Process for cementing semiconductor discs onto a carrier plate

An apparatus and process for cementing semiconductor discs onto a carrier ate, wherein after the carrier plate is coated with a layer of adhesive substance, the discs are applied to the adhesive layer and a vacuum-tight sealing dome is placed over the carrier plate to define a sealed, hollow space. A chemically-inert elastic diaphragm is clamped within the dome 0.2 to 1.5 mm above the free surface of the discs, to divide the hollow space into two sections or subchambers which are initially simulataneously evacuated. Then, air pressure is applied to the upper section, so that the discs are pressed by the diaphragm onto the cement layer. Air pressure is then subsequently fed to the lower hollow space, so that the dome can be removed from the carrier plate.

The present invention relates to a process for cementing on discs, in which 
the discs are placed onto and pressed against a carrier-plate coated with 
a cement substance. 
When semiconductor discs are polished, the individual discs must be held in 
a fixed position on the support during polishing. Even if only one disc 
becomes detached, its broken fragments will result in the destruction of 
the entire batch, and of the polishing cloth. The discs are only securely 
anchored in the cement substance when they are pressed uniformly onto the 
cement layer during cementing on. When discs are cemented on using a rigid 
press, however, due to the fact that their thicknesses are not alike, some 
discs are pressed on too strongly, some are pressed on too lightly, and 
some discs are not pressed on at all. 
There is a second problem wherein the discs, which are usually placed onto 
the cement layer with suction tweezers, can cause an occlusion of air 
under the discs, to produce stresses in the discs when the discs are 
subsequently rigidly pressed against the cement layer and the layer has 
set. When the discs are removed after polishing, they spring back into 
their original state, thus forming a wavy surface. Wavy semiconductor 
discs, especially when using photolithographic processes, result in lack 
of definition in the exposure of the photosensitive resist applied to the 
surface of the disc. It is therefore not possible to produce from wavy 
discs, components having a high circuit packing density. 
The problem underlying the invention was therefore to find a cementing-on 
process in which even discs of different thicknesses could be reliably 
pressed into the cement layer, without the occlusion of air bubbles. 
This problem was solved in accordance with the present invention, wherein 
the carrier plate, coated with a suitable cement substance is, after 
applying the discs, covered with a vacuum-tight sealing dome. The dome, 
together with the carrier plate, define a hollow space in which an 
elastic, air-impermeable, chemically-inert diagragm is clamped 
horizontally towards the discs. When the dome is closed, the diaphragm 
lies approximately 0.2 to 1.5 mm above the free surface of the discs. The 
hollow space, divided into two by the diaphragm, is evacuated to a 
pressure of 10 to 10.sup.-5 torr. Subsequently, the discs are pressed by 
the diaphragm into the cement layer by feeding air to the portion of the 
hollow space formed between the diaphragm and the upper wall of the dome. 
After subsequently feeding air to the lower portion of the hollow space 
between the cement layer and the diaphragm, the dome is removed from the 
carrier plate.

