Rapidly Renewable Lap: Theory and Practice

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  • Rapidly Renewable Lap: Theory and Practice

    Chris J. Evans (21, Robert E. Parks, David J. Roderick, Michael L. McGlauflin National Institute of Standards and Technology, Gaithersburg, MD 20899, USA

    Received on January 8,1998

    The Rapidly Renewable Lap (RRL) uses a textured substrate over which thin films are slumped. The substrate provides the geometry of the lap and a localized texture, depending on the film thickness, properties, and means by which it is deformed over and adhered to the substrate. Abrasives, added to the film, lap or polish without touching or changing the substrate geometry. Depending on process parameters, the RRL gives brittle or ductile (two-body) lapping. This paper has two major themes: it describes the RRL and some applications; and it shows that some relatively simple lapping models predict of process characteristics.

    Keywords: Lapping, polishing, abrasion

    1. Introduction Lapping and polishing are probably the oldest manufacturing professions, with applications dating back to Neolithic man1 and with published process descriptions starting in the 16th century2. Today, these processes are cruaal to the semiconductor3 and many other industries. Not surprisingly, lapping and polishing have been widely investigated and were the subject of two recent CIRP keynote papers45.

    Lapping and polishing processes (except "non- contact" processes such as float polishing6 and elastic emission machining-/) involve the mechanical interaction of three elements: the workpiece surface, the sluny partide, and the surface of the lap. The workpiece geometry is determined by the lap shape, which may change in-process, while surface finish and sub-surface damage are affected by numerous parameters such as load, speed, work and lap material properties, slurry temperature and chemistry. Often the lap properties which optimise finish are not ideal for form control.

    The Rapidly Renewable Lap8 (RRL) separates the functions of figure and finish control and can be used for a broad range of lapping and polishing processes9 (Table 1). It is highly repeatable, and hence makes process investigations easier. In developing and applying the RRL technology, we have gained experience that is summarized in the first part of this paper. That experience indicated RRL characteristics that are contrary to expectations based on classical optical shop experience and on the published literature. The second part of the paper, therefore, desuibes a simple process model defined to help understand system behavior and a set of experiments aimed at validating that model.

    2. Essential features of the RRL The basic idea of the RRL is a lap comprising two parts: a stable textured substrate onto which is held a film that conforms or partially conforms to the substrate's local texture. Abrasives may be embedded in the film, or a sluny applied. The substrate may be flat or have a long radius of curvature; sharply radiussed substrates will cause film puckering.

    A number of implementations of the basic RRL idea are possible. We have focussed on the use of vacuum to both hold the film in place and cause it to partially conform to the

    texture in the substrate (Figure 1). This has been implemented on two kinematically different polishing machineslo; a 300 mm diameter conventionall 1 over-am polisher and a three station 300 mm lapper. In each case, the only required machine modification was to drill out the spindle and add a vacuum union. Implementation on a 600 rnm diameter polisher is nearing completion.

    Polymer or metal Impervious film or pad

    Backing plate

    Spindle G Figure I: The rapidly renewable lap concept

    Table 1: RRL applications have used a number of combinations of materials Film

    Mylar Polyethylene Polyetherether- ketone (PEEK) Themal shrink wrap Kyanar Polyurethane Abrasive papers

    Abrasive

    Alumina Diamond Silicon carbide Chrome oxide Cerium oxide Colloidal silica Colloidal alumina

    Work material Copper Aluminum Anodized aluminum Niobium Stainless steel Electroless nickel Filter glass Fused quark Laser phosphate glass Silicon Sapphire Silicon carbide Silicon nitride

    Annals of the ClRP Vol. 47/1/1998 239

  • For much of our work we have used porous ceramics as the lap substrates. These are available with pore sizes up to 5 mm. Particularly*convenient are foamed silicon carbides12 with 4-12 poredun , which can readily be ground with a resin bond diamond wheel to create a plane of small plateaux. Once the substrate is mounted on the polishing machine, final adjustments to figure can be made using diamond abrasives and appropriate laps. In some polishing applications, control of temperature is considered Critical, leading to the introduction of cooling channels in the cast iron lapping plates of commercially available machines. We have shown that the ceramic substrates can be replaced with metal plates which are compatible with cooling channels and have texture machined into the surface.

