Industry Proximity Linear (IPL) Technology
For Flat panel Automated Optical Inspection
Industrial Inspection Background
The conventional document scanning with CCD requires a lens, which might cause image distortions, e.g. pin cushion distortion, especially for large sized documents. Scanning with CIS usually generates 1:1 images without any distortion.
The flat panel display (FPD), generally made with LCD or OLED technology, serve as the primary interface embedded into consumer products ranging from appliances, automobiles, medical, and the ever increasing popularity of smart phones and televisions. Flat panel displays have become increasingly popular due to most electronics on the market utilizing them. As consumers demand bigger, better displays at lower prices, manufacturers are tasked to meet these demands with continual improvement in quality and yield. To maintain customer satisfaction, it is imperative to maintain high definition image quality by making sure finished panels must not have any defects.
Flat panel displays are made from glass material on the LCD production lines. Thus, in order to make high quality flat panel displays, the inspection on the glass material as well as the whole LCD process line is very important in order to improve the production yield and reduce the manufacturing costs. If there is any defect on the glass substrate or process during the manufacturing process, the yield will be low. To solve this problem, the glass substrate and each process step on the production line needs an efficient method to inspect the defect and then remove it. The most effective and efficient method is used Automated Optical Inspection (AOI) technology. The AOI technology allows the manufacturers to inspect any defect, reduce the overall time of inspection, while providing greater precision than other technology.
Conventional AOI Method: One camera mechanical scanning
Figure 1: Single CCD Camera Conventional AOI Inspection System.
First conventional method is used a single CCD camera. Since one camera cannot cover a big flat panel due to its limited field of view, figure 1 described the camera is moved from left to right, and top to bottom across the panel. After a lot of image was taken, then, using stitching technology combine to a single image.
Scanning Conventional methods: Multiple cameras
Second conventional method is utilized multiple linear cameras to cover the entire area of the flat panel (see figure 2). This takes into account the fact that each linear camera can’t cover a very wide length. For large size flat panels (LCD, OLED), several linear cameras need to align precisely to form a linear plane. Then, the whole panel is moving to form a panel image.
Each linear camera is attached to its own respective PC. Each PC executes the image processing using a neighbor-comparison algorithm. Illumination (generally LED sources) is needed to maximize the visibility of various surface and body defects on the FPD. Together, the system helps detect and determine defects and assess other performance parameters like contrast, brightness, and color uniformity.
Figure 2: CCD Line Scan Camera Setup
Problems with conventional method
There are three main issues with the conventional AOI inspection system. The first is image alignment problem. The second is image distorted caused by vignette (Lens cosine 4th effects). The third is the need to have the same lens magnification for all cameras.
The first major issue in that the conventional AOI system is prone to misalignment. If the cameras are incorrectly lined up and not accurately placed, this can cause the CCD camera's field of view to overlap, thus causing an image of the flat panel display that is not correct. Errors can still occur even at the smallest of margins. For example, a ±1 pixel of misalignment can still cause major performance issues when FPD line scanning which can result in very costly fixes later on.
From Figure 2, the arrays of cameras are lined up side by side in single line formation to create a long linear scanning camera. For 200 and 400 dpi resolutions in the panel, each pixel size is about 127 and 63.5 um, respectively. It is very difficult to precisely align multiple cameras to form an image plane within 127 or 63.5 um range. For example, the Gen 8.5 LCD production line requires at least 10 linear cameras to cover a whole sheet of glass. Each camera is independent and organized accordingly by a human operator.
The focal length of the conventional CCD linear camera is long. Therefore, the overall system is big and requires a lot more resources to maintain. This also generates issues for maintenance as well. If one camera becomes non-fully operational, the maintenance required to replace it with a new one is very tedious.
The CCD linear cameras use conventional lenses that perspective distorts reproduced images. The outer edges of the image are at a different magnification than the center. This lens system can suffer from the following lens distortions from image gauging: barrel and pincushion distortion (see figure 3). These two distortions can hurt the process of creating and scanning a proper image of the possible defects on flat panels. Causes include improper camera alignment, difference in camera's angle for field of view, and many more variables.
Figure 3 Barrel distortion and pincushion distortion on the conventional lens system.
Another undesired effect that can occur is vignette which is usually caused by the limitations of the lens itself. Vignette is the reduction of an image's brightness (illumination fall off by the cosine fourth law of illumination falloff) towards the edge / periphery compared to the center brightness of the image. Though it can be used to add a creative touch to photos, it hurts the performance of scanning flat panel displays. An example is the lowered visibility of defects that fall in the edge of the camera's field of view. If this situation occurs, defects can go unnoticed thus lowering the quality of the manufacturer's yield.
