Semiconductor device

(54) Name: Semiconductor device in the form of a disc shaped semiconductor tablet

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The present invention relates to semiconductor devices which are manufactured in a single thin semiconductor disk in order to increase its yield. Power semiconductor elements with a conductivity of from a few amps to a few tens of amps of the currently widely used type are of the so-called mesas type with glass passivation. These elements are advantageous in that the production costs are low, while the surface is extremely stabilized and the electrical properties are excellent. These elements can therefore be encapsulated in formal plastic moldings. However, semiconductor elements of this type are disadvantageous in that the thin semiconductor disc contained in the device is easily broken both as a result of the disc being provided with grooves and as a result of mechanical residual stresses which arise in the disc due to different coefficient of thermal expansion between the particular semiconductor material such as silicon and a coordinated passivation material such as glass. Thereby, a limit is set for the diameter of the semiconductor boards and for the possibility of further reducing the production costs.

The invention is therefore based on the task of developing a semiconductor device made in a single thin semiconductor disk, so that the cost of production is reduced by increasing the device's output without impairing its electrical properties.

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In a semiconductor device in the form of a disk-shaped semiconductor tablet with a pair of opposite main sides, in which at least one pn junction is located and at which a separating groove is formed in each main surface to separate the transition, this task is solved by the invention in that the separating groove in one main surface forms a closed loop and surrounds a major portion of the tablet and in turn is surrounded by a peripheral portion of the tablet and that peripheral portion projects beyond this one major surface of the major portion of the tablet.

The invention is described in more detail below by way of example with reference to the accompanying drawing, in which Fig. 1 shows a cross-sectional view of a tablet for a mesas type glass glazing thyristor constructed in a conventional manner; Fig. 2 is a perspective view of part of a thin semiconductor cable , before it is divided into a number of tablets of the type shown in Fig. 1 and after all the manufacturing steps which touch the disc have been completed, fig. 3 a perspective view of a portion of a semiconductor board according to an embodiment of the invention; Fig. 4 shows another embodiment of the invention; and Figures 5a-5e are cross-sectional views of a portion of a thin semiconductor board during various stages of manufacture to produce the embodiment shown in Fig. 4.

Fig. 1 shows a power transistor with a current carrying capacity of up to a few tens of amps, which is mesas type with glass passivation. The device shown comprises a semiconductor tablet, which is generally designated 10 and contains a semiconductor matrix 12, i.e. a thin n-type silicon wafer with a pair of opposite main sides, a pair of p-type semiconductor layers 14 and 16, which are made on the wafer. 12 main sites by diffusion therein of a p-type interferer to form each pn junction therebetween, and an n-type semiconductor layer 18, which is provided with a center aperture and is formed on the layer 16 in a known manner by selective diffusion, so that a pn junction is formed therebetween and a center aperture or center window G remains for which the layer 16 is exposed.

As shown in Fig. 1, the semiconductor tablet 10 is chamfered along its entire edge portion 20 from its two main sides, all transitions together with portions of the layer 12 being exposed at the ends of the chamfered edge portion 20 formed by one half of a plateau-forming groove. A passivating glass layer 22 is provided in the groove or on the beveled edge portion to protect the pn junctions in their

7704781-9 exposed ends. The device further comprises a metallized layer 24a, which is located on the entire surface of layer 14, and an apertured metallized middle layer 24b which is formed on layer 18 in a conventional manner. A circular metallized layer 24c is also located on the exposed surface of the layer 16, so that it is at a distance from the layer 24b.

Various electrodes not shown are intended to be attached to the metallized layers 24a, 24b and 24c by some soldering material.

A plurality of semiconductor tablets of the type shown in FIG. 1 can be simultaneously manufactured from a single thin semiconductor disk with a large surface content of FIG.

2, the corresponding reference numerals as in Fig. 1. According to Fig. 2, the semiconductor wafer comprises a number of rectangular semiconductor tablets 10 arranged in rows and slots, with grid forming grooves 20 being arranged. on each of the disc's main faces between adjacent tablets to keep the pn junctions made in each tablet 10 separate from the junctions in adjacent tablets 10. The grooves 20 on one main face of the disc are located opposite the grooves on the other main side in the thickness direction of the disc. The figure also shows p-type diffusing layers 14 and 46, as well as diffused, apertured layers 18. A passivating layer 22 of glass is applied to each groove 20, while the layers 24a, 24b and 24c are made by electroplating. The semiconductor disk is arranged along the center line at the bottom of each groove 20 on one of the main sides of the disk, so that it is divided into a number of semiconductor tablets 10. The tablet 10 therefore has the following advantages:

1) The machinability is good and reduced manufacturing costs are possible since the essential manufacturing steps including the passivation of the pn junctions and the metallization are carried out with respect to a single thin slab of semiconductor material of large diameter.

2) The surface achieves extremely stable condition and the resulting electrical properties are very favorable, since an electrically insulating material such as glass for passivation of the pn junctions is enameled or baked on the semiconductor plate at high temperature.

3) It is possible to mold the tablet into a plastic material.

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However, the semiconductor sheet produced in the manner described above has the disadvantage that it has a high tendency to break due to mechanical residual stresses therein. This is because grooves are made in the board for dividing it into tablets and that the passivating layer consists of an electrically insulating material with a different coefficient of thermal expansion than the semiconductor material in the board. Therefore, it has been possible to increase the diameter of the semiconductor boards only to a certain limited extent to keep the manufacturing costs low. Although it would be possible to increase the diameter of the semiconductor disk, its thickness must also be increased. This measure has not been considered favorable, since the resulting element has deteriorated electrical properties and, in particular, increased voltage drop across the same.

