Method for generating structured metal contacts on a semiconductor substrate

Abstract

This invention provides a method for generating structured metal contacts on a semiconductor substrate. [Solution] The method involves depositing a continuous layer of multiple metal contact materials across the entire surface of a semiconductor substrate, applying further metal contact material to a given contact region on the continuous layer, and performing a heat treatment by scanning the continuous layer in the contact region with a laser beam to form low-resistance contacts from Schottky contacts. The wavelength of the laser beam, the metal contact material of the uppermost layer of the continuous layer, and the further metal contact material are matched to each other such that the metal contact material of the uppermost layer of the continuous layer has at least 1.3 times the reflectivity of the further metal contact material to the laser beam. This allows for self-aligned heat treatment without the need for further protective masks.

Description

Technical Field

[0001] The present invention relates to a method for generating a structured metal contact on a semiconductor substrate, comprising depositing and structuring a layer of one metal contact material or a continuous layer of a plurality of metal contact materials that at least partially form the contact on the semiconductor substrate, and subsequently subjecting it to a heat treatment to alloy one or more contact materials with the semiconductor substrate to form a low-resistance contact from a Schottky contact between one or more metal contact materials and the semiconductor substrate.

[0002] In order to manufacture semiconductor components with low on-resistance, it is necessary to form contacts with the lowest possible resistance. For example, in vertical components on a 4H-SiC wafer, the structured metal contacts on the front side of the wafer typically consist of a nickel-containing metallization (e.g., NiAl(2.6 wt%) or pure nickel) in the case of n-type contacts and a titanium-aluminum-based metallization in the case of p-type contacts. Similarly, the entire backside contact usually consists of a nickel-containing metallization. Since the deposited metal only forms a Schottky contact on many substrate materials, particularly on 4H-SiC, a heat treatment of the metallization is required.

Background Art

[0003] Standardly, an RTP step (RTP: Rapid Thermal Processing) is used for the heat treatment, in which the wafer is heated, for example, at about 1000 °C for 2 minutes, thereby alloying the metal contacts.

[0004] In recent years, in addition to this technique, laser processing using a UV short-pulse laser, which scans the entire back surface of the wafer, has also been established for the treatment of the entire back surface of the back contact. In this case, extremely high temperatures are generated locally in a short time, which similarly causes the contact to alloy. This method yields low-resistance contacts similar to RTP. However, this laser processing is not suitable for alloying structured front contacts because the layers between the contacts are damaged by the laser irradiation, rendering the component unusable.

[0005] Patent Document 1 describes a method for generating structured metal contacts on a semiconductor substrate, in which, before alloying the metal contacts by laser radiation, a mask that reflects laser radiation is deposited on the semiconductor substrate, and the mask is structured so that it has openings only where the contacts are to be alloyed. Subsequently, the contacts are alloyed with the substrate by laser processing through these openings. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] U.S. Patent Application Publication No. 2023 / 0155000 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The object of the present invention is to provide a method for generating structured metal contacts on a semiconductor substrate, which enables the formation of low-resistance contacts with high geometric accuracy in a short time without requiring an RTP step. [Means for solving the problem]

[0008] The above problem is solved by the method described in claim 1. Advantageous forms of the method are the subject matter of the dependent claims or will become apparent from the following description and embodiments.

[0009] In the proposed method, a layer of one metal contact material or a continuous layer of multiple metal contact materials is deposited over the entire surface of a semiconductor substrate, forming at least partially contacts. Then, additional metal contact material is applied (coated) to this layer or continuous layer in a given contact region that will determine the geometry of the subsequent metal contacts. This is done in the first step by depositing and structuring a photoresist layer on the layer or continuous layer so that the photoresist layer on the given contact region has continuous openings. Then, in the second step, additional metal contact material is deposited over the entire surface, at least partially filling the openings with this material. Subsequently, in the third step, the photoresist layer is removed again, for example using a lift-off method, and additional metal contact material is applied only to the given contact region on the layer or continuous layer. In principle, additional metal contact material can also be applied locally on the layer or continuous layer in a given contact region using other techniques.

