Method for producing structured metallic contacts on a semiconductor substrate

Abstract

In one embodiment of a process for producing structured metallic contacts on a semiconductor substrate, a layer sequence consisting of several metallic contact materials is deposited over the entire surface of the semiconductor substrate, and a further metallic contact material is applied to the layer sequence in predefined contact areas. Subsequently, the layer sequence is subjected to thermal treatment in the contact areas to form a low-resistance contact from a Schottky contact. This thermal treatment is performed by scanning the layer sequence with a laser beam.In this process, the wavelength of the laser beam, the metallic contact material of the top layer of the layer sequence, and the other metallic contact materials are aligned such that the metallic contact material of the top layer of the layer sequence has a reflectivity for the laser beam that is at least 1.3 times higher than that of the other metallic contact materials. This results in a self-adjusting thermal treatment without the need for an additional protective mask.

Description

Technical application area

[0001] The present invention relates to a method for producing structured metallic contacts on a semiconductor substrate, in which a layer of one or a sequence of layers of several metallic contact materials, from which the contacts are at least partially formed, is deposited on the semiconductor substrate, structured and subsequently subjected to a thermal treatment in which the contact material(s) are alloyed into the semiconductor substrate to form a low-resistance contact from a Schottky contact between the metallic contact material(s) and the semiconductor substrate.

[0002] To manufacture semiconductor devices with low on-resistance, the formation of contacts with the lowest possible resistance is required. For vertical devices on, for example, 4H-SiC wafers, the structured metallic contacts on the wafer's front side typically consist of nickel-containing metallizations (e.g., NiAl (2.6 wt%) or pure nickel) for n-type contacts and titanium-aluminum-based metallizations for p-type contacts. The full-surface back-side contacts also generally consist of nickel-containing metallizations. Since the deposited metal forms only Schottky contacts on many substrate materials, especially 4H-SiC, thermal treatment of the metallization is necessary. State of the art

[0003] By default, an RTP step (RTP: Rapid Thermal Processing) is used for thermal treatment, in which the wafer is heated to approximately 1000 °C for e.g. 2 minutes, causing the metal contacts to alloy.

[0004] In addition to this technique, laser processing using short-pulse UV lasers has become established in recent years for processing full-surface backside contacts. In this process, the entire backside of the wafer is scanned by the laser. This generates very high local temperatures for a short time, which also causes the contact to be embedded. This method produces contacts with the same low resistance as RTP. However, this laser processing is not suitable for embedding structured frontside contacts, as the layers between the contacts are damaged by the laser irradiation, rendering the components unusable.

[0005] US Patent 2023 / 0155000A1 discloses a method for producing structured metallic contacts on a semiconductor substrate, in which, prior to the alloying of the metallic contacts, a laser-reflecting mask is deposited on the semiconductor substrate using laser radiation and structured such that it has openings only at the contacts to be alloyed. The contacts are then alloyed into the substrate through these openings by laser processing.

[0006] The subsequently published EP 4 485 504 A1 describes a method for producing structured metallic contacts on a semiconductor substrate, in which a layer or a sequence of layers of one or more metallic contact materials, from which the contacts are at least partially formed, is deposited over the entire surface of the semiconductor substrate, and a further metallic contact material is deposited in predetermined contact areas, which define the geometry of the metallic contacts, on the layer or sequence of layers made of the metallic contact material(s). The layer or sequence of layers is then subjected to a thermal treatment by laser scanning to form low-resistance contacts.

[0007] From US patent 2008 / 0258183A1, a method for manufacturing a semiconductor device is known in which one or more metallization layers are deposited onto a semiconductor substrate and locally heated in one or more areas by means of laser radiation to form a bond. The remaining areas are then removed by etching.

[0008] The object of the present invention is to provide a method for producing structured metallic contacts on a semiconductor substrate which does not require an RTP step and enables the formation of low-resistance contacts with high geometric accuracy in a short time. Description of the invention

[0009] The problem is solved by the method according to claim 1. Advantageous embodiments of the method are the subject of the dependent claims or can be found in the following description and the exemplary embodiment.

[0010] In the proposed method, a layer of a metallic contact material, or a sequence of layers of several metallic contact materials from which the contacts are at least partially formed, is deposited over the entire surface of the semiconductor substrate. A further metallic contact material is then applied to this layer or sequence in predetermined contact areas, which define the geometry of the subsequent metallic contacts. This is preferably achieved by first depositing and structuring a photoresist layer onto the layer or sequence, such that the photoresist layer has continuous openings in the predetermined contact areas. The further metallic contact material is then subsequently deposited over the entire surface in a second step, such that the openings are at least partially filled by this material.Subsequently, in a third step, the photoresist layer is removed, for example using a lift-off process, so that the additional metallic contact material is only applied to the layer or layer sequence in the specified contact areas. In principle, however, the additional metallic contact material can also be applied locally to the layer or layer sequence in the specified contact areas using other techniques.

