WO2001022474A2 - Method for producing a semiconductor device consisting of silicon-carbide and comprising a schottky contact - Google Patents

Abstract

The invention relates to the production of a SiC semiconductor device with a Schottky contact. A SiO2-protective layer (150) is produced on a surface (21) of an α-SiC semiconductor area (11) by means of dry thermal oxidation. After at least one additional procedure step, the SiO2-protective layer (150) is removed again in the area of a contact window (160). A Schottky contact layer (110) is subsequently mounted to the surface (21) in the area of the contact window (160).

Description

description

Method for producing a silicon carbide semiconductor device with a Schottky contact on a surface pretreated with thermal oxidation, and use of the method

The invention relates to a method for producing a semiconductor device made of silicon carbide with a Schottky contact, in which a semiconductor region made of α-silicon carbide (SiC) is provided with a surface, an oxide layer is produced on the surface, at least one further method step for producing the semiconductor device is carried out, a contact window to the surface is produced by at least partial removal of the oxide layer, a Schottky contact layer is applied to the surface in the region of the contact window. Such a method is known from US 5, 895, 260.

No. 5,895,260 describes a method for producing an SiC Schottky diode. A first surface of the SiC region is first covered with a dielectric film, e.g. covered with an oxide layer not specified. Then an ohmic back contact in the form of a nickel (Ni) layer is applied to a second surface of the SiC region facing away from the first and formed at a forming temperature of 950 ° C. After implantation of a material inactive in SiC in the edge region of the first surface, a contact window to the first surface is exposed in the dielectric film. Before applying one

Schottky metallization in the area of the contact window can also result in implant healing at a healing temperature between 350 ° C and 400 ° C. Ti / Al or Ni / Al not specified in more detail is used as the metal for the Schottky contact. US 5, 270, 252 and US 5, 471, 072 each disclose a Schottky diode realized in β-SiC. Platinum (Pt) is used as the base metal for the Schottky contact. An ohmic contact made of a titanium (Ti) / gold (Au) layer structure is formed before the Schottky contact is applied at about 750 ° C., the area for the Schottky contact being covered by a passivating oxide layer. This oxide layer is produced in an oxygen and water-containing atmosphere at an oxidation temperature of 1150 ° C, ie by a moist thermal oxidation. After the ohmic contact formation, a contact window for the Schottky contact is created by removing the oxide layer in regions using a buffered HF solution.

DE 196 12 692 Cl describes the production of an oxide layer on silicon carbide by means of dry and moist thermal oxidation. The oxide layer then serves in particular as gate insulation of a MOS (metal oxide semiconductor) structure. It is expressly pointed out that an oxide layer produced using a moist thermal oxidation process has significantly better interface properties than an oxide layer produced using a dry thermal oxidation process. For this reason, an oxide produced by means of a moist thermal oxidation is preferably also used at the interface with the SiC semiconductor region.

EP 0 380 340 A2 discloses a Schottky diode realized in α-SiC and its production. Platinum (Pt) serves as the base metal for the Schottky contact. An ohmic backside contact made of nickel (Ni) is formed at about 1000 ° C. before the Schottky contact is applied, the area for the Schottky contact not being particularly protected.

From the technical paper “Electrical Contacts to Beta Silicon

Carbide Thin Films "by JA Edmond et al., J. Electrochem. Soc, Vol. 135, No. 2, 1988, pp. 359-362 is a process for The method according to the invention for producing a semiconductor device made of silicon carbide with a Schottky contact is a method in which at least one semiconductor region made of α-silicon carbide is provided with a surface, an oxide layer is produced on the surface, and at least another The method step for producing the semiconductor device is carried out, a contact window to the surface is produced by at least partially removing the oxide layer, and a Schottky contact layer is applied to the surface in the region of the contact window, a protective layer being produced as the oxide layer by means of dry thermal oxidation consists of silicon dioxide.

