X / Y axes, defining the energy distribution in the positive and negative directions of the X-axis and the positive and negative directions of the Y-axis respectively; Ux and Uy are used to describe the spectral polarity in the X and Y axis directions respectively; the specific definitions of Ux and Uy are as follows:

[0020]

[0021]

[0022] The beneficial effects of this invention are:

[0023] The DMD digital lithography exposure system and its assembly and inspection method provided by this invention inspect the assembly and adjustment quality of the exposure system by observing the energy distribution, shape, and size of the spectral surface. By rationally selecting the aperture and focal length of the rear lens based on the numerical aperture and focal length of the exposure system, the spectral surface of the exposure system is imaged onto the focal plane of the rear lens. A digital camera is then placed on the focal plane of the rear lens for observation. If the focal length of the exposure system is unknown, the digital camera can only observe a portion of the spectral surface image; however, the image can be stitched together by moving the digital camera. This invention's inspection system, composed of a rear lens and a digital camera, has a simple structure and is convenient to design. The spectral surface reveals more exposure details and better demonstrates the matching issues between the illumination and projection systems during the assembly and adjustment of the exposure system. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the structure of the DMD digital lithography exposure system of the present invention;

[0026] Figure 2 This is a schematic diagram of the spectral ellipticity of the present invention;

[0027] Figure 3 This is a schematic diagram of the light intensity imbalance in the x-direction of the spectral plane of the present invention;

[0028] Figure 4 This is a schematic diagram of the imaging of the photolithography objective lens and the superposition of defocused light intensity in this invention;

[0029] Figure 5 This is a schematic diagram of the spectral plane eccentricity of the present invention;

[0030] Figure 6 This is the imaging optical path diagram of the eccentric photolithography objective lens of the pupil plane in this invention. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0032] Example 1

[0033] like Figure 1 As shown, this embodiment provides a DMD digital lithography exposure system, including a light source, an illumination system, a DMD, a projection system, a rear lens, and a digital camera arranged sequentially. The digital camera is mounted on the rear focal plane of the rear lens and can move on the rear focal plane. The light emitted by the light source is uniformly collimated and shaped by the illumination system to illuminate the object surface DMD. The DMD and the image plane are conjugate to form an image. The pattern of the DMD is imaged on the image plane by the projection system. The rear lens is positioned behind the image plane.

[0034] Optionally, the projection system is a dual telecentric system, and here a 4F optical system is used for model building.

[0035] Among these, the projection system achieves ideal imaging with no aberrations. The matching between the illumination system and the projection system is the main factor affecting the resolution of the exposure system. A common method for matching the illumination system and the exposure system is to ensure that factors such as the field of view, telecentricity, and numerical aperture of the illumination system and the projection system are matched during the optical design stage. However, during actual installation, mechanical tolerances and assembly / adjustment errors cause inconsistencies in the matching between the two systems. Since commonly used optical system designs are dual telecentric systems, companies focus on the assembly and adjustment accuracy of individual systems during optical engine assembly and adjustment, but the matching between the two systems is usually just a simple mechanical fixation.

[0036] This invention determines the quality of system assembly by focusing on the spectral plane matching between the illumination and projection systems. Both the illumination and projection systems have their own spectral planes. Theoretically, the two spectral planes form conjugate images, meaning the illumination spectral plane clearly projects onto the projection spectral plane. If the two systems are not perfectly matched, the image seen at the projection system's spectral plane will be blurry, distorted, and unevenly distributed. However, in design and production, due to size limitations, direct digital observation on the projection spectral plane is impossible. This invention utilizes the 4F system imaging principle, adding a rear lens behind the image plane. The rear lens and the optical system behind the projection spectral plane form a 4F structure. The distance from the image plane to the rear lens is equal to the focal length f of the rear lens. The digital camera is mounted on the rear focal plane of the rear lens. From... Figure 1 It can be seen that for a digital camera to capture a complete image, the aperture D of the rear lens has certain requirements, determined by the image-side aperture angle u' of the projection system, the exposure field of view y', and the focal length f of the rear lens, where D > 2(f * tan u' + y'). The field of view of the rear lens image may be larger than the chip size of the digital camera, which can be integrated through image stitching.