[0011] Since the contact materials of the layer or layer sequence form a Schottky contact with the underlying semiconductor substrate in the proposed method, a thermal treatment is required in which the contact material(s) alloy into the semiconductor substrate in the specified contact areas to transform the Schottky contact into a low-resistance (ohmic) contact. In the proposed method, this is achieved by scanning the layer or layer sequence with the locally applied additional metallic contact material using a laser beam. The present method is characterized by the fact that the metallic contact material of the layer (in the case of only one layer) or of the topmost layer of the layer sequence, the additional metallic contact material, and the laser wavelength of the laser beam are precisely matched to ensure that the metallic contact material of the layer or layer sequence is precisely defined.The topmost layer of the layer sequence has a reflectivity for the laser beam that is at least 1.3 times higher, preferably at least 1.5 times higher, than that of the other metallic contact material. This results in significantly stronger reflection of the laser beam in the area between the specified contact regions, thus preventing these regions from heating up as much as the contact regions themselves. By appropriately selecting the laser power, at least the metallic contact material(s) in contact with the substrate, or even all of them, are alloyed into the semiconductor substrate in the specified contact regions, while the necessary temperature is not reached in the intermediate regions. The metallic contact material of the layer, or of the topmost layer of the layer sequence, is preferably selected to have a reflectivity R of ≥ 80% for the laser beam.The unalloyed material of the layer or layer sequence between the specified contact areas is subsequently removed. This removal of the layer or layer sequence between the specified contact areas can be carried out using known wet chemical techniques. However, other known ablation techniques are also possible.

[0012] The proposed method enables the production of structured, low-resistance metallic contacts, particularly front-end contacts in semiconductor manufacturing, with high geometric accuracy without real-time processing (RTP). In contrast, targeted, purely local laser processing for alloying the contact materials would have significant disadvantages, as the adjustment of the semiconductor substrate would need to be in the micrometer range, and the laser beam would have to be guided with high precision. Furthermore, the laser beam diameter, due to its size, is not universally suitable for all contact sizes. The proposed method is also applicable regardless of the contact geometry and can be used not only for rectangular but also, for example, for round contacts. Compared to applying an additional protective mask in the intermediate areas, such as a hard oxide mask, the proposed method can be implemented in a shorter process time.

[0013] In the proposed method, the layer or layer sequence is preferably scanned across its entire surface with the laser. In principle, scanning could also be performed only in areas containing the predetermined contact zones, since the geometry of the contacts is determined not by the laser, but by the additional metallic contact material applied.

[0014] The term "structured contacts" refers to geometrically defined contact areas that are in contact with each other. The term "full-surface deposition" or "screening" means covering or screening the entire substrate or layer surface without any remaining gaps. The term "contact material" simply indicates that it is a material used to create the contact and is therefore a component of that contact.

[0015] For the thermal treatment, a laser beam with a wavelength in the UV range is preferably used, for example, at a wavelength of λ = 355 nm. The layer sequence preferably consists of a layer of titanium and a layer of aluminum, which forms the uppermost layer of the sequence. Further intermediate layers are also possible. Aluminum has a reflectivity R of approximately 92% at a wavelength of 355 nm. Titanium or nickel is preferably used as the additional metallic contact material in this layer sequence, exhibiting a reflectivity R of approximately 55% (titanium) and approximately 59% (nickel), respectively, at a wavelength of 355 nm. Aluminum thus has a reflectivity for the laser beam that is approximately 1.67 times higher than that of titanium and approximately 1.56 times higher than that of nickel.The semiconductor substrate can, for example, be made of 4H-SiC, where the metallic contacts are alloyed by silicideation. Both a continuous-wave (CW) and a pulsed laser can be used. The wavelength is not limited to the UV range specified above.

[0016] The method can be used in all areas of semiconductor manufacturing where low-resistance metallic contacts need to be created on a semiconductor substrate, which, without thermal treatment, would only form Schottky contacts between the metallic contact material and the semiconductor substrate. The proposed method is particularly advantageous for the fabrication of structured p-type contacts on a 4H-SiC semiconductor substrate. Brief description of the drawings

[0017] The proposed method is explained in more detail below using an exemplary embodiment in conjunction with the drawings. These show Fig. Figures 1A-1G schematically represent different steps in an exemplary fabrication of p-type contacts on a 4H-SiC semiconductor substrate according to the invention. Ways to implement the invention

[0018] The proposed method utilizes a self-tuned process to alloy structured metallic contacts into a semiconductor substrate. This process leverages the reflectivity of the metal layers used for the contacts with the specific laser type employed. The laser wavelength and the metallic contact materials or metals are appropriately matched.

[0019] In the present embodiment, structured p-type contacts are fabricated on a 4H-SiC wafer. Titanium and aluminum, the metals or contact materials typically used for these p-type contacts, are employed for the metallizations or layers. As mentioned above, titanium and aluminum exhibit very different reflectivities in the UV range at λ = 355 nm.

