Lapping machines

Sept. 5, 1961

M. S. LIPKINS

2,998,680

LAPPING MACHINES

Filed July 21, 1958 —44

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INVENTOR

Morton S. Lipkins

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ATTORNEY

United States Patent Office

2,998,680

Patented Sept. 5, 1961

2,998,680 LAPPING MACHINES

Morton S. Lipkins, Malverne, N.Y. Filed July 21,1958, Ser. No. 749,923

Claims. (Cl. 51—131)

The present invention relates to lapping machines and the like.

A known form of lapping machine has an apertured nest for keeping the articles distributed about the lapping plate. The articles are moved across the lapping surface while pressure is maintained against them, commonly by a pressure plate. The nest usually is in the form of a thin member having a distributed pattern of holes whose edges encircle the articles being lapped. The nest is thin enough so that the articles resting on the lapping surface can project above the nest an ample amount for engagement by the pressure plate. An object of the present invention is to provide an improved lapping machine adapted to handle articles that are thinner than the minimum practical thickness of a nest.

Articles to be lapped are commonly carried relatively along the surface of the lapping disc in an orbital path that is off-center relative to the axis of the disc. The lapping plate tends to wear unevenly, and commonly has to be resurfaced after only a limited number of lapping operations. I have found that it is important to provide for considerable “overhang” of the nest and the article beyond the edge of the lapping ring or disc during part of the orbital path for, with an empirically determined extent of overhang, such unequal wear can be practically eliminated. In the case of relatively small articles, the best overhang may well amount to one or more piece diameters.

Accordingly, a further object of this invention resides in the improvement of lapping machines having overhang in the lapping path. A feature of the invention in this connection is in the provision of a vacuum holder to retain the articles during the overhang portions of their lapping paths. In the illustrative embodiment of the invention described in detail below, a nest and the articles to be lapped bear against the flat lapping surface of a rotary lapping ring or disc, and pressure against the articles is applied by projecting elements into the holes in the nest. In practice the wafers are normally of unequal thicknesses at the start. The projecting elements are formed on a single rigid member, and the faces of the projecting elements that apply pressure are coplanar. This feature promotes reduction of all the wafers to the same finished thickness.

The nest in the embodiment below includes a circular pattern of article-retaining apertures; or apertures arranged in multiple concentric circles may be used where a large number of articles are to be handled. In order to help keep the nest in close face contact with the lapping surface (to guard against thin wafers slipping under the nest) the nest is made rigid, and preferably heavy, and is much thicker than the wafers. The pressure member is loosely complementary to the nest, in that the projections of the pressure member fit loosely in the article-confining holes in the nest, and the pressure member has a recessed pattern corresponding to the nest which it accommodates. In such an organization, the wafers are prevented from slipping out of the confining holes in the nest and under the nest, and despite the relative thickness of the nest compared to the thin wafers, the necessary pressure is applied for the lapping operation.

The nest and the pressure member move as a unit relative to the lapping member, moving each wafer in an orbital path which at times carries the wafers into overhanging positions. This is highly advantageous in pre10 serving the flatness of the lapping surface, as noted above (or in preserving the sphericity of a lapping surface if such were used). Vacuum ports are provided in the pressure member of the illustrative apparatus. The pressure member therefore both presses the wafers against the lapping surface and firmly holds the wafers in the pressure plane even during the overhang portions of their travel.

The nature of the invention and its further objects and features of novelty will be more apparent from the following detailed description of the illustrative embodiment shown in the accompanying drawings which form part of this disclosure. In the drawings:

FIG. 1 is a vertical cross-section of illustrative embodiment of the invention, shown largely in cross-section along the line 1—1 in FIG. 2; and

FIG. 2 is a cross-section of the apparatus in FIG. 1 viewed from the line 2—2 in FIG. 1.

In the drawings, a lapping member 10 is shown as a disc having a supporting shaft 12 on which it is rotatably supported, and it is operated by any suitable rotary drive means 14 that is diagrammatically illustrated.