The invention is based on the experimentally confirmed knowledge that a silicon dioxide (Si0 ₂ ) produced by means of dry thermal oxidation leads to a significantly better blocking behavior of the Schottky contact in the semiconductor field made of α-silicon carbide (α-SiC) than in a production via a moist thermal oxidation. Compared to the production of a gate insulation for a MOS structure, the other type of thermal oxidation, ie the dry oxidation, delivers significantly better results, especially when the best possible blocking behavior is desired. The favorable interface properties for a MOS structure achieved by means of moist thermal oxidation consequently do not play a decisive role in a Schottky contact. Rather, it could be shown experimentally that the moist thermal oxidation massively deteriorates the property of the surface to which the Schottky contact is to be applied, while an almost ideal Schottky contact behavior can be achieved with dry thermal oxidation. This can be seen from the low reverse current, which assumes a value of less than 10 ^"7 Acπf ² , and also from the favorable temperature behavior. Manufacture of a Schottky contact on a semiconductor field made of β-SiC known. Before the Schottky contact layer is applied, the surface is cleaned using a dry thermal oxidation process. However, it is expressly pointed out several times that the contacting methods for β-SiC and for α-SiC differ greatly. In particular, the methods are also not transferable.

In the SiC review article by J. B. Casady and R. W. Johnson in "Solid-State Electronics", Vol. 39, No. 10, pp. 1409-1422, 1996, has a section on the Schottky contacting of SiC. Accordingly, nickel (Ni), nickel chromium (NiCr ), Gold (Au), platinum (Pt), titanium (Ti), magnesium (Mg), cobalt (Co), aluminum (Al), hafnium (Hf) and palladium (Pd) have been used for Schottky contact on SiC The height of the Schottky barrier that can be achieved depends not only on the metal used but also on the surface quality of the silicon carbide, the deposition process, the SiC polytype, the conductivity (n- or p-type) and the orientation of the SiC surface - Flat (Si or C side), but no further details, for example about a particularly favorable manufacturing process, are given.

The invention is based on the object of specifying a method of the type described in the introduction with which a Schottky contact can be produced, the quality of which is largely independent of the other method steps for producing the semiconductor device. In addition, it should be possible to produce a Schottky contact with reproducibly high quality.

To achieve the object, a method is specified according to the features of claim 1. Equally good Schottky contact behavior cannot be achieved with any other protective layer based on current knowledge. By using the protective layer made of SiO produced by dry oxidation, reproducible good surface properties can also be set before the Schottky contact is applied.

Since the surface is quasi sealed by the Si0 ₂ protective layer, any other process steps that are carried out for the production of the semiconductor device cannot exert a negative influence on the surface quality and thus also not on the quality of the Schottky contact to be produced. With the use of the silicon dioxide produced by means of dry thermal oxidation, it is now possible for the first time to preserve the surface properties and in some cases even improve them. By contrast, other protective layers used hitherto have at least partially led to a deterioration in the surface properties which are decisive for the Schottky contact behavior.

Special embodiments of the method result from the dependent claims.

A favorable embodiment is one in which the dry thermal oxidation takes place in a practically pure oxygen atmosphere. Only oxygen (0 ₂ ) is supplied from the outside into the corresponding reaction space in which the oxidation takes place. In contrast, water is not introduced into the reaction space. Rather, it is beneficial to keep the remaining amount of water as low as possible. Therefore, the oxidation atmosphere contains an oxygen component that is at least 10 times larger than the water component. The Si0 ₂ is then produced at an oxidation temperature between 800 ° C and 1500 ° C. The oxygen in the oxidation atmosphere reacts chemically with the SiC on the surface of the semiconductor region to form silicon dioxide and volatile carbon oxide. In this thermal oxidation process the SiC material is used up from the surface and the Si0 ₂ protective layer grows into the SiC semiconductor region.

The quality of the Schottky contact subsequently produced is now higher, the purer the oxidation atmosphere in the production of the Si0 ₂ protective layer. Therefore, in a further preferred embodiment, an oxidation atmosphere is provided, the oxygen content of which exceeds the water content by at least 10 ⁶ times.