[0010] In the proposed method, since the contact material of a layer or continuous layer forms a Schottky contact with the underlying semiconductor substrate, a heat treatment is required to alloy one or more contact materials with the semiconductor substrate in a given contact region, thereby forming a low-resistance (ohmic) contact from the Schottky contact. This is done by scanning the layer or continuous layer with locally applied additional metallic contact material using a laser beam. The method is characterized by matching the metallic contact material of the uppermost layer (if there is only one layer) or continuous layer, the additional metallic contact material, and the laser wavelength of the laser beam used with respect to each other such that the metallic contact material of the uppermost layer or continuous layer has a reflectivity of at least 1.3 times, preferably at least 1.5 times, that of the additional metallic contact material with respect to the laser beam. As a result, the laser beam is reflected much more strongly in the region between a given contact region, and therefore this region is not heated as strongly as in the given contact region. By appropriately selecting the laser power, at least the metal contact material in contact with the substrate, or all of the metal contact material, in a given contact region is alloyed with the semiconductor substrate, but the intermediate region does not reach the temperature required for this. Here, it is preferable that the metal contact material of the uppermost layer of the layer or continuum is selected such that its reflectivity R to the laser beam is 80% or more. Subsequently, the unalloed material of the layer or continuum between the given contact regions is removed again. This removal of the layer or continuum between the given contact regions can be carried out by a known method using a wet chemical process. However, other known removal methods are also possible.

[0011] The proposed method also enables the production of structured, low-resistance metal contacts, particularly front contacts, with high geometric precision in semiconductor manufacturing without the use of RTP. On the other hand, performing purely localized laser processing as intended to alloy the contact material has significant drawbacks, as it requires adjusting the semiconductor substrate in the micrometer range and guiding the laser beam with high precision. Also, the diameter of the laser beam is not universally suitable for all contact sizes due to its size. The proposed method can also be used regardless of the contact geometry, and is not limited to rectangular contacts, but can also be used for circular contacts, for example. Compared to applying an additional protective mask, such as an oxide hard mask, to the intermediate region, the proposed method can also be performed in a shorter process time.

[0012] In the proposed method, it is preferable that the layer or continuous layer is scanned entirely by the laser. In principle, since the geometry of the contact is determined not by the laser but by the additional metal contact material applied, scanning may be performed only in the region where a given contact area exists.

[0013] The term "structured contact" is understood herein to mean geometrically determined contact areas that are spaced apart from one another. The term "to deposit or scan completely" means to cover or scan the entire substrate surface or layer surface without leaving any gaps. The term "contact material" is intended simply to mean that the material is used to create a contact and is therefore a component of that contact.

[0014] For heat treatment, it is preferable to use a laser beam with a wavelength in the UV region, for example, a laser beam with a wavelength of λ = 355 nm. The continuous layer consists of a titanium layer and an aluminum layer, with the aluminum layer preferably forming the uppermost layer of the continuous layer. Further intermediate layers are also permitted. Aluminum has a reflectance R of approximately 92% at a wavelength of 355 nm. In this continuous layer, it is preferable to use titanium or nickel as a further metal contact material, with a reflectance R of approximately 55% (titanium) or approximately 59% (nickel) at a wavelength of 355 nm. Therefore, the reflectance of aluminum to the laser beam is approximately 1.67 times that of titanium, and the reflectance of nickel to the laser beam is approximately 1.56 times that of nickel. The semiconductor substrate can be formed from, for example, 4H-SiC, to which metal contacts are alloyed by silicide formation. A continuous-wave laser or a pulsed laser can be used. In this case, the wavelength is not limited to the UV region as described above.

[0015] This method can be used in any area of ​​semiconductor manufacturing where it is necessary to create low-resistance metal contacts on a semiconductor substrate. Without heat treatment, these contacts would only form Schottky contacts between the metal contact material and the semiconductor substrate. The proposed method is particularly advantageous because it can be used to create structured p-type contacts on 4H-SiC semiconductor substrates.

[0016] The proposed method will be described in more detail below, based on one embodiment and with reference to the drawings. [Brief explanation of the drawing]

[0017] [Figure 1] This is a schematic diagram of different steps in the exemplary generation of p-type contacts on a 4H-SiC semiconductor substrate according to the present invention. [Modes for carrying out the invention]

[0018] In the proposed method, a self-alignment process is used to alloy structured metal contacts on a semiconductor substrate. This process utilizes the reflectivity of the metal layer used for the contacts, depending on the type of laser used. At this time, the laser wavelength and the metal contact material or metal are appropriately matched (tuned).

[0019] In this embodiment, a structured p-type contact is generated on a 4H-SiC wafer. Here, titanium and aluminum, which are metals or contact materials commonly used in this p-type contact, are used as metallization or layers. As already mentioned above, titanium and aluminum have significantly different reflectivity in the UV region at λ=355nm.