The invention is now described in more detail with reference to FIG. 3, where FIG.

and 2 corresponding components are designated in the same manner. The device shown comprises a thin plate 12 of n-type semiconductor material such as silicon, which differs from the one shown in Fig. 2 only in that, in Fig. 3, each groove 26 on one of the main sides, in the present case the upper main side of the Fig. 3, of the disc 12 is designed as a closed loop and surrounds the majority of each rectangle-shaped semiconductor tablet 10 so that grid-forming semiconductor zones 28 remain, which extend between adjacent grooves 26 and serve as combined connection and reinforcement elements. The zones 28 are connected to a peripheral zone 28 which remains along the circumferential edge of the disc.

Here it can be pointed out that the zones 28, with their exposed surfaces, are substantially at the height of adjacent tablets 10.

On the other hand, each groove 22 on the second or lower main side of the semiconductor disk 12 is constructed in the same manner as the grooves shown in Fig. 2, but the width of the groove is substantially equal to the total width of the grooves 26 and the immediately above the semiconductor zone 28. It can be pointed out to the device shown in Fig. 3 that the effective thickness of the disc 12 is greater than the thickness of the disc shown in Fig. 2 and becomes as strong as a glass plate fixed to a window frame.

After the metallized layers not shown are produced on predetermined portions of the sheet, as described above with reference to Figures 1 and 2, the apparatus of Figure 3 is ready for division into tablets.

This division into squares takes place along the center line of each element 28,

7704781-9 as indicated by dashed lines 30 in Fig. 3. By way of example, a number of semiconductor tablets in squares with each side were made 3.5 mm in a thin semiconductor disk having a thickness of 220 microns, with upper grooves such as grooves 26 and connection elements such as the elements 28 were arranged on the upper main side of the disc with widths of about 0.2 and 2, respectively. 0.4 mm. Lower grooves such as grooves 22 had a width of about 0.8 mm and a depth of 65 microns. While conventional technology allows processing of semiconductor discs with a maximum diameter of 40 mm, it has now been found that the diameter of semiconductor discs within the structure shown in Figure 3 can be increased to 55 mm without the risk of fracture or cracking.

Fig. 4 shows a modified embodiment of the invention which differs only from that shown in Fig. 3 in that in Fig. 4 the combined connection and reinforcing element 28 protrudes slightly outside the coordinated main surface of the semiconductor tablets 10. The 4. The semiconductor wafer shown has increased strength. For example, one has found that by allowing the element 28 to project from the coordinated main surface of the tablets, about 60 microns of 65 mm diameter conductor discs can be used without the risk of rupture or cracking, while the remaining components have unchanged dimensions as compared to those of FIG. 3 shown.

The apparatus shown in Fig. 4 can be prepared according to a method illustrated in Fig. 5, wherein like components are designated by the same reference numerals as in Fig. 4.

Fig. 5a shows a portion of an n-type silicon wafer 12 having a specific resistance of from 15 to 25 ohms cm, a thickness of 280 microns and a diameter of 65 mm. One of the opposing main surfaces, in this case the upper one of the one shown in FIG. 5a, selectively masked with any suitable wax to form band-shaped masked zones 28 having a width of 0.5 mm according to a grid pattern of equal spacing of 3.5 mm to form the elements 28 and also forming the zone 28 on the disk 12 peripheral part, while the disk on the other side or the lower side is masked in completely the same way. Then, by etching the sheet to a depth of about 60 microns, the strip-shaped projections 28 are formed in the grid pattern, which is surrounded by a strip-shaped peripheral projection.

The resulting structure is shown in Fig. 5b, where the peripheral projection is not shown. According to conventional methods, a pair of p-type semiconductor layers 14 and 16 are formed on the two main sides of the disk 12 over the entire surface to form opposing continuous pn junctions therebetween, whereupon the n-type semiconductor layer 18 is formed on the portions of the the layer 16, which is limited by the grid-forming projections, also designated 28, to form discrete pn junctions therebetween, one for each layer 18.

The resulting structure is shown in Fig. 5c. Then, on the upper main side, the disc 12 is provided with a plurality of plateau-forming grooves 26 in the form of closed loops immediately adjacent the projections 28 to surround the semiconductor layers 18, one of which for each layer 18 These grooves 26 extend through the layer 16. until they reach the layer 12. At the same time, the lower main side is provided with a number of grooves 20 which are immediately below the projections 28 and reach the layer 12. Each of the grooves 20 has substantially the same width as the total width of the projection 28 and two adjacent over the same grooves 26. The grooves 20 and 26 separate the pn junctions below each of the layers 18 from the transitions below adjacent layers 18 to form of a number of thyristor tablets 10 in the disc 12. After coating a passivating layer of glass in each of the grooves 20 and 26, the structure shown in Fig. 5d is obtained or the device shown in Fig. 3.

According to Fig. 5e, a metallized layer 24a, a central apertured metallized layer 24b and a small metallized layer 24c are provided in barrier-free contact with the p-type layer 14. The n-type central apertured layer 18 and the p-type exposed surface 16 are prepared in a conventional manner. Thereafter, the semiconductor disk shown in FIG. 5e is divided along dashed lines 30 extending into the center of the projections 28 of FIG. 4 or FIG. 5e, so that the disk is divided into a number of tablets 10.

The invention is, of course, not limited to the described and shown embodiments. Thus, the invention can equally well be applied to transistors, diodes, etc. except at the described thyristors. The passivating layer may also consist of any other suitable material other than glass, e.g. silica (SiO-) in the form of film.

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