JPS6231487B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an electrode forming method in which an ohmic electrode is formed on an n-type semiconductor layer containing Ga. In manufacturing a field effect transistor (SBFET) having a Schottky barrier gate, it is extremely important to form source and drain electrodes with good ohmic reproducibility from the viewpoint of electrical characteristics, yield, etc. Conventionally, as shown in FIG. 1, as a method for forming this type of ohmic electrode, a 1200 Å thick Au-Ge layer consisting of 88% by weight of Au and 12% by weight of Ge is deposited on the surface of an N-type GaAs semiconductor substrate 1. It is generally known to sequentially form layer 2 and Ni layer 3 with a thickness of about 400 Å by vacuum evaporation, and then perform a heat treatment for alloying at 400° C. to 500° C. for several minutes. in this case,
Since GaAs is easily decomposed thermally, it is desirable to alloy it at as low a temperature as possible, and for this reason, the method described above cannot
As the composition ratio of the Au-Ge layer 2, a eutectic composition Au:Ge=88:12 (eutectic point 356° C.) with the lowest melting point is used. In addition, molten Au-Ge is poorly compatible with GaAs, and if the GaAs surface is particularly hydrophobic, it may partially bulge up (ball up).
To prevent this, the Ni layer 3 is overcoated. However, in reality, especially when the surface of GaAs is hydrophilic, the surface of the alloy metal layer formed by alloying tends to become rough, and sufficient consideration must be given to optimizing the alloying conditions. When surface roughness occurs, it increases contact resistance and causes significant deterioration in the characteristics of SBFET in particular. After considering the causes of such surface roughness, the following facts were found. In other words, those involved in the above-mentioned alloying are actually Au, Ge, Ni,
It is a 5-element system of Ga and As, and the eutectic composition in this 5-element system is not necessarily 12% by weight as described above for Ge. Assuming that Ni forms a compound with As, it becomes a ternary system, and it is necessary to experimentally determine the ternary phase diagram of Au, Ge, and Ga, and at least determine the metal structure based on this. . Figure 2 shows its ternary phase diagram, which shows that the ternary eutectic point is Ge
in the range of 1 to 2% by weight. This composition ratio is exactly the same as the ratio of Ge to Au in the binary system of Au and Ge.
This roughly corresponds to the solid solubility limit of . Note that in FIG. 2, at% represents the atomic ratio. Therefore, as in the method described above, Ge is applied to Au.
It is rather natural that as long as the content is 12% by weight, excess Ge will precipitate or form primary crystals, resulting in the above-mentioned surface roughness in the alloy layer. Furthermore, even if alloyed under conditions that do not cause surface roughness, it is expected that the structure will change due to aging as shown in FIG. 2. To explain this surface roughening phenomenon in more detail with reference to Figure 2, as Ga is dissolved in the binary composition of Au:Ge = 88:12 (corresponding to point A in the figure), 100% by weight of Ga
Although the composition itself changes along the line connecting point B and point A, the liquidus line of the ternary system changes as shown by the thick line in the figure. Therefore, if we consider the case of cooling at, for example, point C on the AB line, Ge first precipitates as primary crystals and reaches point D, where β'-(Au-
Heading towards point E while precipitating Ga) and Ge. This E
At the point, β'-(Au-Ga), γ-(Au-Ga) and
The process ends with the precipitation of a ternary eutectic of Ge. On the other hand, if we start from point F on the AB line, we will reach point G while similarly precipitating the primary crystals of Ge, and follow the liquidus line to point H.
AuGa and Ge are precipitated and γ-(Au-
The process ends with the precipitation of a ternary eutectic of Ga), AuGa, and Ge. In this way, since Ga diffuses and dissolves into the Au-Ge layer, conventional methods cannot avoid the primary crystallization and precipitation of Ge, and therefore Ge inevitably appears on the surface. This surface roughness increases the resistance as described above, and since the degree of the roughness varies, there is a lot of variation and the reproducibility becomes extremely poor. Special surface treatment can be applied to prevent this, but this is difficult due to the manufacturing process. The present invention was invented to correct the above-mentioned defects, and in the electrode forming method described at the beginning, elements that form a compound with constituent elements other than Ga of an n-type semiconductor layer containing Ga are mainly used. a lower layer of Au and
An upper layer consisting mainly of Ge and having a ratio of the total amount of Ge to the sum of the total amount of Au and the total amount of Ge is 1.2 to 2.0% by weight is formed on the lower layer, and then 300%
An alloyed ohmic electrode containing Au, Ge and Ga is formed on the semiconductor layer by heat treatment at a temperature of ~500°C (most preferably 320°C). This relates to an electrode forming method. With this configuration, surface roughness can be prevented, ball-up phenomenon can be prevented, and contact resistance can be lowered, so electrical characteristics such as electrical resistance and reproduction can be improved. can improve sex. In the present invention, the above ratio of the total amount of Ge in the upper layer is selected to be 1.2 to 2.0% by weight, and the reason is as follows. That is,
