High Speed Lapping & Polishing
The prototype Lapper Machine is used to abrasively lap and polish semiconductor
wafers, sapphire displays and hard-material industrial components at very high
speeds. The large, heavy, and rigid high-speed prototype lapping machine uses
fixed-abrasives bonded to flexible island-type discs. It has a speed range from
0 to 3,000 rpm which far exceeds the typical speed of 50 to 100 rpm used for
conventional liquid abrasive slurry lapping and polishing. Material removal rates
(MRR) for very hard materials such as sapphire are more than 10X faster than that
of abrasive slurries.
Lapping & Polishing Experiments
The lapper is used to develop and test the overall abrading system. One specific
use is to test the performance of abrasive media used for high speed lapping and
polishing. Tests include optimizing the configuration of the island annular discs and
the use of different abrasive bead coatings techniques on the disc islands. Other
developments are for optimizing the manufacture of and the erodibility of the
abrasive particle-filled beads. Also, the lapper is used to evaluate abrading speeds,
water coolant systems, hydroplaning of wafers, chemical additives, abrading
pressures, material removal rates (MRR), wafer surface finish and wafer flatness.
Wafer Head Mounted on Lapper
A wafer head is attached to a rigid high-speed precision machine-tool spindle. This rotary spindle is attached to a carriage that is movable in a vertical direction. The vertical carriage is moved upward to load wafers and then moved downward to provide wafer contact with the platen abrasive. This whole assembly is supported by a heavy and rigid secondary carriage that is movable horizontally. The horizontal carriage allows different-sized (either 12” or 18” diameter) abrasive discs to be used on the prototype lapper.
Change Abrasive Discs Quickly
Both 12” and 18” abrasive island discs can be changed very quickly with the platen's vacuum attachment system. Wafers remain attached to the wafer head as fixed-abrasive discs are changed on the platen. Use of progressively smaller abrasive particles provides optimum polishing results. Typically, three different-sized abrasive particles are used to flatten and then polish wafers. Also, different abrasive materials can simply be changed by changing the discs (from diamond to silica for instance).
Fixed-Abrasive Island Discs
Use of the water-cooled fixed-abrasive island discs allow these extraordinary abrading speeds without hydroplaning of the wafers. Wafer hydroplaning causes wafer separation from the abrasive disc and causes wafers to tilt, which results in non-flat surfaces or causes it to not abrade at all. Each disc has an annular band of abrasive coated islands. Both the platen and wafer head are rotated in the same direction and rpm to provide uniform abrading of wafer surfaces (and wear-down of the abrasive). Also, the water-cooled abrasive discs eliminate the clean-up mess of liquid slurry lapping.
Avoid Hydroplaning of Wafers
Water spray applied to the top surfaces of the disc abrasive islands is used to remove the heat generated by the friction action of the abrasive in moving contact with the wafer surface and to wash away removed workpiece material. Non-island discs cause hydroplaning at high abrading speeds where the wafer is lifted off the abrasive surface by a film of water attached to the moving abrasive. This effect is similar to hydroplaning of bald auto tires when driving at high speed on a wet roadway. The abrasive disc islands prevent hydroplaning the same way that tire tread lugs prevent it. When a wafer loses abrading contact with the disc abrasive surface, no material is removed from the wafer surface. Hydroplaning can also tilt the wafer which causes non-flat wafer surfaces. By comparison, conventional liquid abrasive slurry and non-island fixed-abrasive discs must be operated at very low abrading speeds to prevent hydroplaning.
Large Wafers or Groups of Small Wafers
Single large wafers (or groups of multiple smaller sapphire wafers) can be quickly attached to the special-design wafer head with vacuum.
Wafer (or Part) Head
The rotatable wafer (or lapped part) head is adjusted vertically to position wafers in flat-surfaced contact with the platen abrasive surface. A spherical bearing in the wafer head, having an off-set center of rotation, provides assurance that an adjustable abrading pressure is applied uniformly across the wafer surface. The off-set spherical center of rotation also prevents tilting of wafers by abrading forces. Horizontal positioning of the head allows the use of different-diameter abrasive discs.
Lapping Industrial Components
Industrial components such as tungsten carbide sealing rings can be quickly attached to the wafer head with vacuum and flat-lapped to provide precision-flat and smooth surfaces.
Various chemicals can be added to the coolant water to soften the silicon or sapphire wafer surfaces and promote faster cut rates. These are the same chemicals that are presently used for conventional wafer polishing.
Air Bearing Spindle Platen
The air bearing spindle platen on the prototype lapper is flat to within 0.00003”. Interchangeable precision-thickness discs used on the precision-flat platen allows full flat-surfaced contact of the wafer and abrasive even at high abrading speeds. A spindle spherical mount allows the air bearing platen surface to be precisely aligned perpendicular to the wafer head axis of rotation. The platen is driven by a powerful, speed controlled, internal dc torque motor.
