SEMICONDUCTOR DEVICE AND METHOD FOR ITS MANUFACTURE

Abstract: A semiconductor device has a substrate and an active layer (5) of silicon arranged on the substrate, in which active layer the active parts of the device are formed. Between the substrate (1) and the active layer (5), a layer (3) of diamond is arranged and between the diamond layer and the active layer, a layer (4) of silicon dioxide is arranged.

Description: Semiconductor device and method for its manufacture TECHNICAL FIELD The present invention relates to a semiconductor device comprising a substrate and an active layer of silicon arranged on the substrate, the active parts of the semiconductor device being produced in this layer. BACKGROUND ART It is known to manufacture individual semiconductor devices and integrated circuits in so-called SOI technology, which provides great flexibility when forming and using the device or the circuit by the devices or circuit parts being insulated from the substrate and from each other. According to this technique, the circuit or device is produced in a layer of a semiconducting material, usually silicon, which is arranged on an electrically insulating substrate. This substrate usually consists of a body of semiconducting material, for example silicon, on which an electrically insulating layer, usually silicon dioxide, is arranged. To obtain a sufficient electrical insulation between the substrate and the device/circuit, the silicon dioxide layer has to be made relatively thick, typically with a thickness of at least one or a few μm. However, the silicon dioxide has poor thermal properties, especially low thermal conductivity. This results in a limited power handling ' capacity of this kind of devices. Further, they are sensitive to radioactive/ionizing radiation. If a device of this kind is subjected to such radiation, hole-electron pairs are formed in the oxide, the holes remaining in the oxide and causing charging of the oxide layer as well as surface states at the junction betweeen the active layer and the oxide layer. Both of these phenomena have a negative influence on the function of the device or the circuit produced in the active layer arranged on the oxide layer. It is further known that diamond combines good electrical insulating capacity with good thermal properties in the form of high thermal conductivity and high heat capacity. It has therefore been proposed to use diamond as substrate or to use diamond layers for obtaining electrical insulation between a substrate and devices or circuits arranged on this substrate. The direct junction between the diamond material and the active silicon layer in such circuits may, however, give rise to incompletely controlled surface states, may have an adverse effect on the function of the devices or circuits produced in the active layer. SUMMARY OF THE INVENTION The invention aims to provide a semiconductor device of the kind mentioned in the introduction, which exhibits high power handling capacity and good radiation hardness and in which the adverse effect of incompletely controlled surface states at the interface between the active layer and the insulating layer is avoided. Further, the invention aims to provide a method for the manuf cture of such a semiconductor device. What characterizes a semiconductor device and a method according to the invention will be clear from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention will be described in greater detail with reference to the accompanying Figures 1 and 2. Figure 1 shows an example of a semiconductor device according to the invention. Figure 2 illustrates an example of the method according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a semiconductor device according to an embodiment of the invention. The device has a substrate 1 in the form of a monocrystalline silicon wafer. The substrate supports an active silicon layer 5, in which the active parts of the device are produced in a manner known per se. The active layer may, for example, have a thickness of 0.6 μm. Possibly, the substrate may support several separate active layers located beside each other. Between the substrate and the active layer (or the active layers) a polycrystalline diamond layer 3 is arranged, which provides the necessary electrical insulation and separation between the substrate and the active layer. Between the diamond layer 3 and the active layer 5, a thin layer 4 of silicon dioxide is arranged. Between the diamond layer 3 and the substrate, a thin silicon layer 2 is arranged. This layer serves a certain purpose in the manufacturing method which will be described below but may be omitted, for example if a different manufacturing method is used. To obtain high radiation hardness, the silicon dioxide layer 4 is made as thin as possible, which means that only a small number of charges will be generated in the layer upon irradiation. The thickness of the layer should not exceed 0.05 μ for the desired high radiation hardness to be obtained, and according to a preferred embodiment of the invention, the thickness of the layer is at most 0.02 μm, which provides exceedingly good radiation hardness. The radiation hardness may be further increased by choosing such a process technique for the manufacture of the silicon dioxide layer that the tendency of this layer to capture charges generated upon irradiation will be small. Such a suitable process technique is thermal oxidation in moist oxygen gas and subsequent heat treatment in an inert atmosphere, for example 2, at temperatures below 900°C. The thickness of the polycrystalline diamond layer 3 is optimized in view of the desired thermal impedance and in view of the influence on the active layer from the underlying substrate. To obtain optimum power-distributing thermal impedance between the active layer and the substrate, the thickness of the layer 3 should be chosen greater than the thickness of the active layer, but sufficiently low to limit the growth time and mechanical stresses. To prevent breakdown in the active layer because of too high electrical field strength, the thickness of the diamond layer must not be lower than a first minimum value, which, among other things, is dependent on the intended operating voltage between the substrate and the active layer. To prevent disturbing so-called MOS effects (field effect) in the active layer from the underlying substrate, the thickness of the diamond layer must also not be lower than a second minimum value, which is dependent, among other things, on the intended operating voltage and on the maximally allowed induced surface charge in the active layer. Finally, the thickness of the diamond layer should also not be lower than a third minimum value, which depends on the maximum permissible capacitive coupling to the substrate for the device in question. Which of the three above-mentioned minimum values is greatest and hence provides the smallest allowed thickness of the diamond layer depends on the intended operating voltage and on the type of circuits/devices which are produced in the active layer. In an application of the invention in which the circuits produced in the active layer consisted of fast CMOS circuits for low operating voltage, it has proved to be suitable to give the diamond layer 3 a thickness of about 1 μm. In another application of the invention, in which the device produced in the active layer consisted of a switching transistor for an operating voltage of about 1000 V, it proved to be suitable to give the diamond layer a thickness of about 10 μm. In the first-mentioned example, it proved to be the demands on avoidance of MOS effects and capacitance that were dimensioning, whereas in the second example it was the demand for avoidance of breakdown in the active layer that was dimensioning. In the embodiment described above, the substrate consists of silicon. This has proved advantageous since then the substrate has the same thermal coefficient of expansion ιas the active layer, which provides the least possible mechanical stresses on the active layer upon temperature variations. Further, silicon has good thermal conductivity, which is important for an efficient removal of heat from the active layer and hence for being able to allow a high power load on the devices or circuits produced in this layer. Alternatively, however, the substrate may consist of some other material, for example sapphire. In the embodiment described above, the diamond layer is polycrystalline but it may, alternatively, be monocrystal- line. In a semiconductor device according to the invention, very good power handling capacity of the devices/circuits produced in the active layer is obtained because of the high thermal conductance and heat capacity of the diamond material. The power handling capacity within a large dynamic range is typically twice as great or several times as great as in prior art circuits. Further, because of the low tendency of the diamond material to capture charge carriers upon irradiation, and because of the small thickness of the silicon dioxide layer, very good radiation hardness is obtained. Further, with the aid of the thin silicon dioxide layer arranged between the diamond layer and the active layer, a reduction is obtained of the influence on the active layer exerted by uncontrolled surface states at that interface of the active layer which faces the substrate, and by charge effects in those insulating layers which are arranged between the active layer and the substrate. Figures 2a-2f show a number of successive steps in a preferred method for the production of a semiconductor device according to the invention. The starting-point for the production is the body A of monocrystalline silicon shown in Figure 2a. That surface of the body A which in the finished device is to face the substrate, is provided, as shown in Figure 2b, with a silicon dioxide layer 4, for example by heat treatment in the presence of oxygen. Figure 2c shows how a polycrystalline diamond layer 3 is generated on the silicon dioxide layer 4, for example by CVD (Chemical Vapour Deposition) , or with the aid of a plasma jet method. Figure 2d shows how a thin layer 2 of silicon is applied on the layer 3, for example with the aid of a CVD method. 'To provide a good result of the subsequent bonding, the surface of the layer 2 is ground and/or polished so that the surface is given a high planeness and surface smoothness. Figure 2e shows how the body A is brought into concact with the layer 2 against the surface of the substrate 1. By a subsequent heat treatment, the layer 2 is brought to adhere to the substrate, for example by heat bonding. Thereafter, as shown in Figure 2f, such a quantity of the upper part of the body A in Figure 2f is removed, for example by etching, that of the body remains the active layer 5 with a suitable thickness for producing therein the active devices or circuits. Finally, the desired active devices or circuits are produced in the layer 5 in a manner known per se, after which the necessary connection members are produced and the device is enclosed.

