Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-04-14
2001-02-13
Kim, Robert (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S244000, C356S450000
Reexamination Certificate
active
06188485
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to the general field of interferometry and apparatus for testing surfaces. In particular, it provides a novel disk-mount device for successively and automatically testing computer-drive disk blanks for quality control purposes in an assembly line.
2. Description of the Prior Art
As computer equipment becomes smaller and subject to wear and tear associated with portability, the precise flatness of drive disks becomes more and more critical to high-speed performance and reliability. Moreover, any scratch or other imperfection on the surface of a disk is cause for discarding it during manufacture. Thus, conventional aluminum disks are increasingly being replaced by glass disks, which are lighter and less subject to warpage and, therefore, preferable for most applications. The flatness of both kinds of disks is typically tested for quality control by interferometric measurements that identify damaged or substandard areas on either of the two parallel surfaces of a disk. For complete quality control, the flatness of one surface of the disk is first measured on an instrument such as a Fizeau interferometer and the disk is then flipped over to test the opposite surface. During high-volume manufacture, though, disk blanks are processed at a rate of many thousands per hour; therefore, tests are often performed on a limited number of samples from each batch and only on one side of the disk. For example, current disk production needs require the ability to test about 3,600 blanks per hour.
As well known in the art, the surface geometry of an aluminum or glass computer-drive disk can be measured with a Fizeau-type interferometer with reference to a flat reference surface (known in the art as a transmission flat). As illustrated in schematic representation in
FIG. 1
, the interferometric device
30
comprises a light source
10
(normally a laser operating in the single mode) producing a beam of light
12
that is passed through a microscope objective
14
and a spatial filter
16
, such as a pinhole. The light
12
is then collimated by a very-well corrected collimating objective
18
and directed through a transmission flat
20
(comprising the reference surface
21
) toward the test surface
23
(consisting of one side of a computer-drive disk
22
) positioned collinearly (with respect to the light beam) and substantially in parallel to the reference surface at some distance within the coherence length of the light source
10
. The light reflected by the test surface
23
interferes with the light reflected at the reference surface
21
and, according to the principle of superposition, bright interference fringes are produced corresponding to all points on the reference surface where the optical path difference (OPD) of the light is equal to a multiple of its wavelength. A beam splitter
24
is placed between the spatial filter
16
and the collimating objective
18
in order to reflect the fringes to the side, so that they may be observed on a screen or directed to a camera
26
through appropriate lenses
28
for display, and/or to other instrumentation for recording and data processing. The interference fringes so produced are used to provide a measure of the flatness of the tested disk surface.
Prior-art equipment for testing disk blanks utilizes mounting devices that permit the placement of the disk
22
on a sample stage in a desired predetermined spatial relation with respect to the transmission flat
20
. It is essential that the disk surface mounted on the stage be positioned as desired and then held still for interferometric measurements. It is also essential that each successive disk placed on the mount device be fixed in the same exact position. Appropriate mechanisms for adjusting the tip and tilt of the disk surface are also provided, as well known in the art, operating on the back surface of the disk, rather than directly on the test surface.
Pressure applied by the mounting mechanism may cause disk deformation, which in turn results in misalignment with respect to the reference surface. In addition, vibrations in the disk mount can cause measurement errors. Accordingly, these devices are not very stable while performing repeated measurements. U.S. Pat. No. 5,689,337, herein incorporated by reference, discloses a self-aligning disk mount consisting of a hub capable of retaining a disk in precise alignment with the transmission flat by hanging its center hole on two support posts and resting the disk on three pressure tips on the hub. A system of push-pull adjustment screws is provided to set the proper tip/tilt of the disk. The plane established by the three pressure tips provides a fixed, aligned plane against which the test surface of successive disks is directly positioned in alignment with the reference surface. By utilizing gravity to position the disk against the three pressure tips in substantially vertical disposition, deformation of the disk is minimized.
All prior-art testing equipment is designed for manual feed of disk blanks and is not suitable for high-volume on-line testing. Even devices that ensure the alignment of successive disks placed in test position, such as the apparatus described in the '337 patent, require that each disk be handled in and out of the retaining structure that holds the disk in place during the test procedure. This requirement slows the operation down and renders current equipment unsuitable for automatic, high-speed testing. Therefore, there is still a need for a more versatile solution.
BRIEF SUMMARY OF THE INVENTION
It is therefore an objective of this invention to provide a mounting mechanism that is more suitable than prior-art devices for automated, high-speed positioning of computer-drive disk blanks in alignment with interferometric test apparatus.
Another objective of the invention is an automatic mounting mechanism capable of sequentially processing disks at a rate of one every few seconds.
Another goal of the invention is a test-mount mechanism that does not require manipulation of the disks by solid mechanical means that may damage a disk surface during testing.
A further objective of the invention is a mounting mechanism that minimizes manipulation of the test disk by extraneous handling equipment.
Still another objective is a mechanism that is suitable for incorporation with existing interferometric instruments.
Finally, another goal is the implementation of the above mentioned objectives in a commercially viable system that maximizes the utilization of existing technology.
In accordance with these and other objectives, the preferred embodiment of the invention consists of a substantially vertical sample stage positioned along an inclined disk trackway. The stage includes three pressure tips that provide a fixed, aligned plane against which the back surface of the disk being tested is positioned in optical alignment with the reference surface of the instrument. The degree of protrusion of two of the pressure tips can be adjusted to set the proper tip/tilt of the disk. Each disk being tested is allowed to roll down the trackway by gravity to a rubber stop in front of the stage. The trackway is also tilted slightly to cause the disk to roll on edge and lean against the sidewall of the trackway. After the disk is stopped in front of the stage, a retractable finger engages the forward edge of the disk and produces a slight backward motion to release the disk from the rubber stop and align it with the stage. An air-stream burst is then applied to the back surface of the disk to create a suction that attracts the disk to the three pressure tips while supported by two edge points in contact with the retractable finger and the inclined trackway. Interferometric testing is carried out and, finally, the finger is retracted and the disk allowed to roll down the inclined trackway away from the stage while another disk is rolled into position for testing.
According to another aspect of the invention, multiple trackways and sample stages are operated
Durando Antonio R.
Kim Robert
Natividad Phil
Veeco Corporation
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