GB2028583A - Electrical lead for a semiconductor device - Google Patents

Abstract

A semiconductor device comprising a semiconductor body portion (1) encircled by a supporting wall (3) of electrically insulating material, e.g. glass, which also provides support for an electrical connecting lead (11 or 13) secured thereto, to a region of the body portion. The s.c. body portion typically includes a Schottky barrier diode wherein the electrical conductor 13 has a reduced area portion contacting a gallium-arsenide epitaxial layer 9 to reduce diode capacitance. A method is described for the simultaneous fabrication of a number of diodes in a single wafer of low resistivity gallium- arsenide. Areas of the water, each containing a diode, are then separated from each other. <IMAGE>

Description

SPECIFICATION Semiconductor devices This invention relates to semiconductor devices.

The invention relates particularly to semiconductor devices wherein connection to one or more regions of the device is made by way of a beam lead.

In such devices a conflict arises between the need for the beam leads to be securely fixed and have good mechanical strength and the need for the stray impedances, especially capacitance, attributable to the beam leads to be kept to a minimum. The conflict is particularly acute, of course, in devices intended for operation at high frequencies e.g. microwave frequencies.

It is an object of the present invention to provider a semiconductor device wherein this problem is alleviated.

According to the present invention there is provided a semiconductor device comprising: a semiconductive body portion; a wall of electrically insulating material encircling the body portion; and a lead making electrical connection with a region of the body portion, which lead extends at least partly across said wall and is secured thereto.

Preferably the lead extends across and is secured to a surface of the wall which constitutes a lateral extension of a surface of the body portion The wall of insulating material preferably provides a surface which constitutes a lateral extension of said surface of the body portion all round the body portion.

In a device according to the invention the encircling wall of insulating material not only provides a base to which the or each lead can be satisfactorily secured without introducing excessive stray impedance, but also confers good mechanical strength on the device.

The invention also provides a convenient method of manufacturing a semiconductor device according to the invention including the steps of forming a trough extending into a semiconductor wafer from a surface thereof and filling the trough with electrically insulating material, thereby to isolate a portion of said surface of the wafer from the remainder of that surface; forming a semiconductor device extending into the part of the wafer beneath said isolated surface; and forming a lead for a region of the device, which lead extends at least partly across and is secured to the surface of the insulating material in said trough.

For the manufacture of discrete devices the method will further include the step of separating from the remainder of the wafer the portion of the wafer containing the device, the surrounding insulating material and said lead.

One semiconductor device in accordance with the invention and its method of manufacture will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a perspective view of the device; Figure 2 is a sectional view of the device; Figure 3 is an enlarged plan view of part of the device; and Figure 4 is a sectional view illustrating a stage in the manufacture of the device.

The device is a Schottky barrier diode for use at microwave frequencies.

Referring to Figures 1, 2 and 3 the device comprises a generally rectangular body portion consisting for the most part of low resistivity gallium arsenide 1. The body portion 1 is encircled by a wall of glass 3, the top and bottom surfaces of the wall being essentially coplanar with, and thus forming lateral extensions of, the corresponding surfaces of the body portion 1, which surfaces are approximately square.

Over a small D-shaped area at the center of one side of one of the square surfaces there is a thin epitaxial layer 5 of high resistivity gallium arsenide. Over the remainder of that square surface there is a thin layer 7 of metal, e.g. a gold/tin alloy, which makes ohmic contact with the underlying low resistivity gallium arsenide. On the surface of the epitaxial layer 5 there is a silicon oxide layer 8 provided with a window through which a small circular area 9 of a suitable metal e.g. titanium makes the required Schottky barrier contact with the underlying epitaxial layer 5.

Connection to the metal layer 7 is made by way of a metal beam lead 1 1 which extends across and is secured to the adjacent part of the top planar surface of the wall 3. Connection to the circular metal area 9 is made by way of a second metal beam lead 13 similarly secured to and extending across the surface of the wall 3. The lead 13 is of reduced width at its inner end to reduce the area of overlap of the lead 13 and the layer 5, and hence reduce the capacitance of the diode.

The bottom of the body portion 1 is provided with a protective metal layer 15.

The leads 1 and 1 3 suitably consist of gold and the layer 15 is suitably a composite titanium/gold layer.

