Method of Manufacturing a Field-Effect Transistor

High-speed FETs are known to require a short channel running time or transit frequency. Short channel running times can only be obtained by using extremely short channel lengths in the sub-.mu.m-range (<0.5 .mu.m) and very high electron drift speeds (>2.10.sup.7 cm/sec). It is therefore most important for high-speed field-effect transistors to have short channel lengths.

It is an object of the invention to provide a method for the manufacture of a field-effect transistor which enables easy manufacture of field-effect transistors having a short channel.

According to the invention, there is produced in a method for manufacturing a field-effect transistor comprising a substrate and an epitaxial layer located thereon, a V-shaped trench extending from the surface of the epitaxial layer through the epitaxial layer into the substrate, with a zone being created by implantation in an area located in front of the V-shaped trench.

The invention will now be described in greater detail, by way of example, with reference to the drawings, in which:

FIG. 1 shows a base member including a substrate with an epitaxial layer;

FIG. 2 shows the V-shaped trench in the epitaxial layer;

FIG. 3 shows the passivation layer on the epitaxial layer and the implanted channel;

FIG. 4 shows a selective oxide layer on the surface after removal of the passivation layer;

FIG. 5 shows a metal layer on the oxide layer;

FIG. 6 shows the electrode which is made from the metal layer; and

FIG. 7 shows the finished field-effect transistor.

In the manufacture of a field-effect transistor according to the invention, one proceeds, for example, from a semi-insulating substrate 1 as shown in FIG. 1, and deposits an epitaxial layer 2 of the n-conductance-type on this substrate. A V-shaped trench 3 is etched into the epitaxial layer 2 as shown in FIG. 2. A photoresist layer serves as etching mask. The trench depth can be controlled by measurement of the current in the epitaxial layer, 2. That is, when the channel current measured reaches zero, this is an indication that the epitaxial layer 2 has just been etched through and the bottom of trench 3 has reached the substrate 1. In the field-effect transistor according to the invention, however, the V-shaped trench 3 should not only extend as far as the substrate 1, but to a certain extent also into the substrate 1. By means of slow-acting etching solutions (e.g. citric acid+H.sub.2 O.sub.2 +H.sub.2 O) the depth a (depth of penetration of trench 3 into the substrate 1) can be easily set at, e.g., 0.1 .mu.m. Fluctuations in this penetration depth do not, however, extend into the thickness of the active channel area which has still to be created. The angle of inclination formed by the walls of the etching trench 3 may be set by suitable crystal orientation and choice of etching solution. In the embodiment, the angle formed by the two trench walls is, for example, 50.degree. to 55.degree..

After the etching trench 3 has been made, the implanted channel is formed. First of all, however, the photoresist layer is removed from the surface of the epitaxial layer 2, the surface is then cleaned and a passivation layer 4 is applied to the cleaned surface of the epitaxial layer 2, as shown in FIG. 3. The passivation layer may, for example, consist of Si.sub.3 N.sub.4. Following this, a zone 5 of the same conductivity type as the epitaxial layer 2 is implanted into the entire surface by the implantation process. After reinforcement of the passivation (or renewed passivation), the implanted zone 5 is annealed at high temperatures (around 800.degree. C.) and the implanted zone is activated.

The implantation and activation result in the formation in the substrate 1 of an implanted channel 6 whose depth (and pinch-off-voltage) is determined exclusively by the implanted profile. The channel length is determined by the channel depth a and will be approximately 0.3 .mu.m.+-.0.15 .mu.m at a=0.1 .mu.m.+-.0.05 .mu.m.

Following the creation of the implanted channel 6, the passivation layer 4 is removed and the entire surface is oxidized in an electrolyte in the dark. As is apparent from FIG. 4, oxide 7 will grow in certain areas only since the anodic oxidation process requires open holes. In the instant invention, the voltage applied must be greater than the break-down voltage of the diode formed by the epitaxial layer 2 and the electrolyte. If this is the case, an oxide will readily start to grow on the N.sup.+ area 2 (epitaxial layer) when a correspondingly low voltage is applied, whereas the surface area of the active channel area 6 remains free of oxide. The thickness of the selective oxide layer 7 depends on the break-down voltage of the electrolyte-diode which corresponds to that of a Schottky diode. With a channel doping of from 1 to 2.times.10.sup.17 cm.sup.-3, the thickness of the oxide layer is, for example, 200 to 300 .ANG., corresponding to a break-down voltage of 10 to 15 volts.

Following the formation of the oxide layer 7, a metal layer 8 is vapor-deposited over the entire surface as shown in FIG. 5. The metal layer consists of the material of the gate electrode. Suitable materials for the layer 8 are, for example, TiAu, MoAu or TaMo. After the metal layer 8 is vapor-deposited, the section of this metal layer which is located on the oxide layer 7 is removed together with the oxide layer 7. This can be done, for example, by means of stripping. Following the stripping, which may, for instance, be carried out in HCl, there remains of the metal layer 7 only the gate electrode 9, as shown in FIG. 6. FIG. 7 shows the finished field-effect transistor which comprises in addition to the gate electrode, the source electrode 10 and the drain electrode 11.