Articles 16 to be lapped are shown round, although they may be of any shape, and they are constrained by nest 18 to move in a circular orbit eccentric to lapping member 10. A circular array of apertures 20 are provided in nest 18, although any expedient pattern of apertures may be used. Conventionally, the apertures are arranged with one in the center of the nest as illustrated, a circle of six apertures for as many articles, a larger circle of 12 apertures and successive concentric circles of 19, 26 . . . apertures, each larger circle containing seven more apertures than the preceding smaller one. Only a center aperture and a circle of six more are shown for simplicity, but a larger number of apertures is of course contemplated for inclusion in the illustrative apparatus where appropriate. The walls of these apertures engage the edges Of wafers 16 to react against the fricttional drag of the lapping surface against the wafers, to force the wafers to travel relatively across the lapping surface during rotation of plate 10.

A pressure member 24 is provided having integral projections 26 extending with a loose fit into openings 20 in the nest. The bottom faces of projections 26 that engage wafers 16 are all in a common plane. These faces contain ports 30 which extend to a vacuum manifold 34 and hollow shaft 38, which communicates, via hole 40 in the shaft to a non-rotatably supported gland 42, and to vacuum line 45. This is connected to a suitable source of weak vacuum, of sufficient capacity to hold wafers 16 against projections 24 without being so hard a vacuum as to deform wafers 16. A pattern of passages 30 may be used in each projections 26 for relatively large or especially thin wafers.

A stationary shaft 44 locates shaft 38 and thereby fixes the axis of pressure member 24, as well as that of nest 18. Shaft 44 is intended to avoid imposing other constraint on members 18 and 34, such as might force them out of their natural resting position or rotational motion. Other equivalent means may be used to arrest the axis of nest 18 and thereby arrest the axis of the pressure plate 24 without interfering with their natural pressure attitudes, and without restraining their rotation. It will be understood that the pressure plate and the nest are caused to rotate by rotation of lapping plate 10, although any suitable drive of member 24 or of member 18 may be used, with the remaining members appropriately supported free of the restraints considered above.

A radiating pattern of grooves 46 are formed in the bottom face of nest 18, to distribute slurry or lapping compound to all parts of the lapping surface. Slurry may

2,998,680 be dropped onto the projecting part of the lapping plate 10, at the right in the drawings.

Pressure member 26 can freely shift up and down relative to the nest IS, to accommodate different thicknesses of wafers, and to maintain pressure against the wafers as lapping reduces their thickness. Pressure is greatest on the thickest of the wafers, which are thereby quickly reduced to the thickness of the thinnest wafers; and then they are then all lapped to a smooth, true finish. Both top and bottom surfaces of the wafers can be lapped if desired. The apparatus is effective with wafers whose finished thickness may be only 0.002 inch or less, much thinner than is practical for the conventional thin nest and flat pressure plate.

Rotation of lapping plate 10 produces a rotation of the assembly 16—18—24 as a unit. Wafers 16 move in a circular path, which carries each of them periodically into the overhang position shown at the left in FIG. 1. This is important in preserving the flatness of the lapping surface against the natural tendency of the system to produce uneven wear. The vacuum system provided in pressure member 24 is important in the overhang regions, for holding the wafers in the lapping plane as they leave and as they enter the area of the lapping surface, and to support them against gravity where they completely leave the lapping surface in the overhang region.

Both the lapping plate and the nest are of cast iron, and the latter is thick enough to be rigid and to remain flat. The nest wears (as does the lapping surface) during the lapping surface, but the nest remains flat and true with the proper extent of overhang.

The foregoing represents what is presently regarded the preferred embodiment of the invention; but it is naturally susceptible to a wide variety of modification in details and of rearrangement, and it is undoubtedly useful in various applications. Consequently, the invention should be broadly construed, in accordance with its full spirit and scope.