Referring to the drawing, the still warm carrier plate 2, consisting of, 
for example, stainless steel or coated or uncoated aluminum, onto which 
the cement layer 3 has been whirler-coated, is placed on a work table 1 
consisting of, for example, aluminum, steel, marble or sinter brick. 
Carrier plate 2 and cement layer 3, which has been whirler-coated on, at 
this stage have a temperature of, usually, about 50.degree. to 160.degree. 
C., preferably, approximately 90.degree. to 110.degree. C. 
It is possible to use as cement substances, paraffin, pizein, wax/colophony 
mixtures or, preferably, additive-modified coumarone/indene resins, phenol 
resins, polyterpene resins, pentaerythritol resin esters or glycerol 
esters of dehydrogenated colophony, and maleic resins, as described in 
German Offenlegungsschrift No. 2,608,427. 
Semiconductor discs 4 are placed onto the cement layer, which is usually 
approximately 15 to 30 .mu.m thick, and is still soft, using conventional 
suction tweezers. A dome 7 is provided over carrier plate 2, which can be 
moved vertically in appropriate guides 6, for example, by means of a 
pneumatic lifting cylinder 5. Dome 7 consists, for example, of aluminum or 
plastic material, such as, for example, polyvinyl chloride, and is 
disposed over carrier plate 2 at the outer edge thereof, as shown in the 
drawing. A gasket 8 consisting, for example, of silicon rubber or rubber, 
is secured to the outer edge of the dome, to provide a vacuum-tight seal 
for the space inside the dome. 
An elastic, air-impermeable diaphragm 10, which is chemically inert towards 
the discs to be cemented on and consists, for example, of silicone-treated 
or rubberized synthetic fabric, is fitted by vulcanization into a two-part 
ring 9 in the interior of dome 7. The upper part of the ring is securely 
joined to the dome wall. Retaining ring 9 clamping the diaphragm is 
arranged in the dome so that when the dome is closed, the diaphragm lies 
approximately 0.2 to 1.5 mm above the free surface of the discs. Above and 
below the diaphragm, are pipe lines 11 and 12, leading into the interior 
of the dome. It is possible for pipe line 12 to be closed by a valve 13. 
There is furthermore arranged on pipe line 11, an air supply connection 
15, closable by means of a valve 14. Pipe lines 11 and 12 lead into a 
common tube 16, the interval pressure of which can be read off at 
manometer 17. By means of a valve 18, pipe 16 is connected to a vacuum 
pump 19 and advantageously, a vacuum-ballast vessel 20, which can be 
evacuated by means of vacuum pump 19. 
The pressing of the discs onto the still soft cement layer 3 on carrier 
plate 2 is performed by evacuating the entire inner space, after closing 
dome 7, to a pressure of approximately 10 to 10.sup.-5 torr, usually 1 to 
5 times 10.sup.-1 torr, by valve 18 being opened while vacuum pump 19 is 
running with valve 13 open and valve 14 closed. Vacuum-ballast vessel 20 
is advantageously approximately 10 to 30 times as large as the hollow 
space defined by dome 7 and carrier plate 2. Vacuum-ballast vessel 20 is 
not absolutely necessary, but by using it, it is possible to accelerate 
enormously the evacuation of the interior of the dome. Vacuum-ballast 
vessel 20 is advantageously already pumped free of air by vacuum pump 19 
before or while discs 4 are placed onto cement layer 3. 
After the hollow space defined by carrier plate 2 and dome 7, has been 
evacuated to a desired residual pressure of preferably approximately 1 to 
5.times.10.sup.-1 torr, the three-way valve 18 and valve 13 are closed, 
and the upper part of the hollow space between dome and carrier plate, 
which is divided in two by diaphragm 10, is supplied with air by opening 
valve 14. Under the action of the external air pressure, diaphragm 11 
presses discs 4 uniformly into cement layer 3. Subsequently, valve 13 is 
also opened and dome 7 is lifted from carrier plate 2, by means of 
pneumatic lifting cylinder 5. Once the cement has set, discs 4 are 
reliably fixed on carrier plate 2. Carrier plate 2, with the cemented-on 
semiconductor discs 4 can then be used in the polishing machine. 
The cementing-on process, according to the instant invention, is obviously 
not limited to cementing on semiconductor discs of, for example, silicon, 
germanium, gallium arsenide or gallium phosphide, but can equally be used 
for polishing or lapping sapphire or ruby substrate discs or, for example, 
optical lenses. 
EXAMPLE 
12 silicon discs having a diameter of 75 mm and a thickness of 380.+-.10 
.mu.m, were placed on a round carrier plate of aluminum having a diameter 
of 495 mm, after this had been whirler-coated with a 20 .mu.m thick layer 
of cement, consisting of a mixture of wax and colophony. Then, an aluminum 
dome having an internal diameter of 505 mm, which had a gasket consisting 
of silicone rubber at its bearing edge, was placed over the edge of the 
carrier plate onto the table surface. When the dome was closed, the 
diaphragm, consisting of a fabric-reinforced, 1.5 mm thick, air-tight 
rubber fabric having a knopped underside, fitted by vulcanization into the 
two-piece retaining ring, was disposed approximately 1 mm above the 
surface of the semiconductor discs, whereas the distance between the 
diaphragm and the upper wall of the dome was approximately 5 mm. 
Subsequently, the space sealed by the dome was evacuated on both sides of 
the diaphragm to a pressure of approximately 0.5 torr, the vacuum-ballast 
vessel connected in a series upstream of the oil pump having a volume of 
0.02 m.sup.3. By feeding air to the hollow space above the diaphragm, 
under the action of the external air pressure, the diaphragm was pressed 
onto the semiconductor discs and these were thus pressed firmly into the 
soft cement layer having a temperature of approximately 90.degree. C. Then 
the space beneath the diaphragm was also supplied with air and the dome 
was removed again. After the cement had hardened and the remainder of the 
cement between the individual semiconductor discs was removed, the carrier 
plate was inserted into a polishing machine. During the polishing 
operation, none of the silicon discs came loose from the underside. The 
polished discs were absolutely wave-free when removed. 
While only a single embodiment is shown and disclosed, it is obvious that 
many changes and modifications may be made thereunto, without departing 
from the spirit and scope of the invention.