METHOD FOR INSPECTING INTEGRATED CIRCUIT CHIPS

Abstract: Integrated circuit chips (1), mounted by solder mounds (3) to a substrate carrier (2), can be elastically deformed by impact forces during manual or tool handling by plastic creep of the solder mounds. In order to detect the deformation of the chips and accordingly the integrity of the chips as well as that of the solder mounds, they are inspected by reflecting an illuminated grid (22) from a (mirror polished) backside of the chips, and viewing the reflected image (23) for distortion of the grid lines (24, 25).

Description: METHOD FOR INSPECTING INTEGRATED CIRCUIT CHIPS This invention relates to semiconductor device fabrication and, more particularly, to the determination and identification of integrated circuit chips which become misjoined or damaged in facedown relationship to a supporting substrate carrier. US Patent No. 3 993 123 describes a package for encapsulating semiconductor devices with provision for cooling of the devices or for dissipation of heat during operation of the device. In this patent, a gas encapsulated cooling unit is provided for one or more heat generating components, e.g. integrated circuit chips, mounted on a supporting substrate carrier of suitable dielectric such as alumina ceramics, glass-ceramic, and the like. The mounting specifically described in this patent is by the flip-chip or facedown solder-bonding techniques. The components are packaged on the substrate by a heat conductive cap which is sealed to the substrate to enclose the heat generating components. The wall of the cap opposite the substrate contains elongated openings or bores which extend toward the heat generating components and on the same centers with respect thereto. A resilient member, such as a compression spring, is disposed in bores in the rear thereof. A thermal conductive element, e.g. a piston, is inserted in the bores, and is biased by the springs into thermal contact with the heat generating components. The integrated circuit chips are normally concurrently formed in a mother wafer substrate from which they are separated or diced. The original wafer is initially polished for planarity and thickness which results in a mirror surface, which is retained in the diced chips having substantially mirror polished backsides, with the opposite side covered with an insulator, such as quartz, with suitable terminal solder contacts for the face-down or flip chip bonding to solder joinable sites on a supporting substrate carrier. The chip carrier solder joinable sites are surrounded by nonsolderable or non-wettable areas so that when the solder between the carrier sites and semiconductor device contact sites melt and merge, surface tension of the solder causes it to fall in a shape to support the chip in spaced relation above the carrier. In handling of the above cooled package, since the pistons are moveable under bias within the bores, these pistons are free to move in retraction against the bias springs, and return under the force of the spring so as to impact the semiconductor chips. Such piston impact can produce plastic deformation in the solder joints and accompanying deformation or bending of the semiconductor chips to place them under stress. Such deformation of the chips can be conducive to fracture of the solder joints or their separation from the chip or carrier surfaces, to stress of the quartz insulating layer on the chips, as well as alteration of the electrical characteristics of the integrated circuit. As will be appreciated, such deformation can also be induced in the chips by various tool handling means, as for example in the fabrication of the package. Similarly, stresses can be induced in the mother wafer during processing of the integrated circuits, as for example during diffusion and coating operations with compositions having varied expansion coefficients. Since these problems can affect the integrity of the devices, it is desireable to provide means for the detection of potentially malfunctioning units. Presently available means can involve complex electrical testing techniques which require comparable complex testers, which not only can fail to insure complete reliability of detection, but also detract from the economic viability of the manufacturing operation. One optical method for detecting deformation in planar surfaces is described in US Patent No. 2 867 149 which is directed to the inspection of singular surfaces as contrasted to multi-surfaces of a plurality of chips as employed in the above described cooled chip package. Application of this patent's technique to the detection of deformed chips utilizing a coarse grid (e.g. a window screen) and a diffraction grating (13,400 lines per inch or about 530 lines per mm) failed to detect any bent chips by any one of eight operators. Although the diffraction grating may cast shadows, there was no definition of lines or they were invisible. Although the window screen did cast shadows which were big enough to see, the only position where they could be seen with ease is by looking through the screen, as shown in the patent. Also the shadows were only sufficiently distinct on a rough or granular surface (e.g. ceramic). Heretofore, there is no known way to screen or inspect chip products, thereby allowing for unknown failure rates in field products. The above drawbacks in inspecting bent or deformed semiconductor devices solder mounted face-down on a supporting carrier substrate can be overcome by a novel technique of reflecting a grid pattern from a mirror polished surface of the device substrate, and observing the reflected image for distortion. Increased sensitivity