Claims: CLAIMS 1. A semiconductor device comprising a substrate (1) and an active layer (5) of silicon arranged on said substrate, the active parts of the device being produced in said active layer, characterized in that between the substrate and the active layer a layer (3) of diamond is arranged and between the diamond layer and the active layer a layer (4) of silicon dioxide is arranged. 2. A semiconductor device according to claim 1, characterized in that the silicon dioxide layer (4) is considerably thinner than the diamond layer (3) . 3. A semiconductor device according to any of the preceding claims, characterized in- that the thickness of the silicon dioxide layer (4) is at most 0.05 μm. 4. A semiconductor device according to claim 3, characterized in that the thickness of the silicon dioxide layer (4) is at most 0.02 μm. 5. A semiconductor device according to any of the preceding claims, characterized in that the thickness of the diamond layer (3) is at least 1.0 μm. 6. A semiconductor device according to any of the preceding claims, characterized in that the diamond layer (3) is thicker than the active layer (5) . 7. A semiconductor device according to any of the preceding claims, characterized in that the substrate (1) consists of silicon. 8. A method for manuf cturing a semiconductor device with a substrate (1) and an active silicon layer (5) arranged thereon, the active parts of the device being produced in said silicon layer, characterized by the following method steps : on a surface of a silicon body (A) a silicon dioxide layer (4) is produced; on the silicon dioxide layer a layer (3) of diamond is produced; the silicon body (A) is bonded to the substrate (1) with the diamond layer (3) against the substrate; the silicon body (A) is removed with the exception of that part of the silicon body which is located nearest the silicon dioxide layer (4) , which part constitutes the active layer (5) ; the active parts of the device are produced in the active layer. 9. A method according to claim 8, characterized in that after the production of the diamond layer (3) and prior to bonding the silicon body (A) to the substrate (1) , a silicon layer (2) is generated on the surface of the diamond layer, whereupon a high surface smoothness is imparted to the silicon layer.

Inventors: SVEDBERG PER, SVEDBERG, PER
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1991
Main classification: H01L21
IPC codes: H01L21, H01L27