[0020] In this example, based on the Fig. The process described in point 1 is carried out in the first step of the Fig. 1A first an epitaxial layer 2 of n-type (7-9 × 10 15 cm -3 ) grown on the n-type 4H-SiC wafer 1. In the second step of the Fig. 1B a p + -Implantation of Al into epitaxial layer 2, thereby increasing the p +-Implantation layer 3 is formed. In the next step, the contact metal stack is deposited, consisting of a lower layer 4 of titanium 4 and an upper layer 5 of aluminum. This standard titanium-aluminum metallization is deposited across the entire front surface of the wafer, as shown in the diagram. Fig. 1C is recognizable. Subsequently, a photoresist layer is applied to this layer sequence and structured in such a way that the resulting openings geometrically define the specified contact areas. A layer 7 of titanium (as another metallic contact material) is then vapor-deposited onto this structured photoresist layer 6. Fig. Figure 1D shows the structured photoresist layer 6 with the applied titanium layer 7, which completely fills the openings in the photoresist layer. The photoresist is then removed using a lift-off process, leaving structured titanium contacts 8 on the full-surface titanium-aluminum layer sequence 4, 5 ( Fig. 1E). Subsequently, the wafer surface or the surface of the applied layer sequences is processed across its entire area with the laser, as described in Fig. 1F is indicated by the arrows. At the points where the titanium layer 7 is on top, the laser radiation couples in and the titanium-aluminum-titanium stack reacts with the SiC substrate (alloying). At the points between the specified contact areas, where the aluminum layer 5 is on top, most of the laser radiation is reflected, thus preventing a chemical reaction with the SiC substrate. After laser processing, the aluminum layer 5 and titanium layer 4 between the specified contact areas are removed by wet chemical means. The titanium layer 4 can be removed, for example, using a dilute mixture of HNO3 and HF, and the aluminum layer 5, for example, using a dilute mixture of H3PO4, HNO3, and HAc. This leaves behind the silicidized titanium-aluminum-titanium stack as the structured p-type contact 9 to be produced, as shown in Fig. 1G zu erkennen ist.

[0021] In the example above, only metallic contact materials are used that are currently standard for manufacturing p-type contacts on 4H-SiC. Therefore, in this example, the process advantageously does not introduce any additional materials into the process, but merely changes the stacking sequence of the metallizations. Reference symbol list 1 n-Typ 4H-SiC Wafer 2 n-Typ epitaktische Schicht 3 p + -Implantationsschicht 4 Titan-Schicht 5 Aluminium-Schicht 6 strukturierte Fotolackschicht 7 Titan-Schicht 8 strukturierte Titan-Kontakte 9 strukturierter p-Typ-Kontakt

Claims

[1] Method for producing structured metallic contacts (9) on a semiconductor substrate (1, 2) wherein - a layer or a sequence of layers (4, 5) of several metallic contact materials, from which the contacts (9) are at least partially formed, is deposited over the entire surface of the semiconductor substrate (1, 2), - another metallic contact material (7) in predetermined contact areas, which define a geometry of the metallic contacts (9), on which layer or layer sequence (4, 5) of the metallic contact material(s) is deposited, - the layer or layer sequence (4, 5) is subsequently subjected to a thermal treatment in which the contact material(s) are alloyed into the semiconductor substrate (1, 2) in the specified contact areas in order to form a low-resistance contact from a Schottky contact between the metallic contact material(s) and the semiconductor substrate (1, 2), and - the layer or layer sequence (4, 5) is subsequently removed again between the specified contact areas, - wherein the thermal treatment is carried out by scanning the layer or layer sequence (4, 5) with a laser beam and the metallic contact material of the layer or a top layer of the layer sequence (4, 5), the further metallic contact material (7) and a laser wavelength of the laser beam are matched such that the metallic contact material of the layer or the top layer of the layer sequence (4, 5) has a reflectivity for the laser beam that is at least 1.3 times higher than that of the further metallic contact material (7). [2] Method according to claim 1, characterized by , the metallic contact material of the layer or the topmost layer of the layer sequence (4, 5) has a reflectivity R of ≥ 80% for the laser beam. [3] Method according to claim 1 or 2, characterized by , that a laser beam with a wavelength in the UV range, in particular with a wavelength of 355 nm, is used for the thermal treatment. [4] Method according to any one of claims 1 to 3, characterized by , that the layer sequence (4, 5) is formed from a layer of titanium (4) and a layer of aluminum (5), wherein the layer of aluminum (5) is the top layer of the layer sequence (4, 5). [5] Method according to claim 4, characterized by , that titanium or nickel is used as another metallic contact material (7). [6] Method according to any one of claims 1 to 5, characterized by , that the semiconductor substrate (1, 2) is a 4H-SiC semiconductor substrate. [7] Method according to any one of claims 1 to 6 for producing structured p-type contacts on a 4H-SiC semiconductor substrate. [8] Method according to any one of claims 1 to 7, characterized by, that after the deposition of the layer or layer sequence (4, 5) from the metallic contact material(s), a photoresist layer is applied and structured onto the layer or layer sequence (4, 5) in order to subsequently apply the further metallic contact material (7) only in the specified contact areas onto the layer or layer sequence (4, 5), wherein the photoresist layer is subsequently removed by a lift-off process. [9] Method according to any one of claims 1 to 8, characterized by , that the removal of the layer or layer sequence (4, 5) between the specified contact areas is carried out using wet chemical methods.

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