Parallel Mode Confocal System for Wafer Bump Inspection

Transcription

1 Parallel Mode Confocal System for Wafer Bump Inspection ECEN5616 Class Project 1 Gao Wenliang wen-liang_gao@agilent.com 1. Introduction In this paper, A parallel-mode High-speed Line-scanning confocal system for 3D Measurement is described. Unlike conventional confocal systems that use laser-scanning point by point to form a 3D profile of target, In this system, A collimated beam from white light source goes through a beam-splitter and incident on the surface of micro-lens array. It has 1024 x 100 micro-lenses with same focal length, and a pinhole at each micro-lens focal point. The micro-lenses focus the collimated beam into spots and the light goes through the pinholes to an objective lens -- a dual telecentric lens and onto the specimen surface. The light reflected from the surface goes back to objective lens, pinhole, micro-lens and beam-splitter to a re-image lens and images onto a CMOS camera. When the specimen is at objective focal plane, the light will go through the pinholes and reach the camera, and if it is at de-focused plane, the light go through pinhole decrease rapidly, so 3D information can be obtained by detecting the position with highest intensity at CMOS camera output. In order to separate the illumination path and image path, linear polarizer is used after light source and before CMOS sensor. A quart wave-plate is used between objective lens and specimen to reduce stray reflection. The micro-lens and pinhole array is slant placed to avoid axial direction movement of specimen or system during the inspection. The inspection speed can be achieved by using a high-speed customized CMOS camera with 2000fps. Preliminary result shows that at FOV 50mm, 40um accuracy axial sectioning can be achieved. Better accuracy can be achieved by various method such as reducing FOV, double scanning etc. The targeted application is on-line CSP/BGA and wafer bump inspection. This paper will mainly focus on optics aspects including system, subsystem and components. 2. System Configuration 2.1 Conventional: Laser scan confocal microscope Wenliang Gao ECEN5616 Project_1 Page 1 of 10

2 As shown in Fig. 2.1, A typical laser scanning confocal system is to use point source and point detector to limit the depth of field, so that we can only see the focused point, so the height information can be obtained by axially scan the object or the optical module. The whole image can be obtained by lateral scanning the whole surface point by point. The advantage is high resolution can be achieved since small beam spot size,typical system such as Keyence has 0.1um axial resolution, and for 3D inspection, 3D movement is required, so the scanning mechanism is complicated and scanning speed is slow, it can only be used off-line inspection and verification. 2.2 Array-based Confocal system: By using pinhole array, we can have different version of parallel mode confocal systems, typical systems include Nipkow disc-based system (1), micro-lens array-based sytem (2), (3) ; These are shown in Fig.2.2. and Fig 2.3 Pinhole, Detector Photo Detector Pinhole, Source Light Source Mirror Light Source Collimating Lens Imaging Lens Beam splitter Photo Detector Objective Lens Beam Splitter Microlens Array Pinhole, detector Focal Plane Focal plane Specimen Fig 1.1 Principle of Confocal System Fig1.2 (a) Microlens array-based parallel system Obviously for all the above systems, in order to get Z-sectioning information, a separated axial direction movement and a lateral scanning are required. This 2 axis mechanical motion will limit the speed of the system 2.3 Parallel Mode Lateral Scanning System with Embedded Axial Scanning As shown in Fig 2.3, This basically a microlens array based parallel mode system, but the microlens array is slantplaced with an angle matching the axial scanning dynamic range requirement. Wenliang Gao ECEN5616 Project_1 Page 2 of 10

3 Light Source Collimating lens Polarizing Beamsplitter Photo detector Microlens, pinhole array Re-imaging lens Dual Telecentric lens 1/4 waveplate Focal plane Fig1.2 (c) Another Microlens array-based parallel system Light Source Collimating lens Linear Polarizer Beamsplitter Photo detector Microlens, pinhole array Re-imaging lens Linear Polarizer 1/4 waveplate Dual Telecentric lens Focal plane Fig1.3 SVO's confocal inspection system Wenliang Gao ECEN5616 Project_1 Page 3 of 10