Referring to Figure 4, in manufacture a number of diodes are fabricated simultaneously on a single wafer 17 of low resistivity gallium arsenide having a high resistivity epitaxial layer 19 on one main face. At each location on the wafer 17 where a diode is required to be formed, a square trough 21 is first formed using a photolithographic etching technique, the trough 21 extending into the wafer 17 from the side carrying the epitaxial layer 19, and encircling a portion 23 of the wafer which will constitute the body portion 1 of the completed device.

The trough 21 is then filled to the surface of the wafer 17 with a low melting point glass frit of approximately micrometre sized glass particles in a suitable suspension medium, for example photoresist. The glass frit is then vitrified by heating. The metal layer 7 is then deposited in conventional manner, through a window in a photolithographically defined silicon oxide mask, the epitaxial layer 19 being removed before metal deposition in the area exposed through the oxide mask so that the metal layer 7 contacts the low resistivity portion of the wafer.

The Schottky barrier contact metal area 9 is then deposited in similar manner on the surface of the remaining part of the epitaxial layer 19 within the area enclosed by the glass filling 25 in the trough 21.

The beam leads 1 1 and 13 are then formed in conventional manner by photolithographic definition of an evaporated layer and subsequent electroplating to bring the leads up to the desired thickness.

The reverse side of the wafer is then lapped or etched until the bottom Qf the glass filling 25 in the trough 21 is clearly exposed. The metal protective layer 1 5 is then formed by evaporation.

Finally, the various areas of the wafer each containing a diode are separated from one another, and excess gallium arsenide around the outside of the glass wall 3 of each diode is removed by etching, leaving a completed diode as shown in Figure 1.

If necessary, to protect the glass in the trough 21 from etchants used during subsequent processing, the walls of the trough and the exposed surface of the glass in the trough maybe provided with protective layers, e.g. of silicon oxide, to provide a protective coating all around the glass.

It will be appreciated that the invention is applicable not only to Schottky barrier diodes but to virtually any semiconductor device eniploying beam leads, or leads of such a form that they may introduce undesirable stray impedance.

Claims (16)

1. A semiconductor device comprising: a semiconductive body portion; a wall of electrically insulating material encircling the body portion; and a lead making electrical connection with a region of the body portion, which lead extends at least partly across said wall and is secured thereto.

2. A device according to Claim 1 wherein the lead extends across and is secured to a surface of the wall which constitutes a lateral extension of a surface of the body portion.

3. A device according to Claim 2 wherein the wall of insulating material provides a surface which constitutes a lateral extension of said surface of the body portion all round the body portion.

4. A device according to Claim 3 wherein the height of the wall is substantially equal to the thickness of the body portion.

5. A device according to any one of the preceding claims wherein said lead is in the form of an electrically conductive layer.

6. A device according to Claim 5 wherein the part of the lead making electrical connection with a region of the body portion is of smaller width than the part of the lead extending across said wall.

7. A device according to any preceding claim wherein said body portion is of generally rectangular form.

8. A device according to any preceding claim wherein said insulating material is a glass.

9. A device according to any preceding claim wherein the lead extends right across and beyond said wall to form a beam lead for the device.

10. A semiconductor device substantially as hereinbefore described with reference to Figures 1 and 2.

11. A method of manufacturing a semiconductor device according to Claim 1 including the steps of forming a trough extending into a semiconductor wafer from a surface thereof and filling the trough with electrically insulating material, thereby to isolate a portion of said surface of the wafer from the remainder of that surface; forming a semiconductor device extending into the part of the wafer beneath said isolated surface; and forming a lead for a region of the device, which lead extends at least partly across and is secured to the surface of the insulating material in said trough.

12. A method according to Claim 11 wherein said trough is filled by placing particles of said insulating material in the trough and fusing them.

13. A method according to Claim 1 1 or Claim 12 further including the step of separating from the remainder of the wafer the portion of the wafer containing the device, the surrounding insulating material and said lead.

14. A method according to Claim 13 wherein prior to the separating step the thickness of the wafer is reduced from the surface opposite said isolated surface by an amount sufficient to expose the bottom of the insulating material in the trough.

15. A method of manufacturing a semiconductor device according to Claim 1 substantially as hereinbefore described with reference to Figure 3.

16. Asemiconductordevice manufactured bya method according to any one of Claims 11 to 1 5.