[0037] It's important to note that DMDs are based on semiconductor manufacturing technology and consist of a high-speed digital light-reflecting switch array. The projection shape of the DMD is determined by computer-controlled micromirror deflection angles and times. It is a fast, flexible, and digital display device that integrates a reflective micromirror array and CMOS SRAM onto a single chip using MEMS technology. The pixel size and overall dimensions of DMDs are constantly evolving. Early pixel sizes were 16μm with a deflection angle of 10°. Later, TI continuously upgraded DMDs, increasing the deflection angle to 12° while continuously reducing the pixel size, which is now available at 10.8μm, 7.6μm, and 5.4μm. The overall size of a DMD is obtained by multiplying the pixel size by the number of array elements. Commonly used DMDs have between 500,000 and 2 million pixels, with a pixel spacing of approximately 1μm. DMDs have a lifespan of up to 100,000 hours and can be flipped back and forth one million times. Each micromirror unit has three steady states: +12° or +10° (on), 0° (no signal), and -12° or -10° (off). In actual industrial production, the DMD micromirror in the 0° state is not powered on, which is an unstable state and may introduce errors into the system; therefore, it is generally not used. Figure 2 As shown, when the computer sends a "1" signal to the DMD, its deflection is +12° or +10°. The reflected light is imaged onto the exposure surface through the projection lens, forming a photolithographic pattern. When the computer sends a "0" signal to the DMD, the reflector deviates from its equilibrium position by -12° or -10°, causing the reflected light beam to deviate from the projection system and not be imaged onto the exposure surface. The computer sends multiple bits of "1" and "0" signals to the DMD, causing each pixel to be in its corresponding "on" and "off" state. When a given sequence of graphic data control signals is written into the CMOS circuit, the incident light is modulated by the DMD, and the graphic can be displayed on the image plane.

[0038] There is a difference in light energy in the X and Y directions on the spectral plane, such as... Figure 2 As shown. Ellipticity is used to characterize the difference in the longitudinal and transverse distribution of light energy in the spectrum. Spectral ellipticity is defined as follows:

[0039] Spectral polar balance describes the symmetry of light intensity energy along the x / y axes at a given frequency spectrum. Based on the distribution of light intensity along the x / y axes, the energy distributions in the positive and negative x-axis and the positive and negative y-axis are defined respectively. Ux and Uy are used to describe the spectral polar balance along the x and y axes, respectively. The specific definitions of Ux and Uy are as follows:

[0040]

[0041]

[0042] like Figure 4As shown, there is a schematic diagram of the optical path of a conventional illumination lithography objective imaging system with a highly unbalanced spectrum in the Y direction, and the superposition of the image plane when out of focus. The light intensity distribution on the image plane is the superposition of the light intensity projected onto the image plane from the positive and negative Y directions of the spectrum. However, when the image plane is out of focus, the light intensity after the superposition of the positive and negative spectra will no longer be symmetrical about the center of the field of view, that is, the image will shift. Similarly, the highly unbalanced spectrum will also cause different light intensity distributions in the horizontal and vertical directions of the image plane, resulting in differences in the linewidth in the horizontal and vertical directions.

[0043] During system assembly, there may be an angle between the hardware of the lighting system, light source, and projection system. This can lead to a mismatch between lighting and projection, manifesting as a deviation between the shape of the projection lens's spectral plane and its ideal shape. Figure 5 As shown. This deviation exists in three directions. In the x / y plane, it can be represented by the offsets Ex and Ey of the spectral X-axis and Y-axis, respectively. The change in the z-direction is reflected in whether the objective lens spectral plane has a clear boundary.

[0044] A brief analysis of the principle of spectral eccentricity is provided, such as... Figure 6 As shown, assuming the projection system is a perfect ideal system with no aberrations, spectral eccentricity causes the spectral center to be off-center from the field of view, and the principal ray of the optical path to have an angle with the optical axis. Since the projection produces perfect images, the eccentricity will not affect the image plane. However, if defocus occurs, because the principal ray has an angle with the optical axis, the center of the image will no longer be the ideal image center, and the image will shift.

[0045] The above phenomena are all caused by poor matching between the lighting system and the projection system. By observing the spectrum graph, we can find out the quality of the matching between the two systems and how to improve it.

[0046] Example 2

[0047] This embodiment provides a method for assembly and inspection of a DMD digital lithography exposure system. The method uses a DMD digital lithography exposure system provided in Embodiment 1 above and includes the following steps:

[0048] Step 1: Install the lighting system and projection system using the initially designed mechanical positioning, then install the position of the rear lens according to the 4F optical principle, and install the digital camera on the rear focal plane of the rear lens;

[0049] Step 2: The digital camera records the spectral image of the exposure system, recording the size and clarity of the spectrum, and extracts the energy distribution of the spectrum through digital grayscale.

[0050] Step 3: When coordinating the rear lens with the projection system to solve the matching problem between the lighting system and the projection system, keep the projection system unchanged and change the lighting mode and position of the lighting system; observe whether the spectrum image is rotationally symmetrical based on the center of the aperture stop (assuming the optical axis passes perpendicularly through the center of the aperture stop). This indicates whether the optical axis of the lighting system matches the optical axis of the projection system. If not, adjust the tilt angle of the lighting system or the DMD.

[0051] Step 4: If the spectrum is blurry, it means that the distance between the lighting system and the projection system in the direction of light propagation is not appropriate. Change the distance between the lighting system and the DMD and observe the clarity of the spectrum.

[0052] Step 5: Extract grayscale values ​​from the spectrum using a digital camera, ensuring that the light intensity energy at each point in the spectrum is rotationally symmetrical about the center of the aperture. If the grayscale values ​​are not symmetrical about the center, check the cleanliness of the internal components of the lighting system and the lens, and adjust the tilt angle of the light source and DMD.

