Optical system uses machine vision to align wafers
Combination microscope-based inspection and alignment system meets the task of wafer alignment.
Combination microscope-based inspection and alignment system meets the task of wafer alignment.
By Chris Brais
To align a stack of silicon microelectromechanical-system wafers with a precision of 1 µm, Ben Wells, president of Wells R&D (Lincoln, MA), created a combination microscope-based inspection and alignment system using off-the-shelf image-processing components. Developed on behalf of Nova Scientific (Sturbridge, MA), the system aligns wafers so that multiple layers can be bonded together.
RIGHT. Ben Wells created a combination microscope-based inspection and alignment system using off-the-shelf image-processing components.
While commercial equipment is available to perform this function, Wells wanted a cost-effective "one-off" solution for performing the alignment of a handful of wafer stacks. To do this, he developed a solution with two major elements: a wafer manipulator and a microscope viewing system.
While the manipulator allows wafers to be moved relative to each other with a precision of less than a micron, the microscope viewing system displays magnified images of both wafers for alignment. After alignment, a thin layer of bonding agent keeps the wafers at a fixed distance and location.
A large granite surface plate provides a stable base for the assembly, as well as a flat reference plane for the xy motions. All assembly components have vacuum grooves on their bottom surface and can be rigidly locked in position when motion is not desired. A rigid stainless-steel outer cage forms the reference point for all other components. Normally the cage is locked in place by vacuum.
The lower wafer carrier is located within the outer cage. When the assembly is in operation, a partially assembled stack of wafers rests on the top surface of the lower wafer carrier, held firmly in position by vacuum (see Fig. 1). The x, y, and q positions of the lower wafer carrier can be adjusted with micron precision by three very-fine-pitch pusher screws manufactured by ThorLabs Inc. (Newton, NJ).
RIGHT. FIGURE 1. To align a stack of silicon microelectromechanical-system (MEMS) wafers with a precision of 1 µm, Wells R&D combined a machine-vision system with dual microscopes (top). The wafer manipulator allows wafers to be moved relative to each other with a precision of less than a micron. A microscope viewing system displays magnified images of both wafers for alignment. Wafers are held in place inside the assembly by a vacuum. The x, y, and z positions of the lower wafer carrier are adjusted with fine-pitch pusher screws.
The upper wafer carrier is a removable plate that rests on top of the outer cage. This upper carrier is used to position single wafers atop the stack for bonding. A vacuum holds the wafer firmly against the upper wafer carrier until the bonding process is complete. The height and tip of the plate can be adjusted by another set of three pusher screws, but a kinematic mount prevents any other motions. In combination with the x, y, and z motion of the lower wafer carrier, control over the full 6° of separation is attained.
Once the alignment between the new wafer and the stack is satisfactory, the upper wafer carrier an be temporarily removed to apply a bonding agent between the wafers. The kinematic mount allows the plate to be quickly and precisely returned to the previously aligned position. This arrangement allows Nova Scientific to experiment with different adhesives under near-contact and pressure-loaded-contact conditions.
Each wafer bears a set of fiducial marks that are aligned to the corresponding marks on the wafer underneath. To do this, Wells used two microscopes with a common optical axis—one looking up, the other down. The granite base allows the microscope tower to be moved but locked in place with vacuum when desired. To view fiducials at various locations on the wafer surface, the microscope tower can be repositioned with pusher screws.
Wafer images are displayed in adjoining windows on a computer, with crosshairs representing the common optical axis. As the stack of wafers is adjusted using the manipulator assembly, the fiducial marks are brought into alignment with the crosshairs (see Fig. 2). Repeatability of this measurement is better than 1 µm.
At first, Wells intended to assemble the microscopes using catalog components. However, initial tests with dark-field illumination from a ring of LEDs did not produce the contrast expected. To overcome this, Wells designed a custom microscope body with vertical illumination from a beamsplitter prism. To reduce the total height of the microscopes, the microscope light path was also folded.