can be obtained by adjusting the grid size, grid-tosubstrate distance, and substrate-to-operator distance. For example, using a 1 cm grid size, 50 cm grid-tosubstrate distance and 50 cm substrate-to-operator distance, a warpage or bend in the device substrate of about 15002 or 0.15 pm is readily detectable. This grid technique permits evaluating the surface of the device substrate for warpage to a specification of within + 1500 or + 0.15 #m (or better if desired). The inspection can be performed in short times (e.g. few minutes) by relatively unskilled operators. In its broad concept, this invention comprehends its application to inspection of the mirror polished surfaces of a variety of substrates (e.g. stress-deformed processed semiconductor wafers). One way of carrying out the invention is described in detail below with reference to the drawings which illustrate an embodiment of the invention, wherein: Fig. 1 is a cross-sectional view illustrating a multi-chip cooled module package employed for purposes of illustrating the application of this invention. Fig. 2 is a cross-sectional view illustrating a simplified version of Fig. 1, showing a single thermal conductor arrangement for cooling a single chip. Fig. 3 is a schematic cross-sectional view of a semiconductor chip solder joined in normal bonding to a supporting carrier substrate. Fig. 4 is a schematic cross-sectional view illustrating, in conceptional exaggeration, the deformation induced in a solder joined semiconductor chip during handling of device package. Fig. 5 is a schematic view in perspective illustrating the application of this invention to the detection of warpage in solder mounted semiconductor chips. Fig. 6 illustrates the reflected image, of the grid employed in an embodiment from an un stressed chip solder mounted to a carrier substrate. Fig. 7 illustrates the reflected grid image from a surface of a stress deformed chip. Fig. 8 is-a Tallystep Trace of an impact deformation of a chip solder mounted to a carrier substrate. Fig. 9 graphically shows data for a plurality of chips mounted to a carrier substrate. Fig. 10 is another schematic representation of the application of this invention to the inspection of a processed stressed mother wafer. Referring to Fig. 1, there is shown a cross-sectional view of a gas encapsulated module 2 for providing cooling of a plurality of solid state electronic chips 1 to be cooled. As is well known, the chips consist of solid state integrated circuits and devices which are densely packed on each chip. The power consumed in the circuits within the chip generates heat which must be removed from the chips. Since the various circuits have different power requirements, and since the integrated components thereof must be maintained within certain temperature ranges for reliable operation, the cooling must be of such character as to maintain the chip temperatures within the required operating range. The chips 1 are mounted by solder mounds 3, on one side of a dielectric carrier substrate 2, generally made of alumina ceramic, glass-ceramic and the like, which has pins 4 extending from the other side thereof. These connecting pins 4 provide for plugging of the module into a mother board (not shown) which normally can carry auxiliary circuits, etc. A container or cap 5 is attached to the carrier 2 by means of a flange 7 which extends from the periphery of the substrate 2 to the cap 5. The cap 5 is made of a heat conductive material such as copper, aluminum and the like. The cap 5 is sufficiently thick to provide the bores 8 therein, and in which are inserted a compression bias spring 10 and slidably mounted heat conductive piston members 11 which are biased to abut in surface engagement with the backsides or top surfaces of the chips 1. The heat accumulated in the cap 5 from each of pistons 11 is transferred to a cold plate 12 which is attached to the cap. This cold plate has a cooling liquid circulating within internal channels 13 via the respective inlet and outlet 14 and 15. Fig. 2 depicts a simplified version. A single chip 101 is shown mounted on a dielectric carrier substrate 102 which has pins 103 extending from the opposite side for plugging the unit into a board. The cap 104 is sealed to the substrate 102 to enclose the chip 101. The cap has a central extension 105 which extends from the top of the cap into the encapsulated region. The extension has a bore in which is contained a compression spring 106 and a slidable heat conductive piston 111 which is biased against the backside of chip 101. The encapsulated region within cap 104 is evacuated and an inert gas, such as helium, is placed therein under slight positive pressure to prevent leakage of the external ambient into the cap interior. Fig. 3 shows the normal mounting of the chips 1 by solder mounds 3 to the dielectric carrier substrate 2. Since the pistons 11 are slidably mounted in the cap bores, they are free to move against the bias springs 10 during handling, vibration and shock, with resultant return to impact on the chips 1. These impact returns of pistons 11 against chips 1 can produce plastic deformation of the solder mounds 3 with concurrent elastic deformation of the chips, as shown in Fig. 4, which can alter the electrical characteristics of the devices and place its passivating coating in stress. Such stressing of the coating, e.g. quartz, can induce cracking thereof with malfunction of the electric package. The non-planarity of the backside of the chip is an indication of the severity of the impact blows which this invention can identify the deformed chips from a plurality thereof as shown in Fig. 1. In its broad