# US 5,703,684 A · Apparatus for optical differential measurement of glide height above a magnetic disk > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,703,684 A - Document ID: US5703684A - Status: Granted - Kind: A - Application number: 08/710,818 - Filed: 1996-09-23 - Granted: 1997-12-30 - Earliest claimed: 1996-09-23 - Assignees: International Business Machines Corporation - Inventors: Huizong Lu; Ali Reza Taheri - Web page: https://patentgrep.com/patent/US5703684A ## CPC - G01B 11/14: for measuring distance or clearance between spaced objects or spaced apertures (G01B11/26 takes precedence; rangefinders G01C3/00) - G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS ## Abstract A glide height tester for inspecting a surface of a spinning magnetic disk includes a glide plate, which is mounted to pivot above the disk surface. A reflective surface fastened to the glide plate is imaged by interferometric apparatus which produces a calculated output indicating a first angle of the glide plate. The interferometric apparatus includes a Wollaston prism, which can be rotated 90-degrees to provide an output indicating a second angle of the glide plate, perpendicular to the first angle. ## Claims ### Claim 1 Inspection apparatus for inspecting a surface of a disk, wherein said apparatus comprises: means for rotating said disk about a center thereof; a glide plate movably mounted to extend adjacent said surface of said disk, wherein air dragged by rotation of said disk causes movement of said glide plate; a reflective surface mounted to move with said guide plate; and optical means for measuring rotation of said reflective surface in a first pivoting direction wherein said optical means comprises: a laser producing a coherent, linearly polarized beam; optical apparatus, wherein said coherent, linearly polarized beam is decomposed into first and second projected sub-beams, with said first projected sub-beam being linearly polarized in a first direction, with said second projected sub-beam being linearly polarized in a second direction, perpendicular to said first direction, wherein said first projected sub-beam is projected to a first test spot on said surface under test, wherein said second projected sub-beam is projected to a second test spot on said surface under test, with said first and second test spots extending along said surface under test in a spaced-apart relationship, wherein said first and second projected sub-beams, after reflection from said first and second test spots, are recombined into single, elliptically polarized return beam; a polarizing beamsplitter in which said elliptically polarized return beam is split into a first return sub-beam polarized in a third direction and a second return sub-beam, polarized in a fourth direction, perpendicular to said third direction; a first photodetector measuring intensity of said first return sub-beam; and a second photodetector measuring intensity of said second return sub-beam; wherein a portion of said optical means is rotatable about an optical axis extending perpendicular to said reflective surface to measure rotation of said reflective surface in a second direction, perpendicular to said first pivoting direction. ### Claim 2 (depends on 1) The inspection apparatus of claim 1, wherein said optical apparatus includes: a Wollaston prism, wherein said coherent, linearly polarized beam is decomposed into said first and second projected sub-beams and wherein said first and second projected sub-beams, after reflection from said first and second test spots, are recombined into said single, elliptically polarized return beam; and an objective lens, disposed between said Wollaston prism and said test surface, wherein said objective lens focusses said first projected sub-beam on said first test spot and said second projected sub-beam on said second test spot. ### Claim 3 (depends on 2) The inspection apparatus of claim 2: wherein said Wollaston prism includes a first segment having a crystal axis extending in said third direction and a second segment having a crystal axis extending in said fourth direction; wherein said coherent, linearly polarized beam is directed into said optical apparatus being polarized in said third direction; and wherein said optical apparatus additionally includes a half-wave plate through which said coherent, linearly polarized beam is directed into said Wollaston prism, with said half-wave plate rotating polarization of said coherent, linearly polarized beam into a direction mid-way between said third and fourth directions. ### Claim 4 (depends on 3) The inspection apparatus of claim 3, wherein said Wollaston prism is rotatable through a 90-degree angle about an optical axis perpendicular to said third and fourth directions to rotate said first and second test spots also through said 90-degree angle, so that rotation of said reflective test surface in a second pivoting direction, perpendicular to said first pivoting direction, is measured by said optical means. ### Claim 5 (depends on 4) The inspection apparatus of claim 4, wherein said glide plate is mounted to pivot in said first and second pivoting directions. ### Claim 6 (depends on 1) The apparatus of claim 1, wherein said glide plate is mounted to pivot in said first pivoting direction. ### Claim 7 Inspection apparatus for simultaneously inspecting both flat surfaces of a disk, wherein said apparatus comprises: a spindle rotating said disk about a center thereof; a glide plate movably mounted to extend adjacent each said surface of said disk, wherein air dragged by rotation of said disk causes movement of said glide plate; a reflective surface mounted to move with each said guide plate; and optical means for measuring rotation of each said reflective surface in a first pivoting direction, wherein said optical means comprises: a laser producing a coherent, linearly polarized beam; optical apparatus, wherein said coherent, linearly polarized beam is decomposed into first and second projected sub-beams, with said first projected sub-beam being linearly polarized in a first direction, with said second projected sub-beam being linearly polarized in a second direction, perpendicular to said first direction, wherein, said first projected sub-beam is projected to a first test spot on said surface under test, wherein said second projected sub-beam is projected to a second test spot on said surface under test, with said first and second test spots extending along said surface under test in a spaced-apart relationship, wherein said first and second projected sub-beams, after reflection from said first and second test spots, are recombined into single, elliptically polarized return beam; a polarizing beamsplitter in which said elliptically polarized return beam is split into a first return sub-beam polarized in a third direction and a second return sub-beam, polarized in a fourth direction, perpendicular to said third direction; a first photodetector measuring intensity of said first return sub-beam; and a second photodetector measuring intensity of said second return sub-beam; wherein a portion of said optical means is rotatable about an optical axis extending perpendicular to said reflective surface to measure rotation of said reflective surface in a second direction, perpendicular to said first pivoting direction. ### Claim 8 (depends on 7) The inspection apparatus of claim 7, wherein each said optical apparatus includes: a Wollaston prism, wherein said coherent, linearly polarized beam is decomposed into said first and second projected sub-beams and wherein said first and second projected sub-beams, after reflection from said first and second test spots, are recombined into said single, elliptically polarized return beam; and an objective lens, disposed between said Wollaston prism and said test surface, wherein said objective lens focusses said first projected sub-beam on said first test spot and said second projected sub-beam on said second test spot. ### Claim 9 (depends on 8) The inspection apparatus of claim 8: wherein each said Wollaston prism includes a first segment having a crystal axis extending in said third direction and a second segment having a crystal axis extending in said fourth direction; wherein, within each said optical apparatus, said coherent, linearly polarized beam is directed into said optical apparatus being polarized in said third direction; and wherein said each optical apparatus additionally includes a half-wave plate through which said coherent, linearly polarized beam is directed into said Wollaston prism, with said half-wave plate rotating polarization of said coherent, linearly polarized beam into a direction mid-way between said third and fourth directions. ### Claim 10 (depends on 9) The inspection apparatus of claim 9, wherein each said Wollaston prism is rotatable through a 90-degree angle about an optical axis perpendicular to said third and fourth directions to rotate said first and second test spots also through said 90-degree angle, so that rotation of each said reflective test surface in a second pivoting direction, perpendicular to said first pivoting direction, is measured by each said optical means. ### Claim 11 (depends on 10) The inspection apparatus of claim 10, wherein each said glide plate is mounted to pivot in said first and second pivoting directions. ### Claim 12 (depends on 7) The apparatus of claim 7, wherein each said glide plate is mounted to pivot in said first pivoting direction.