4 Linear polarized collimated beam from polaring beam splitter goes on to the surface of micro-lens array, the light focused by the micro-lens goes through an aligned pinhole array on the back focal plane of the micro-lens array, and then to specimen surface through the objective lens. The dual telecentric objective lens is used for mapping the micro-lens focal plane onto the specimen surface. Light reflected from specimen surface goes through the objective lens, in the end, only the reflection from the focal plane can go through the pinhole array and reach the photo-detector (CMOS sensor); Only very small portion of light reflected from de-focused points can go through the pinhole. The quarter wave-plate and two linear polarizers are used to block the noise reflected from all surfaces before the quarter wave-plate. By laterally scanning the whole specimen, for a specific point at specimen surface, all the N number of pinholes scan through it and N number of data can be obtained, where N is the number of microlens/pinholes in scanning direction. The axial position of this point can be located by finding maximum intensity output and the corresponding pinhole element position. And by combining all the data points, a 3D map of specimen surface can be obtained by only one lateral scanning. 3. Wafer bumper properties Here are some typical key specifications and inspection requirements from wafer fabrication companies or packaging companies. bump diameter: 80um~300um bump height: 70um~200um pitch: >300um Resolution: 2um (height), 5um (diameter and pitch) Speed: 30K bumps per hours. Let s analyse some of optical surface properties of wafer bump, most of the wafer bump are solder balls, it s close to a specular surface, the reflected light angle from wafer ball is very large, while the objective lens NA is about 0.2 for a abberration well controlled lens. so only very small area at the ball tip can be captured, but it s good enough to know the height and co-planality. We can translate the above wafer bump data to optical system specifications. Axial dynamic range: 250um Axial resolution: 2um that means we need at least 250/2=125 micro lens-lets in slanted direction Lateral resolution: 35/2x=17um This decided by microlens fabrication limit, fine resoltution can be realized by double Scanning using indexing 8.5um instead of 17um. FOV: 1000 microlens: 1000x35um/2x = 17mm Microlens Slanted Angle: Atan(2um x (2x) 2 /35) = 13 degree. Wenliang Gao ECEN5616 Project_1 Page 4 of 10

5 4. Optical subsystem and key components Fig4 shows the equivalent optical path of the system with single microlens. camera CCD Re-imaging lens Polarized Fiber Laser Beam expander Anamorphic Beam shaping PBS Microlens pinhole array Dual telecentric lens with a ¼ wave-plate at stop Surface under inspection Fig2.1.2 Equivalent Optical path of single microlens Due to the limitation of microlens fabrication, the pitch and diameter of the microlens is 35um and pinhole is 7um, there are 150 micro lens-lets in the slanted direction and 1000 direction in other direction. The objective lens lateral magnification is 2x. Wenliang Gao ECEN5616 Project_1 Page 5 of 10

6 4.1 Light Source The light source can be coherent or incoherent. The advantage of laser is easy to collimate, less stray light, but it has speckle problem. It can be solved by vibrating the fiber at high frequency. While white light source such as DC Halide source coupled out by rod lens and fiber guide does not have speckle problem, but it s difficult to get a good collimated beam. The actual light source used is Coherent laser diode bar array with fiber output, out put power is 4W. The required beam size at Microlens surface is length 1000x35um=35mm, width = 5.25um, consider design margin, let the beam size be 40mm x 10mm 150x35um So the original beam size 4mm diameter from fiber output has to be expanded and anamorphic shaped. 10x beam expander and prism pair module are used for this. 4.2 Microlens Pinhole Array In order for lenslet focal point to match the pinhole position, aspherical lenslet design is required. The following is the details of the design Wenliang Gao ECEN5616 Project_1 Page 6 of 10

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10 4.4 Stray light rejection Basically use polarization to reject all the reflection except only the reflection from object surface. The linear polarized light goes through the Polarized Beam Splitter, to microlens array to objective lens with built-in ¼ waveplate, it becomes right handed circular polarized, after the reflection from object surface, it becomes left handed circular polarized, through ¼ waveplate again, it becomes linear polarized again but 90 degree with the incident polarization direction. When the reflected light goes through PBS, it directs the light to re-imaging lens and to camera. 4.5 Confocal FWHM estimation: By using geometrical paraxial optics estimation: I output = I input * (K * Z) if K <1 I input if K 1 Where: K = (a 2 * M obj-lens 2 * NA micro-lens 2 ) -1 Z: Object De-focal distance A: pinhole diameter M: Objective Lens Magnification NA: Micro-lens Numerical Aperture 5. Conclusion: This report is only a brief discription of the system as well as components, actual system performance such as cosstalk, confocal effect (FWHM), field curvature correction, speed, noise, peak detection algorithm and the image acquisation systems were not mentioned. The achieved axial resolution is 2um, reapeatabilty is 5um. The actual project was done in 2000 when I was with Agilent Technologies SVO (Singapore Vision Operation). Wenliang Gao ECEN5616 Project_1 Page 10 of 10

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