    A variety of polymeric and metal films have been used. Most of the polymer films deform elastically over the substrate texture, although 75 pm polytetrafluorethylene (PTFE) crept and failed in a matter of minutes. Aluminum foils (12 and 25 pm thick) deform plastically. Polyurethane and similar pads currently used for industrial chemomechanical polishing (CMP) applications have also been used. Conventional thickness pads performed in the same manner as when used on a conventional lap; the substrate texture propagated through specially thinned pads (0.5 mm), offering the prospect of providing textures on different spatial scales.

    Clearly the construction of the RRL leads to some specific and desirable characteristics: 1. 2.

    3.

    4.

    5.

    6.

    3.

    The substrate defines the geometry of the finished part; The abrasive never touches the substrate, so its shape never changes; No adhesives are used, so the film may be changed in seconds, allowing rapid changes of abrasive size or type knowing that the part will "fit"; Varying the texture in the substrate changes the spatial scale of lap surface texture; Varying the applied vacuum changes the amplitude of the lap surface texture Additional control of lap surface texture can be obtained by changing the micro-texture of the film

    Experience with the RRL As indicate above, the RRL has been applied to a broad range of lapping and polishing processes (Table 1). Rather than try to follow conventional, inconsistent, process nomenclature, we will class@ processes by the three primary modes of material removal observed in RRL applications to date.

    3.1 Fracture mode lapping In the fabrication of glass, ceramic and crystalline components most of the material removed is via the initiation and propagation of fracture. In the dassical optics shop, fixed abrasive roughing operations are typically followed by fine "grinding" using alumina or silicon carbide abrasives and a cast iron tool. Similar lapping operations are used, for example, in silicon wafer processing and the manufacture of silicon carbide automotive water seals for automobile uses.

    Similar processes have been implemented on the RRL using 75 pm thick Mylar films. Silicon carbide abrasives up to 25 pm have been used to lap sapphire and alumina used to lap a range of glasses, from a soft filter glass to fused silica. Data on removal rates for the latter process are given in Figure 11. Two qualitative observations differentiate surfaces prepared in this mode on the RRL from those prepared more conventionally: 1. The 'gray' glass surfaces have finer scale fracture, observed from the surface, than those prepared on a cast iron tool using the same abrasive size;

    Pores intersected by randomly selected lines on the surface

    240

    2. These surfaces clean up quickly and uniformly when changing from lapping with alumina to polishing with ceria. Gray fused silica surface prepared using 9 pm alumina can be polished uniformly to a 1 nm rms finish13 in less than 10 minutes, although it is not clear that all the sub-surface damage has been removed at this stage.

    Note also that, unlike the 2-body ductile diamond lapping process described below, this process involves significant wear of both the work material and the lap film. Figure 2 shows scanning electron micrographs of wear on Mylar film. Experience shows that film life under representative conditions on the 300 mm lapper (30 rpm, 0.75 glmm2) is about 8 hours. The process described here may fall between the classic definitions of two- and three-body abrasive wearl4-15 :

    "Two-body abrasive wear occurs when a rough surface or fixed abrasive particles slide across a surface to remove material. In three-body abrasive wear, the particles are loose and may move relative to one another, and possibly rotate, while sliding across the wearing surface."

    From observation of the surfaces produced, there is no way to determine what proportion of the abrasive particles interacting with the surface are sliding or rolling. Whatever the mechanism, the surfaces produced are nearly isotropic, fine- scale fractured surfaces.

    Figure 2

    3.2 Two-body ductile lapping Over a broad range of materials, we have obsewed that rapid stock removal with low damage is obtained when diamond abrasives are used in a "two-body" rather than a 'threebody" material removal (or abrasion) regime. Such a removal regime is assumed to occur when abrasive grits become embedded in the lap so that no rolling occurs. This mechanism has been referred to as "closed threebody abrasive weat"6; in the application, however, the analogy to grinding suggests that it be described as a twGbody process.

    Twebody ductile lapping has been achieved on the RRL on metals, single crystals such as silicon and sapphire, and ceramicsg. The newly generated surfaces are characterized by a network of continuous smooth fracture free "scratches". Other removal mechanisms seen in some circumstances are: (a) three-body abrasion, characterized by linear surface tracks of fluctuating width and depth obviously caused by rolling of the abrasive; and (b) spalling out of material as a result of brittle fracture. Here we will describe primarily our experience with silicon lapping.