Figure 4 Vignette effect on the conventional lens system.
For flat objects or documents that contain text, chromatic aberration can occur while using conventional linear scan cameras. Figure 5 displayed that the characters to be blurred due to focus failure from the refraction of different wavelengths through slightly different angles.
Figure 5 The character distortion caused by chromatic aberration
As a result, image distortion is undesired because it does not give an accurate representation of the flat panel being scanned as well as false and inaccurate information regarding the defects discovered.
The third issue with conventional AOI systems is that since they are using a lens, the magnification performance is limited by the lens as well. Each lens must be able to support the proper magnification to properly show accurate details of the object being viewed. If there are certain defects in the glass panel in an assembly line, it may require a more powerful magnification in order even find it. If not, the defect will go unnoticed. For example, the multiple cameras in the conventional AOI system setup shown in Figure 2 must have the same magnification ability and setting. If not, a certain part of the huge glass panel may have defects that go unnoticed because one of the cameras responsible for that area may not have the ability to discover the defects.
CMOS Sensor Inc. IPL Industrial Camera
In order to solve common issues occurring in the inspection industry, CMOS Sensor Inc developed the IPL Industrial Camera. The IPL utilizes a single package to house multiple linear sensors. Figure 6 shows the IPL camera which combined several linear sensor chips together to form a single camera. CMOS Sensor Inc produces a series of the product lines for our customers. For IPL camera, each unit of the image length is 0.25 meter. Two units is 0.5 meter. The IPL camera is able to extend up to 3 meter long.
Figure 6 IPL linear camera scanning system.
The IPL camera used two different sensor placement methods for different applications. One is silicon butting, the other is silicon stagger.
First method is silicon butting which shown in figure 7. Each sensor chip is placed in a line and each chip is butting up against the other sensor chip next to it. All of the sensor chips are mounted on a PCB substrate and are not physically touching each other. There is about 20 to 40 um optical gap in between each sensor chip to prevent any physical touch without hindering device performance.
Figure 7 Butting Method
Second method for creating a longer image sensor array is by silicon staggering (see figure 8). Unlike the butting method, staggering has each sensor chip are put together with the ends of each sensor chip overlapping its neighbor thus eliminating the optical gap. Compared to the sensor formation in the butting method, the stagger formation is more suited for AOI application. The overlapping of the sensor chips are intentional allowing the sensor chip boards to be adjusted accordingly for proper photo diode alignment.
Figure 8 Stagger Method
With the circuitry and their respective sensors packed** into one unit, this bears many advantages over conventional multi camera setups.
All of the components which included sensors, FPGAs, circuitry, optical lens are all packaged and secured into one module. This removes the possibility of moving components (lens, field of view, variance of distance, etc) that might negatively affect the performance of the camera. In addition, system maintenance is much more easy and simple. Instead of having to remove each individual component in a traditional system and reinstalling everything again, you can simply remove the whole unit and install it back whenever. Each unit has accurate and calibrated predrilled holes to enable accurate mounting for alignment. This removes the tedious hassles of removal and installation as well as added flexibility in scheduled maintenance (no need to recalibrate sensors and less downtime).
** Depending on the specifications required, the length of the IPL line scanner and the amount of components inside will vary.
Industrial Inspection Methods With The IPL Camera
There are two scanning methods and two illumination methods available when using an IPL camera. The first scanning method is a convey line setup where the IPL camera is stationary while the panel (highlighted in red for Figure 9) to be inspected is on a conveyor belt line and passes through the IPL camera.
Figure 9 IPL Camera Convey Line scanning method
The second scanning method is called the flatbed which is the opposite of the convey line. Here, the panel under inspection is stationary while the IPL camera moves from one end of the panel to another. This is illustrated in Figure 10 where the IPL (highlighted in red) would move from position 1 to position 2.
Figure 10 IPL Camera Flatbed scanning system
The panel illumination is a necessary part in scanning for imperfections on flat panel displays. The image sensor of the IPL camera can detect the whole spectrum of the wavelength from UV to NIR light. Therefore, the IPL camera can use a lot of different LED light sources to illuminate the object. The light source can cover from UV, Blue, Green, Red, to Near infrared (NIR) LED. The white light of LED is also available.
There are two illumination methods possible with the use of IPL camera. The first utilizes the reflection method (see figure 11) where the LED light source (either external or internal) is on the same side as the IPL camera. The LED light hits the panel and the information is reflected from the panel and to the sensors inside the IPL camera. The other method uses the transmission method (see figure 12). Here, the LED is on the opposite side of the IPL camera while the flat panel display is between the two.
Figure 11 Reflection type illumination method
Figure 12 Transmission type illumination method