The imaging system consists of two XC-73 CCD monochrome cameras from Sony (Montvale, NJ), each paired with a 20X microscope lens from Rolyn Optics (Covina, CA). The resulting field of view is approximately 300 µm wide. When displayed on the computer monitor at a resolution of 640 x 480, each pixel corresponds to less than 0.5 µm at the wafer, allowing alignment on a scale consistent with the requirements.
Aligning the microscopes
Accurate coaxial alignment of the microscopes was critical. If the optical axes of the two microscopes did not correspond exactly, an error would be introduced into the alignment process. To check the optical alignment, Wells created an optical "sandwich" consisting of a 1-mm-thick glass microscope reticle (crosshair) from Edmund Scientific (Barrington, NJ) bonded to a 1-mm-thick microscope slide. The reticle target in the center of the sandwich could be viewed simultaneously by both the upper and lower coaxial microscopes. The sandwich construction insures that the target remains clean and also provides exactly the same optical path from both sides.
Wells made provision in the software to compensate for small alignment errors, but he wanted to deliver the microscopes with an initial alignment accuracy of better than 5 µm. Setting the physical location of the microscopes this accurately would have been a tough task. Wells sidestepped this challenge by performing the final alignment at the CCD end of the microscope. The 20X microscope lens provides a 20:1 optical "lever arm," turning a 5-µm specification into an easy-to-achieve 100-µm specification.
Concerns over wobble in the vertical motion also had to be addressed. Any deviation from perfectly straight vertical motion would result in a corresponding wafer misalignment. Wells initially considered commercial linear bearings, but all the samples he evaluated had a wobble of several microns, even over short distances. Instead, a custom hardened and ground dovetail Z-slide (a type of linear motion guide) was built to solve the problem.
To view the two microscope images simultaneously, Wells selected the PCVision frame-grabber card from Imaging Technology (Bedford, MA) to digitize the RS-170 signals from the cameras. Wells chose the PCVision for several reasons. First, its ability to accept input from up to four cameras allowed Wells to show both video windows and synchronize the two cameras together. Second, on-board video memory ensures that screen images scroll smoothly as wafers are aligned. Drivers for the Sony XC-75/73 cameras are also supplied with the board.
RIGHT. FIGURE 2. In operation, wafer images are displayed on-screen, with crosshairs representing the common optical axis. As the stack of wafers is adjusted, the fiducial marks are brought into alignment with the crosshairs.
Wells used Delphi from Borland (Scotts Valley, CA) to develop the user interface and create a framework for the application code. Delphi was chosen because of Wells' prior experience with the product and because code from an earlier project also could be used. To render the images in the windows created in the Delphi application, Wells used an OCX control from Stemmer Imaging (Puchheim, Germany). Stemmer's software provided a number of features such as zoom, pan, and resizing of the viewing windows, as well as allowing microscope images to be inverted and flipped so that the images on the screen correspond with those at the fixture. This was critical because the alignment procedure called for physically moving and rotating the microscope assembly to check alignment of the fiducial at the left and right sides of the wafer.
The object-oriented development software tools allowed Wells to create an application that would have been an enormous undertaking 10 years ago. "Under DOS, you would have had a team of programmers working for a year. Today, Visual Basic, Visual C++, and Delphi let you put objects together while saving thousands of lines of code in the process," Wells says.
In creating the system, Wells needed to solve a number of hardware, software, and optical problems. "When time is limited, it is essential to maximize the odds that everything is going to work together. Find out if the vendor has ever done what you're doing. For example, find out if camera drivers are available for the camera you are using and whether the software supports functions such as multiple camera windows."
Wells also suggests making sure that products have already been integrated and tested. For example, one vendor offered a frame-grabber card at an enticing price, with "free" development software. However this vendor's OCX component had never been tested under Delphi. "Everybody said it should work under Delphi, but I didn't want to be the one to debug it for them," says Wells.
"The key to making a successful system is the ability to spot problems as they arise and then deal with them," Wells concludes. "Problem areas that you anticipate can almost always be dealt with satisfactorily. The problems you overlook are the ones that come around to bite you!"
CHRIS BRAIS is manager of applications engineering at Imaging Technology, Bedford, MA 01730; www.imaging.com.
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