A particularly favorable temperature range for the dry thermal oxidation of the Si0 ₂ protective layer is between 1050 ° C and 1250 ° C. In this temperature range, a technically reasonable oxide thickness between 5 nm and 100 nm can be achieved in a reasonable process time of around 10 minutes to a few hours.

In order to maintain the protective effect of the protective layer for as long as possible, it is provided in a further advantageous embodiment that the protective layer in the region of the Schottky contact is only removed immediately before the Schottky contact layer is applied. This ensures that the surface is not exposed to any environmental influences that reduce the quality that can be achieved. The Schottky contact then has a particularly high quality.

An embodiment in which the silicon dioxide of the protective layer in the region of the contact window is removed by wet chemical etching with an HF-based etching solution is also favorable. An etching solution made of buffered hydrofluoric acid (= BHF) is particularly well suited, with an HF content between approximately 10% and approximately 15%. A selective chemical etching of the protective layer is possible with a wet chemical etching process, since the etching process stops automatically when the SiC surface is reached. The original properties of the surface are retained. In contrast, for example dry etching process does not offer this selectivity and also attack the SiC surface.

At least one of the metals nickel (Ni), titanium (Ti) and gold can be used as the contact material for the Schottky contact layer

(Au), platinum (Pt) or aluminum (AI) can be applied. Metallization made of a material with nickel and aluminum as the first and second material components provides particularly good Schottky contact behavior. The volume fraction of aluminum is then preferably at least 20%. In principle, however, any other material which forms a Schottky contact in the SiC semiconductor region can also be used for the Schottky contact layer.

In a further embodiment variant of the method, an ohmic contact layer is produced. In practically all known SiC semiconductor devices, such an ohmic contact layer is required as an electrical connection. Nickel or a nickel-iron (NiFe) alloy are, for example, materials that are well suited for the ohmic contact layer. After the material has been applied, formation is carried out with heating to a formation temperature of at least 500 ° C., in particular of at least 850 ° C. During this temperature treatment, the Si0 ₂ protective layer protects the surface for the Schottky contact. If this surface were exposed unprotected, the surface quality and thus the achievable quality of the Schottky contact would otherwise deteriorate.

Since a semiconductor device usually contains semiconductor regions with different doping, which in particular can also be produced via an ion implantation, implant healing is provided in a further embodiment. This healing is usually carried out at a temperature at which epitaxial layer growth also takes place on SiC, since in addition to activating the implanted dopants during healing the damage caused by the implantation in the SiC crystal lattice is to be reduced. The healing temperature is therefore at least 1200 ° C. The curing is preferably carried out in a temperature range between 1200 ° C. and 1700 ° C. over a period of up to one hour, for example in an argon or hydrogen atmosphere. The cooling after curing then takes place in particular in a hydrogen atmosphere. The Si0 ₂ protective layer also serves as an effective surface protection in the described healing process.

Since both the production of the protective layer and the healing process take place at a comparable temperature, both processes can also take place in parallel in the context of a further advantageous embodiment. This results in a shortened and therefore particularly efficient manufacture of the semiconductor device. The protective effect for the surface of the Schottky contact is unchanged due to the SiO ₂ protective layer that forms.

The semiconductor region to be contacted can consist of α-SiC of different polytypes. In particular, 6H, 4H, or 15R SiC can be used. The 4H poly type is particularly cheap. However, other than the α-SiC polytypes mentioned are also possible. Both the Si and the C side of the α-SiC single crystal used can be used as a contact area for the Schottky contact layer. However, the Si side is particularly favorable. A common misorientation with a misalignment of e.g. is up to 10 ° in these

Orientation information included.

Due to the high quality Schottky contact, in particular due to the low reverse current, the semiconductor device is particularly well suited for use as a fast high-voltage diode. In particular, the method for producing a semiconductor device with an allowable Reverse voltage of at least 3 kV can be used. This is because the method can also achieve a high Schottky barrier of more than 1.5 eV. The fact that the Schottky barrier can also be loaded with a higher field strength than in the case of a conventionally produced Schottky contact has a particular advantage for use in the case of a high reverse voltage.