Vertical, Horizontal Carriages
The lapper has both a vertical and horizontal carriage that facilitates the positioning of the wafer head on the abrasive disc. These carriages are supported by multiple large THK industrial machine-tool ball bearing blocks that slide on precision-ground rails. Each carriage is positioned with the use of a threaded drive screw. After the horizontal carriage is manually positioned for the specific abrasive disc diameter, it is locked in place to the lapper machine frame. A heavy and structurally stiff 500 lb vertical carriage is supported by the horizontal carriage. A dc gear motor attached to the horizontal carriage drives a threaded screw that positions the vertical carriage and wafer head either by manual adjustment or by electrical power. The heavy 600 lb horizontal carriage is very stiff structurally to support the vertical carriage without dimensional distortions due to abrading forces. It also contains damping materials to absorb vibration excitations to provide a stable platform for the wafer head.
Controls, Instruments, Gauges
The lapper has a large assortment of monitoring devices along with electrical, pneumatic, fluid, and mechanical controls. The air bearing platen internal torque motor has a control system that allows different profiles to be used for abrading procedures. Many of the motor operating parameters including speed, torque (abrading friction), power, acceleration and deceleration are displayed during the lapping and polishing operation. Likewise, the wafer head speed is controlled and monitored to match its speed to the abrasive platen. Various independent pneumatic pressure and vacuum systems are manually controlled and monitored with analog gauges. Abrading pressure (applied by the wafer head) can be varied during a polishing procedure by use of a high-speed air-bearing rotary union that can be operated at 3,000 rpm. Also, a precision position read-out device provides information for setting up the abrading process for different thickness wafers if operating in a fixed mode is desired.
Coolant Water Spray System
Coolant water is atomized and then sprayed onto the rotating abrasive island disc
upstream of contacting the wafer leading edge. Sufficient water spray is applied to
cool both the wafer surface and the diamond abrasive that is attached to the island
disc, yet allows the moving abrasive to contact and abrade the wafer. Excess water
carried by the islands is sheared off the island tops when contacting the leading
edge of the wafer. Water is also carried by the moving disc islands from the wafer
leading edge to the interior portions of the rotating wafer. A water collection system
gathers coolant water that is thrown radially off the rotating disc by centrifugal
forces and debris contained in the excess coolant water is partially collected by a
circular enclosure surrounding the platen. The excess coolant is then run through
drainpipes in the bottom of the circular enclosure which routes the coolant and
debris into a settling tank where it is easily collected and removed.
Water Coolant Advantages
Water is the best cooling agent for high speed abrading and is far superior to the oils used in liquid abrasive slurries. Contact between the wafer surface and the abrasive surface during polishing produces friction heat. This heat can degrade both the wafer surface and the diamond abrasive particles. Brittle wafer materials are susceptible to uneven frictional heating (and resultant temperature variations) across the surface of the wafers. Large thermal expansion stresses can occur when large temperature differences exist over a wafer surface which can cause wafer distortions and other problems. Also, abrading frictional heat can cause localized overheating of the sharp cutting edges of individual diamond abrasive particles reducing their life and effectiveness. Use of coolant water prevents both of these problems. Water has a unique cooling capability such that whenever a localized temperature exceeds 212F, the water contacting that hot area boils and changes to steam. The tremendous heat of vaporization required to change the water from liquid to vapor is supplied by the heated surface area. This heat transfer mechanism prevents any heated area from exceeding the relatively low temperature of 212F which is not harmful to either a wafer or the diamond abrasive particles.
The protype lapper has a system of strong and tough polycarbonate and fiberglass guards that allow observation of the abrading process while providing full protection for the operator. A durable fiberglass circular enclosure surrounds the platen to collect coolant water and debris that is thrown off the rotating abrasive disc by centrifugal forces. A circular-arc polycarbonate window guard is vertically inserted into slots of the fiberglass enclosure to provide visual access of the abrading process but contain all the excess water and debris within the circular enclosure. In addition, a large ½ inch thick polycarbonate vertical window-slide is mounted at the lapper front with strong bearing slides that are attached to the lapper frame. This clear polycarbonate window is in a closed position along the front of the lapper during abrading which allows full-view visual observation of the abrading procedure. For wafer loading or changing of the abrasive discs, the vertical window is slid horizontally away from the front of the lapper. Another large ½ inch thick polycarbonate vertical side window, positioned at the right side of the platen, is also mounted with strong slides that are attached to the lapper frame. This side window can be moved to provide access to that side of the platen. Finally, a ½ inch thick polycarbonate vertical window, positioned at the left side of the platen, is rigidly mounted to the lapper frame to provide visual access.
Exhaust Ventilation System
An exhaust fan ventilation system collects coolant water vapor and other fumes from the circular fiberglass enclosure that surrounds the platen and exhausts it through building ventilation ductwork. This greatly reduces the amount of water mist and debris that can escape from behind the polycarbonate safety guards.
Robotic Wafer Loading
The prototype lapper is designed with the front access capability to add a robotic loading device that can insert or remove wafers.
Large Lapper Machines
Single or multiple workstations can be positioned around the circumference of larger platens using 24” or 36” diameter abrasive discs to simultaneously abrade groups of small wafers or workpieces. Also, multiple 12” (300 mm) wafers can be processed simultaneously using the same abrasive disc.