If Ge exceeds 2.0% by weight, the effects of preventing precipitation and surface roughening will be substantially reduced. Further, 1.2% by weight of Ge is the solid solubility limit of Ge in Au at 300°C, but if the Ge content is less than 1.2% by weight, the donor effect of Ge becomes weaker, which is not preferable. In addition, before alloying treatment, Au88% by weight,
The first layer is an Au-Ge layer containing Ge12% by weight, and on top of this,
The second layer of 100% Au by weight is stacked,
These first and second layers constitute the above-mentioned upper layer, and in these first and second layers,
It is desirable that the ratio of the amount of Ge to the sum of the total amount of Au and the amount of Ge in the first layer is 1.2 to 2.0% by weight. In other words, part of the Ge in excess of the solid solubility limit of Ge in Au replaces Ga and acts effectively as a donor, but the other part is prevented from precipitating by the second layer of Au. blocked. Moreover, since the first layer contains a relatively large amount of Ge at 12% by weight, the amount of Ge at the interface with GaAs is large, and therefore it is simply an Au-Ge layer (Ge 2.0% by weight).
This is advantageous because the melting point is lower than when only a single layer is formed. In addition, in this case, below the first Au-Ge layer,
It is preferable that the Ni layer constituting the above-mentioned lower layer is vacuum-deposited in advance to form a total of three layers. That is,
Ni forms Ni-As at the GaAs interface, and as a result, excess Ga diffuses into Au, so the presence of Ni determines the amount of Ga diffused into Au, and Ni-As
It is thought that this reduces the interfacial energy and prevents the above-mentioned ball-up phenomenon. The thickness of this Ni layer is preferably 50 to 200 Å; outside this range, the ball-up phenomenon tends to occur. Next, an embodiment in which the present invention is applied to the formation of a GaAs ohmic electrode will be described with reference to the drawings. First, the structure before the alloying process is explained in FIG.
A Ni layer 13 with a thickness of 100 Å), an Au--Ge layer 12 as a second layer, and an Au layer 14 as a third layer are sequentially formed by vacuum evaporation. In this case, the Au-Ge layer 12 is
It consists of 88% Au and 12% Ge, and this Au
The amount of Ge relative to the sum of the total amount of Au and the amount of Ge in the Ge layer 12 and the Au layer 14 is selected to be 1.2 to 2.0% by weight (for example, 1.5% by weight).
As mentioned above, the Ni layer 13 lowers the interfacial energy and determines the amount of Ga diffusion, and the Au-Ge layer 12
can be easily deposited using easily available Au-Ge (Ge 12% by weight). Then, in the state shown in FIG. 3, heat treatment is performed at an alloying temperature of 320 to 350°C (the furnace setting value may be about 455°C). As a result, Ga on the substrate side is diffused into Au, and an alloyed electrode containing Au, Ge, Ga, and Ni is formed on the semiconductor substrate 11. In this alloying process, the Ni of the Ni layer 13 is transferred to the semiconductor substrate 1.
1 forms a compound with As, and excess Ga diffuses into Au, but as mentioned above, the amount of Ge is different from the total amount of Au.
Since the amount is limited to 1.2 to 2.0% by weight based on the sum of the amount of Ge, the ternary eutectic of Au, Ge, and Ga can be precipitated without substantially causing primary crystals and precipitation of Ge, and these Alloy. On the other hand, part of the excess Ge
It effectively acts as a donor by replacing Ga, and even if the other parts try to diffuse to the surface, they are blocked by the Au layer 14 and their precipitation is prevented. Also Au
- The relatively large amount of Ge in the Ge layer 12 lowers the melting point and thus reduces the time required for alloying. Referring again to FIG. 2, according to this example, the amount of Ge is adjusted to 1.2 to 1.2 as described above before alloying treatment.
The concentration is 2.0% by weight, which corresponds to point I where the straight line connecting points B and E in FIG. 2 intersects the binary state system of Au and Ge. Therefore, when Ga enters Au from the substrate side through alloying treatment, the composition ratio changes along the BI line, but it is understood that the eutectic point E is reached without Ge precipitation as in the conventional case. will be done. The ohmic electrode formed as described above is alloyed in an extremely good condition, and
It has a smooth surface with no Ge precipitation (surface roughness) or ball-up phenomenon, almost regardless of the pretreatment or alloying conditions of the GaAs surface. for example,
In the case of the conventional example shown in FIG. 1 mentioned above, surface roughness always occurred, and ball-up phenomenon often occurred. However, in the case of the embodiment shown in FIGS. 3 and 4, no surface roughness or ball-up phenomenon occurred, and a perfect mirror surface was obtained. Therefore, it has sufficient electrical properties, especially when the alloying time is 40
When the time is ~200 seconds, the contact resistance becomes considerably smaller than that of the conventional method, indicating that the contact resistance is excellent in ohmic properties. For example, the average values of the contact resistivity measured by the Cox method of the above conventional example and the above example at a furnace temperature setting of 450° C. were as shown in the following table.