Preferred exemplary embodiments will now be explained in more detail with reference to the drawing. For clarification, the drawing is not to scale and certain features are shown schematically. In detail show:

Figures la to lf manufacture of a Schottky diode using an Si0 ₂ protective layer and

Figure 2 Comparison of the Schottky contact behavior with unoxidized and moist and dry oxidized surface.

Corresponding parts are provided with the same reference numerals in FIGS. 1 and 2.

The manufacture of a semiconductor device in the form of an SiC Schottky diode is illustrated in FIGS. The starting point is a strongly n-conducting 4H-SiC substrate 10 with a dopant concentration between 10 ¹⁷ cm ^"3 and 10 ²⁰ cm ^{" 3} , here in particular from 10 ¹⁹ cm ^"3. A weakly n-conducting 4H is located on the substrate 10 SiC epitaxial layer 11 with a dopant concentration of less than 10 ¹⁷ cm ^{" 3} , here in particular from 3-10 ¹⁵ cm ^{" 3.} Nitrogen serves as the dopant for the n-type SiC. The low doping of the epitaxial layer 11 ensures a high blocking capacity and the high doping of the substrate 10 and a low forward resistance of the Schottky diode to be produced. With the specified doping, the Schottky diode is designed for a reverse voltage of up to approximately 3 kV. In the next method step, a weakly p-conductive edge termination 300 is created by implanting aluminum ions on a surface 21 of the epitaxial layer 11 (FIG. 1b). In the case of the rotationally symmetrical structure assumed in FIG. 1, the implanted edge termination 300 then surrounds the region of the surface 21 in which the Schottky contact is to be applied. The edge termination prevents an excessive electric field in the area of the Schottky contact during operation of the Schottky diode. Such an edge closure is disclosed in WO 96/03774.

The arrangement in FIG. 1b is then subjected to a combined oxidation and healing process (FIG. 1c). This common process takes place at a temperature of 1200 ° C in a pure oxygen atmosphere. On the one hand, the damage to the crystal lattice of the SiC epitaxial layer 11 caused by the implantation of the edge termination 300 is at least partially eliminated, and the implanted aluminum ions are activated. In addition, a dry thermal oxidation takes place simultaneously on the surface 21, which leads to the formation of a protective layer 150 made of an SiO 2 with a thickness of 30 nm to 100 nm. The protective layer 150 formed in this way then covers the surface 21 in particular in the area in which the Schottky contact is applied in a later method step. This means that no degradation of the surface quality can occur in the process steps carried out before the Schottky contact is applied. That is why a very high quality of Schottky contact is achieved. For the achievable quality of the Schottky contact, it is also favorable that the atmosphere in which the oxidation process for the protective layer 150 takes place has practically no water content. The oxygen content exceeds the water content by 10 ⁶ times.

The oxidation of the protective layer 150 and the healing of the edge seal 300 can also be carried out in separate process steps as well. Thereafter, a rear side contact layer 140 is applied to a side (= rear side) of the substrate 10 facing away from the epitaxial layer 11 (FIG. 1d). An oxide layer, which may have also formed on the back during the oxidation process which is actually only intended to produce the protective layer 150, is removed beforehand. The rear side contact layer 140 is used for ohmic contacting of the Schottky diode. A nickel layer is applied as the contact material. In order to finally set a temperature-stable ohmic behavior, the rear side contact layer 140 is subjected to a formation at a temperature of approximately 900 ° C. The surface 21 is effectively protected by the protective layer 150 during this formation.

Only after the formation process is a contact window 160 in the protective layer 150 reaching to the surface 21 opened (FIG. 1). This is done using a wet chemical etching process with buffered hydrofluoric acid (= BHF), which is particularly suitable due to its selectivity. This

This is because the etching process stops as soon as the SiC of the surface 21 is reached. The properties of the surface 21 are thus not changed by the etching process. A Schottky contact layer is then applied in the region of the contact window 160 immediately after the protective layer 150 has been removed.