[0020] In the process described in this example based on Figure 1, in the first step of Figure 1(A), first, an n-type epitaxial layer 2 (7~9×10) is placed on the n-type 4H-SiC wafer 1. 15 cm -3) is grown. In the second step of Figure 1(B), a p+-injected layer 3 is formed by p+ injection of aluminum (Al) into the epitaxial layer 2. Then, in the next step, a contact metal laminate is deposited with a lower layer 4 made of titanium 4 and an upper layer 5 made of aluminum. This standard titanium-aluminum metallization is deposited across the entire front surface of the wafer, as can be seen in Figure 1(C). Subsequently, a photoresist layer is coated on this continuous layer and structured so that the resulting openings geometrically determine a given contact area. Next, a titanium layer 7 is deposited on this structured photoresist layer 6 (as an additional metal contact material). In this regard, Figure 1(D) shows the structured photoresist layer 6 with the titanium layer 7 added, where the titanium layer 7 completely fills the openings in the photoresist layer. Subsequently, the photoresist is removed using the lift-off method, thereby leaving structured titanium contacts 8 across the entire surface of the titanium-aluminum continuous layers 4 and 5 (Figure 1(E)). Next, the entire surface of the wafer or the surface of the applied continuous layer is processed using a laser, as indicated by the arrows in Figure 1(F). Where the titanium layer 7 is located, laser radiation is incident, and the titanium-aluminum-titanium laminate reacts with the SiC substrate (alloying). Where the aluminum layer 5 between the given contact regions is located, most of the laser radiation is reflected, and as a result, no chemical reaction occurs with the SiC substrate. After laser processing, the aluminum layer 5 and titanium layer 4 between the given contact regions are removed by a wet chemical method. The titanium layer 4 can be removed, for example, using a diluted mixture of HNO3 and HF, and the aluminum layer 5 can be removed, for example, using a diluted mixture of H3PO4, HNO3, and HAc. Here, as can be seen in Figure 1(G), the silicided titanium-aluminum-titanium laminate remains as the generated structured p-type contact 9.

[0021] In the above example, only the metal contact materials that have been standardly used so far to generate p-type contacts on 4H-SiC are used. Therefore, in this example, this method has the advantage that it only needs to change the lamination order of the metallization without introducing additional materials into the process.

Explanation of Signs

[0022] 1 n-type 4H-SiC wafer 2 n-type epitaxial layer 3 p+ implantation layer 4 Titanium layer 5 Aluminum layer 6 Structured photoresist layer 7 Titanium layer 8 Structured titanium contact 9 Structured p-type contact

Claims

1. A method for generating structured metal contacts (9) on semiconductor substrates (1, 2), A layer of one metal contact material or a continuous layer (4, 5) of multiple metal contact materials, which forms at least partially the contact (9), is deposited over the entire surface of the semiconductor substrate (1, 2). In a given contact region that determines the geometric shape of the metal contact (9), further metal contact material (7) is deposited on the layer or continuous layer (4, 5) of the one or more metal contact materials. Next, the layer or the continuous layer (4, 5) is subjected to heat treatment to alloy the one or more contact materials in the given contact region with the semiconductor substrate (1, 2), and a low-resistance contact is formed from the Schottky contact between the one or more metal contact materials and the semiconductor substrate (1, 2). Next, the layer or the continuous layer (4, 5) is removed again between the given contact regions. The heat treatment is performed by scanning the layer or the continuous layer (4, 5) with a laser beam, wherein the metal contact material of the uppermost layer of the layer or the continuous layer (4, 5), the further metal contact material (7), and the laser wavelength of the laser beam are matched to each other such that the metal contact material of the uppermost layer of the layer or the continuous layer (4, 5) has a reflectivity of at least 1.3 times that of the further metal contact material (7) to the laser beam.

2. The method according to claim 1, characterized in that the metal contact material of the uppermost layer of the layer or the continuous layer (4, 5) has a reflectance R to the laser beam of 80% or more.

3. The method according to claim 1 or 2, characterized in that the heat treatment uses a laser beam having a wavelength in the UV region, particularly a wavelength of 355 nm.

4. The method according to any one of claims 1 to 3, characterized in that the continuous layers (4, 5) are formed from a titanium layer (4) and an aluminum layer (5), and the aluminum layer (5) constitutes the uppermost layer of the continuous layers (4, 5).

5. The method according to claim 4, characterized in that titanium or nickel is used as the further metal contact material (7).

6. The method according to any one of claims 1 to 5, characterized in that the semiconductor substrate (1, 2) is a 4H-SiC semiconductor substrate.

7. The method according to any one of claims 1 to 6, for generating structured p-type contacts on a 4H-SiC semiconductor substrate.

8. The method according to any one of claims 1 to 7, characterized in that, after depositing the layer or continuous layer (4, 5) with the one or more metal contact materials, a photoresist layer is applied to the layer or continuous layer (4, 5) to structure it, and then the further metal contact material (7) is applied only to the given contact region on the layer or continuous layer (4, 5), and then the photoresist layer is removed again by a lift-off method.

9. The method according to any one of claims 1 to 8, characterized in that the layer or continuous layer (4, 5) between the given contact regions is removed by a wet chemical method.

Images