[Table] Although some of the conventional examples described above exhibit contact specific resistances close to those of the above embodiments, the variation is large and it is not possible to obtain ohmic electrodes exhibiting desired contact specific resistances with good reproducibility. Therefore, as is clear from the above table, the value of the contact specific resistance in the case of the above example was on average about half of that in the case of the above conventional example. When the ohmic electrode according to this example is actually used as the source and drain electrodes of SBFET as shown in FIG.
There was no variation in the distance between 5 and 5, and a fine pattern was effective. Note that FIG. 4 schematically shows the state before alloying treatment. Although the present invention has been described above with reference to one embodiment, it is understood that the present invention is not limited to this embodiment and can be further modified based on the technical idea as described below. Good morning. For example, the entire Au-Ge layer is formed through the Au layer 14 and the Au-Ge layer 12, the surface side is made an Au-rich layer or a 100% Au layer, and the amount of Ge is gradually increased toward the Ni layer 13 side by vapor deposition. You may. In addition, although the lower layer 13, which is mainly composed of elements that form a compound with constituent elements other than Ga of the Ga-containing n-type semiconductor layer, is made of Ni, it can also be made of Ti, Pt, or Cr to obtain almost the same effect. Obtainable. Note that regardless of whether the lower layer 13 is made of Ni, Ti, Pt, or Cr, if the thickness of the lower layer 13 is approximately 300 Å or less, there is no problem in rapidly cooling it as soon as the alloying temperature is reached. There is an advantage. However, even if the film is thicker than 300 Å, the desired purpose can be achieved by maintaining the alloying temperature for a predetermined time depending on the film thickness. Further, in the above embodiment, the semiconductor substrate was made of GaAs, but almost the same effect could be obtained even if it was made of other compound semiconductors containing Ga such as GaP, GaAsP, GaAlAs, InGaAs, etc. That is,
When composed of CaP or GaAsP, the group metal
Since only Ga was used, the results were exactly the same as the phase diagram shown in FIG. Furthermore, in the case of the structure made of GaAlAs, almost the same results as the phase diagram shown in FIG. 2 were obtained. In addition, when composed of InGaAs, the weight percent of Ge can be relatively small within the range of 1.2 to 2.0 weight percent of the total amount of Ge, and
The alloying temperature could be kept relatively low within the range of 500°C. Furthermore, the electrode according to the present invention can also be applied to other semiconductor devices, such as diodes. As mentioned above, in the present invention, the ratio of the total amount of Ge to the sum of the total amount of Au and the total amount of Ge in the upper layer mainly composed of Au and Ge is set to 1.2 to 2.0% by weight. can be prevented. Further, a lower layer mainly consisting of an element that forms a compound with a constituent element other than Ga of the semiconductor layer is interposed between the n-type semiconductor layer containing Ga and the upper layer mainly consisting of Au and Ge. Therefore, the formation of the above-mentioned compound lowers the interfacial energy, thereby making it possible to effectively prevent the ball-up phenomenon, and also reducing the contact resistance by forming the above-mentioned compound. Therefore, according to the present invention, it is possible to suppress the occurrence of surface roughness and ball-up phenomenon that were conventionally unavoidable, and the increase in contact resistance, thereby improving electrical characteristics such as electrical resistance, and improving the reproducibility of electrode formation. can be done.

[Brief explanation of the drawing]

FIG. 1 shows a conventional example, and is a sectional view before alloying treatment. Figure 2 is a phase diagram of the ternary system of Au, Ge, and Ga. 3 and 4 show an embodiment of the present invention, in which FIG. 3 is a sectional view before alloying treatment, and FIG. 4 is a shot key barrier.
FIG. 3 is a cross-sectional view of the FET before alloying treatment. In addition, in the symbols used in the drawings, 12...Au-
They are a Ge layer, 13...Ni layer, and 14...Au layer.

Claims (1)

[Scope of Claims] 1. An ohmic electrode forming method for forming an ohmic electrode on an n-type semiconductor layer containing Ga, the electrode comprising mainly an element that forms a compound with a constituent element other than the Ga of the semiconductor layer. A lower layer is formed on the semiconductor layer, and then consists mainly of Au and Ge, and the ratio of the total amount of Ge to the sum of the total amount of Au and the total amount of Ge is
The alloyed ohmic electrode containing Au, Ge and Ga is formed by forming an upper layer of 1.2-2.0% by weight on the lower layer and then heat-treating at a temperature of 300-500°C. A method for forming an electrode, characterized in that the electrode is formed on a semiconductor layer.