As a result, the surface 21 is exposed to unprotected external influences for the shortest possible time. A nickel-aluminum material with a nickel and aluminum content of 50% each is used as the contact material for the Schottky contact layer 110.

In a last method step, the Schottky contact layer 110 and the rear-side contact layer 140 are each additionally covered by a contact reinforcement layer 131 or 132 made of aluminum. At the same time, the rest of the protective layer 150 that is no longer required is also removed. This last process step affects the existing NEN Schottky contact in any way, since the semiconductor device in particular is also no longer brought to an elevated temperature.

The quality of the Schottky diode produced according to the method shown in FIGS. 1 a to 1 f is now illustrated with reference to FIG. 2. For the quality of the Schottky contact behavior, the use of a protective layer 150 produced by means of dry thermal oxidation is particularly crucial. A comparison of the blocking behavior of Schottky contacts which have been produced on an unoxidized, a moist and a dry oxidized surface is therefore shown in FIG. The logarithmically scaled reverse current is plotted against the linearly scaled reverse voltage in the diagram. The blocking characteristic for the unoxidized surface is designated 50, that for the moist oxidized surface 51 and that for the dry oxidized surface 52.

The favorable influence of using an oxidized protective layer compared to an unprotected surface can be clearly seen. The reverse current for a Schottky diode, which has been manufactured using an Si0 ₂ protective layer produced by dry or wet oxidation, is at least for small reverse voltages almost three orders of magnitude below the reverse current of a Schottky diode manufactured with an unprotected surface. But the advantage of a dry versus a moist oxidation is also clearly expressed. While the use of wet oxidation increases the reverse current at higher reverse voltages and even exceeds the value that can be achieved with an unprotected surface, the reverse current remains at the favorable low value when using dry oxidation even with high reverse voltages. The dry thermal oxidation method is therefore particularly well suited for producing a semiconductor device designed for a high reverse voltage.

Claims

claims 1. A method for producing a semiconductor device made of silicon carbide with a Schottky contact, in which at least one semiconductor region (11) made of α-silicon carbide is provided with a surface (21), an oxide layer (150) is produced on the surface (21), at least one further method step for producing the semiconductor device is carried out, a contact window (160) to the surface (21) is produced by at least partially removing the oxide layer (150), and a Schottky contact layer (110) on the surface in the region of the contact window (160) (21) is applied, characterized in that a protective layer (150) is produced as an oxide layer by means of dry thermal oxidation, which layer consists of silicon dioxide. 2. The method according to claim 1, characterized in that the protective layer (150) by oxidation of the surface (21) of the semiconductor region in a gas atmosphere with an oxygen content that is at least 10 times as high as the water content, at an oxidation temperature between 800 ° C and 1500 ° C is produced. 3. The method according to claim 2, characterized in that the protective layer (150) is produced in a gas atmosphere with an oxygen content that is at least 10 ⁶ times as high as the water content. 4. The method according to claim 2 or 3, characterized in that the protective layer (150) is produced at an oxidation temperature between 1050 ° C and 1250 ° C. 5. The method according to any one of the preceding claims, that the protective layer (150) is removed immediately before the Schottky contact layer (110) is applied. 6. The method according to any one of the preceding claims, that the silicon dioxide of the protective layer (150) in the region of the contact window (160) is removed by means of a wet chemical etching process with an HF-based etching solution. 7. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that at least one of the metals nickel, titanium, gold, platinum, aluminum, in particular a material with nickel and aluminum as the first or second material component, is applied as a Schottky contact layer 8. The method according to any one of the preceding claims, d a - d u r c h g e k e n n z e i c h n e t that as a further Method step, an ohmic contact layer (140) is applied and a formation with heating to a forming temperature of at least 500 ° C. is carried out. 9. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that implant healing is carried out as a further process step at a healing temperature of at least 1200 ° C. 10. The method according to any one of claims 1 to 8, so that implant healing is carried out during the thermal oxidation to produce the protective layer (150). 11. Use of the method according to one of the preceding claims for producing a semiconductor device for a reverse voltage of up to at least 3 kV.