concept, as shown in Fig. 5, this inspection technique comprises the removal of the cap 12 from module 2, and reflection of a light 21 illuminated straight line or grid 22 from the mirror polished back surfaces of the chips 1, and viewing the reflected image 23. The distortion by a bent chip results, as shown in Fig. 7, in a curved line 24 and/or 25 or a "pinched skin" of a square grid 22 (which can be comprised of lines 24 and 25 disposed in normal extension to each other). The grid image distortion is evidenced at each chip (Fig. 7 and Fig. 4) which is stress deformed, whereas the grid images remains square (Fig. 6) in the absence of deformation or bending of the chip as shown in Fig. 3. A distorted reflected image (Fig. 7) will indicate that the chip was bent, e.g. by piston 11 impacts, and therefore the possibility of cracking of the dielectric passivating coating (e.g. quartz) about and adjacent the solder joints 3 is much greater than for unbent or undeformed chips. The set-up for obtaining maximized ease of use of the reflected technique for locating the (elastically) deformed or bent chips is shown in Fig. 5, which comprises the grid 22 and a light source 21. In one application comprising the solder mounting of 100 chips 4.7 x 4.7mm on a 9.0cm x 9.0cm alumina ceramic substrate (in an X-Y distribution), an appropriate grid size was found to be about 305 x 203mm (12 X 8 inches) with a grid line width W (see Fig. 5) about 0.76 mm (0.03 inch). Maximum sensitivity was obtained when one square of the grid is at least smaller than the chip size as reflected from the mirror polished back of the chip. For the chip sizes indicated, one desired reflected grid square side "S" would be about 3.05 x 3.05 mm (0.12 x 0.12 inches). This condition is satisfied if EMI9.1 the terms are as defined in Fig. 5. In conformance, it was found that a grid size of approximately S=6.35mm (S=0.25 inches) with A=B=305mm (A=B=12 inches) and ¯=45 yielded excellent results. This specifically dimensioned system enabled the resolution of angular deflection of the reflected grid lines, of about 0.5 seconds of arc which corresponds to a center deflection of the chips of one Fm or less. For a piston impact test (PIT) of 100cm/second which can be expected to cause about a 2 Fm deflection in the chip center, there is ample resolution to spare. It is noted that the degree of resolution can be significantly increased if the length of the optical lever (A+B) is increased. In order to obtain quantitative data concerning this inspection technique, 9 chips solder mounted on a 3x3 array on an alumina substrate were impacted at three different PIT velocities: 100, 160 and 225 cm/sec. The curvature of the chips was then measured by obtaining a Tallystep Trace across the entire back of the chip (the chips were also profiled before impact to assurre reasonable initial flatness). The profiles in Fig. 8 show exact replication of Tallystep data from one of the chips before and after 100 cm/sec PIT. As shown, this chip had a remnant center deflection tof about 1.77 Sm (70 microinchesf due to the PIT. Additional Tallystep data for chips is shown in Fig. 9. When viewed using the reflected grid technique of this invention, the grid reflection appears as shown in Fig. 6 for an unimpacted chip, while the reflection of the square grid after PIT appears as shown in Fig. 7. The data obtained also showed only a modest increase in tv as the PIT velocity was increased from 100 to 225 cm/sec. The threshold for quartz cracking is well above 100 cm/sec and therefore, any chip with potentially cracked quartz can be readily detected and replaced on the carrier substrate. Fig. 9 graphically shows deflection data for the chips piston-impacted at 100 cm/sec, 160 cm/sec and 225 cm/sec. Fig. 10 shows the application of this invention to the inspection of a wafer 50 for determining location of bent, warped or stressed regions (e.g. chip areas). therein which may have resulted from the various processing stages employed in the fabrication of integrated circuits.

Claims: CLAIMS 1. A method for determining stresses in a semi conductor chip (1) or wafer (50) at various stages of fabrication into integrated circuit devices, said substrate having a reflective polished surface, characterized by providing a grid (22) of vertical and horizontal lines (24, 25) in normal extension to each other, reflecting said grid from said surface, and viewing said reflected grid for deviation of said reflected linesrsaid deviation corresponding to stress deformation of said substrate. 2. A method for analyzing the deformation in a plurality of semiconductor chips (1) mounted face-down on a supporting substrate (2), all said chips having reflective, in particular polished, surfaces on their backside, characterized by providing a grid (22) of vertical and horizontal lines (24, 25) in normal extension to each other, reflecting said grid from said reflective surfaces of said chips, and viewing said reflected grid (22) for determining deformation in any of said chips. 3. The method of claim 1 or 2, wherein said vertical and horizontal lines (24, 25) form squares which are smaller than the surface of each of said chips (1). 4. The method of claim 3, wherein the square sidelength S < 3(A+B)/A, A being the distance between grid and the chip's reflecting surface and B being the viewing distance to the reflecting surface. 5. The method of claim 4, wherein A and B are approximately equal.

Inventors: FOUTS, DAVID PERRY, JASPAL, JASVIR SINGH, ROUSH, WALDEN BENNET, SULLIVAN, MICHAEL JAMES
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1982
Main classification: H01L21
IPC codes: G01B11, H01L21