[0053] Step 6: Repeat steps 2 to 5 above until the spectral image is clear and rotates symmetrical about the center of the aperture. The adjustment is then complete.

[0054] In the fifth step, grayscale extraction of the spectrum is performed using a digital camera. The light intensity energy at the spectrum is rotationally symmetric about the center of the aperture, and the spectral ellipticity is equal to 1. The spectral ellipticity is used to characterize the different distributions of light intensity energy in the longitudinal and transverse directions of the spectrum. The spectral polar balance is Ux = Uy. The aperture plane is divided into four quadrants with the center of the aperture as the origin. The spectral light intensity distribution is divided along the X / Y axes, defining the energy distribution in the positive and negative directions of the X-axis and the positive and negative directions of the Y-axis, respectively. Ux and Uy are used to describe the spectral polar balance in the X and Y axis directions, respectively. The specific definitions of Ux and Uy are as follows:

[0055]

[0056]

[0057] Some steps in the embodiments of the present invention can be implemented using software, and the corresponding software program can be stored in a readable storage medium, such as an optical disc or a hard disk.

[0058] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for assembly and inspection of a DMD digital lithography exposure system, characterized in that, A DMD digital lithography exposure system is applied, comprising a light source, an illumination system, a DMD, a projection system, a rear lens, and a digital camera arranged sequentially. The digital camera is mounted on the rear focal plane of the rear lens and is movable on the rear focal plane. Light emitted from the light source is uniformly collimated and shaped by the illumination system to illuminate the DMD object surface. The DMD and the image plane are conjugate to form an image. The pattern of the DMD is imaged onto the image plane by the projection system. The rear lens is positioned behind the image plane. The assembly and testing method for the DMD digital lithography exposure system includes the following steps: Step 1: Install the lighting system and projection system using the initially designed mechanical positioning, then install the position of the rear lens according to the 4F optical principle, and install the digital camera on the rear focal plane of the rear lens; Step 2: The digital camera records the spectral image of the exposure system, recording the size and clarity of the spectrum, and extracts the energy distribution of the spectrum through digital grayscale. Step 3: When coordinating the rear lens with the projection system, to solve the matching problem between the lighting system and the projection system, keep the projection system unchanged and change the lighting mode and position of the lighting system; observe whether the spectrum image is rotationally symmetrical based on the center of the aperture. This indicates whether the optical axis of the lighting system matches the optical axis of the projection system. If not, adjust the tilt angle of the lighting system or the DMD. Step 4: If the spectrum is blurry, it means that the distance between the lighting system and the projection system in the direction of light propagation is not appropriate. Change the distance between the lighting system and the DMD and observe the clarity of the spectrum. Step 5: Extract grayscale values ​​from the spectrum using a digital camera, ensuring that the light intensity energy at each point in the spectrum is rotationally symmetrical about the center of the aperture. If the grayscale values ​​are not symmetrical about the center, check the cleanliness of the lighting system and the lens, and adjust the tilt angle of the light source and DMD. Step 6: Repeat steps 2 to 5 above until the spectral image is clear and rotates symmetrical about the center of the aperture. The adjustment is then complete.

2. The method according to claim 1, characterized in that, The projection system is a dual telecentric system.

3. The method according to claim 1, characterized in that, The aperture plane of the projection system is also the spectral plane of the DMD digital lithography exposure system, and the light intensity distribution on the spectral plane is the spectral image of the DMD digital lithography exposure system.

4. The method according to claim 1, characterized in that, The aperture D of the rear lens, the image-side aperture angle u' of the projection system, the exposure field of view y', and the focal length f of the rear lens satisfy the following relationship: D > 2(f*tanu' + ​​y').

5. The method according to claim 1, characterized in that, In the fifth step, the spectrum is extracted in grayscale using a digital camera. Grayscale extraction does not require that the light intensity energy be uniform across the spectrum.

6. The method according to claim 1, characterized in that, In the fifth step, the spectrum is extracted in grayscale using a digital camera. It is required that the light intensity energy at the spectrum is rotationally symmetrical about the center of the aperture, and the spectral ellipticity is equal to 1. The spectral ellipticity is used to characterize the different distributions of light intensity energy in the longitudinal and transverse directions of the spectrum.

7. The method according to claim 1, characterized in that, In the fifth step, grayscale extraction of the spectrum is performed using a digital camera, requiring that the light intensity energy at each point in the spectrum be rotationally symmetric about the center of the aperture, and that the spectral polarity is Ux=Uy. The aperture plane is divided into four quadrants with the center of the aperture as the origin. The spectral intensity distribution is divided along the X / Y axes, defining the energy distribution in the positive and negative directions of the X-axis and the positive and negative directions of the Y-axis, respectively. Ux and Uy are used to describe the spectral polarity in the X and Y axis directions, respectively. The specific definitions of Ux and Uy are as follows: 。

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