    3.2.1 Silicon wafer lapping A number of 100 mm diameter silicon wafers have been diamond lapped under a range of conditions. The wafers were mounted using commercial 'Yemplates"l7, which

  • comprise a wetted felt backer with a polymer annulus which constrains the wafer radially but allows the wafer to conform to the lap surface, thus ensuring uniform removal.

    Lapped and etched wafers, with an initial roughness18 ranging from 4.8 to 17.9 pm Rt (0.35 to 1.1 pm Rq), were lapped on Mylar with both oil-based and aqueous diamond slumes at loads up to 2.25 g/mm2. With 3 pm to 17 pm diamonds, surfaces were generated with no evidence of surface fracture (Figure 3). At larger diamond sizes, under these conditions there was some evidence of lines of fracture, possibly introduced by rolling diamonds. Changing to a softer film (polyethylene) allowed ductile lapping with diamonds as large as 45 pm.

    In the course of these tests, we observed that for diamond sizes greater than 6 pm, the time taken to remove the etch pits (ie the removal rate) appeared independent of diamond size. Surface finish, however, is a strong function of abrasive size. Changing from Mylar to a harder film, aluminum foil, and 1 pm diamond gave comparable removal rates and finishes of the order of 1 nm ms.

    Figure 3: Nomarski micrographs of ductile lapping of silicon using 3 pm diamond showing (let?) incomplete removal of etch pits and (right) final surface. Marker bar is 50 pm

    Figure 4: Nomacski micrographs showing (let?) as- lapped silicon suface using 6 pm diamond and (right) the effect of a 3 minute Schimmel etch, which revealed no subsurface fracture. Marker bar is 50 pm

    3.2.2 Subsurface damage in crystalline materials Rroduction of apparently fracture free surfaces is a necessary, but not suffiaent condition in production of semiconductor substrates and some crystalline optics. Previous researchers1 920 have indicated that apparently fracture-free surfaces produced on silicon and other crystalline materials may have substantial dislocation densities. For the 2-body ductile lapping process described here, rocking cunre21, X-ray topography22, etching9 (Figure 4) and our CMP data all indicate a significant dislocation density in the subsurface region.

    3.3 CMP using the RRL As indicated earlier, the RRL can be used for a variety of chemomechanical polishing processes. Ceria has been used with Mylar films to polish glasses, chrome oxide on Mylar to polish silicon nitride, and colloidal silica on polyurethane to polish copper, tungsten, silicon oxide and silicon. Where polymer films have been used, the most obvious advantage of the RRL is the ease of changing process, typically from fracture mode lapping to polishing. Where pads are used, the primary advantage demonstrated so far is that no adhesives

    are used: thus the pad may be changed in a matter of seconds, rather than the 30 minutes or more typical of current practice in the IC industry.

    4. Simple psuedo-static indentation model of ductile lapping In using both the rapidly renewable lap and other laps in the two-body mode, we have observed that there is some empirical relationship between the material properties of the workpiece and those required from a usable lap. Specifically, it seems that the work must always be somewhat harder than the lap. In addition we have observed counterintuitive behaviors compared with common optical shop practice. In this section of this paper, therefore, we present a simple model of the process (see also Brown23) and of the conditions under which hmbody abrasion would be expectedEminate.

    Work Work ............ 70- .::..,\ .... ... . ,:_.. . To-. ._ .. ., . * ,.... , i .. -.. './". .._ ..

    --.

    _,: ,?.'

    Lap Figure 5 Figure 6

    \ /

    Material removed Figure 7

    Consider (Figure 5) a particle lodged between a lap and the harder work. With no relative motion, we can consider this as a simple indentation process. Now, indentation hardness is defined as the pressure on the projected indentation area. Thus, for example, Vickers hardness obtained is derived from the geometry of the indenter as:

    (1) 2F. sin 68'

    tiv = D2

    where F is the load and D the measured diagonal of the square impression. The diamond abrasive will usually be a regular polyhedron; the geometric factors are different, but:

    where throughout this paper ki (i = 1 ..13) is a set of constants combining geometric terms and physical properties such as the densrty of diamond (eg Eq 10 below).

    The penetration depth into elher work (dlhh or lap (do is proportional to the diagonal length D, and, for a regular polyhedron, the geometric constants are the same on both sides of the contact: Thus, for a fixed normal load F:

    7 r

    (3)

    Consider next the effect of applying a lateral force. If no rolling takes place then, for mat...

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