CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to European Patent Application No. 20155173.6, filed on Feb. 3, 2020, which application is hereby incorporated herein by reference.

TECHNICAL FIELD

The disclosure relates to a method of making a silicon carbide electronic device.

BACKGROUND

It is well known that silicon carbide (SiC) possesses attractive chemical, physical, and electronic properties for high-power, high-temperature, and high-frequency electronic devices. In particular, a higher electric breakdown field strength, a higher thermal conductivity, a lower intrinsic carrier concentration and a higher saturated drift velocity in the 4H-polytype of silicon carbide (4H—SiC) and in the 6H-polytype of silicon carbide (6H—SiC) compared to the corresponding values in silicon (Si) make 4H—SiC and 6H—SiC favorable materials for power semiconductor devices.

In order to be able to use silicon carbide in electronic devices, or to connect such devices into electrical circuits, appropriate contacts must be made between the silicon carbide and the metals (e.g., printed circuits, wires, or any other appropriate metal elements) used to carry current. Appropriate contacts allow charge to flow easily in both directions between the metal and the silicon carbide, without blocking due to rectification or excess power dissipation due to voltage thresholds. Such contacts are known as ohmic contacts. Ohmic contacts are characterized by having a linear and symmetric current-voltage characteristic for positive and negative applied voltages and a relatively low resistance. Conversely, a Schottky contact has rectifying properties and allows current flow only for one polarity.

Whether a junction between a metal and a semiconductor behaves as an ohmic contact depends on the height of the Schottky barrier (contact potential), which sets the threshold for the excess energy an electron requires to pass from the semiconductor to the metal. To form an excellent ohmic contact, the Schottky barrier must be minimized, which requires matching the work function of the contact metal to the work function of the semiconductor. More specifically, in order to act as an ohmic contact to a particular semiconductor, the work function of the contact metal should ideally be greater than the electron affinity and the bandgap of the semiconductor.

Silicon carbide has a wide band gap in the range from about 2.8 electron volts (eV) to 3.3 eV for the common polytypes and a high electron affinity of around 4 eV. Thus, its work function is large with values of 6 eV and above, depending on the carrier concentration. In contrast, most metals have a work function in the range from 4 eV to 5.5 eV. Contacts formed by depositing a metal layer on silicon carbide therefore generally result in a residual Schottky barrier height with values of 1 eV and above and thus have a non-ohmic behavior. The same problems exist for other wide bandgap semiconductor materials such as zinc selenide (ZnSe), gallium nitride (GaN), diamond, boron nitride (BN), gallium phosphide (GaP), and aluminum nitride (AlN).

The method for forming an ohmic contact to silicon carbide therefore generally comprises a high temperature annealing process (e.g., at 900° C. or above) after the deposition of the metal layer on the silicon carbide. The annealing causes a reaction of the metal with the silicon carbide and the formation of silicides, carbides, and/or ternary phases. These compounds may have an electron affinity which is close to that of silicon carbide and may consequently lower the Schottky barrier height and/or the barrier thickness such that an ohmic behavior of the contact is obtained.

Various materials for forming an ohmic contact to silicon carbide have been investigated, both in terms of structural characterization and of electrical performances. Nickel (Ni) is typically used for forming an ohmic contact to n-type silicon carbide. Ni films deposited on n-type silicon carbide and subjected to an annealing at temperatures in the range from about 900° C. to 1000° C. can provide good ohmic contacts with specific contact resistance values as low as about 1·10−6Ωcm2.

Formation of an ohmic contact to p-type silicon carbide is more difficult than to n-type silicon carbide because of the larger values of the Schottky barrier height. Aluminum (Al), titanium (Ti) or combinations thereof (e.g., an Al/Ti stack or an Al/Ti alloy) have been considered as potential contact metals to p-type silicon carbide and, after high temperature annealing at 1000° C. or above, can yield an ohmic contact having a specific contact resistance in the range between 1·10−4cm2and 1·10−5Ωcm2(J. Crofton et al., “Titanium and Aluminum-Titanium ohmic contacts to p-type SiC”, Solid-State Electronics 41, (1997) 1725-1729; B. J. Johnson et al., “Mechanism of ohmic behavior of Al/Ti contacts to p-type 4H—SiC after annealing”, J. Appl. Phys. 95, (2004) 5616-5620).

However, already at annealing temperatures above 800° C., the top layer of an aluminum based contact (Al contact) is prone to oxidation, even when performing rapid thermal annealing under gas flow, and this has a negative effect on the electrical properties of the contact (F. La Via et al., “Ohmic contacts to SiC”, International Journal of High Speed Electronics and Systems, 15 04, (2005) 781-820). To form Al/Ti based contacts resistant to oxidation, stack sequences comprising palladium (Pd) have been proposed because of its high resistance to oxidation (K. Vassilevski et al., “Phase formation at rapid thermal annealing of Al:Ti:Ni ohmic contacts on 4H—SiC,” Mat. Sci. Eng. B 80, (2001) 370-373).

Moreover, when forming an Al/Ti based contact to a p-type silicon carbide layer, due to the high annealing temperatures required, there is the problem of spiking. Spiking refers to formation of Al filled cavities (so called spikes) in the p-type silicon carbide layer due to diffusion of silicon (Si) into the metallization and diffusion of Al into the p-type silicon carbide layer. When penetrating through an underlying p-n junction these spikes negatively affect the quality of the p-n junction and may lead to an enhanced leakage current or reduced breakdown voltage.

A further disadvantageous effect of having such high annealing temperature is that it may lead to an increased roughness of the contact and this can give serious problems during photolithography alignments.

Moreover, many semiconductor devices such as metal oxide-semiconductor field-effect transistors (MOSFETS) require a layer of a semiconductor oxide (e.g., silicon dioxide). However, the high annealing temperatures associated with contact metal annealing processes place high stress on these oxide layers, which can damage oxide layers, or the semiconductor-oxide interface. Alternatively, forming the ohmic contact prior to creating the oxide layer is not practical because the oxidizing environment utilized to form the oxide layer has adverse effects on the metal contact.

In view of the above problems, alternative approaches were investigated with the aim to reduce the annealing temperature, however with rather limited success. For example, Ge/Ti/Al contacts showed ohmic behavior (with an average specific contact resistance of 1·10−4Ωcm2) already after annealing at 600° C. in ultrahigh vacuum, however they present a high surface roughness, which is undesired for photoresist processing (T. Sakai, “Ternary TiAlGe ohmic contacts for p-type 4H—SiC”, J. Appl. Phys. 95, (2004) 2187-2189). A structure fabricated by a sequential evaporation of a Si/Co structure, sintered by a two-step annealing process in vacuum (at 500° C. and 900° C. respectively), was also shown to achieve a low specific contact resistance in the 4·10−6Ωcm2range. However, a degradation of the electrical properties of the sintered Co/Si/SiC contacts was observed after a high temperature reliability test at 1100° C., most probably as a consequence of a morphological change of the CoSi2 layer (N. Lundberg et al., “Thermally stable low ohmic contacts to p-type 6H—SiC using cobalt silicides,” Solid-State Electron. 39, (1996) 1559-1565).

From U.S. Pat. No. 5,409,859 it is further known an ohmic contact to silicon carbide in which platinum (Pt) is used as the contact metal instead of Al, and because of its characteristics as a noble metal such a contact metal can withstand annealing while retaining ohmic characteristics of the ohmic contact. Therein, the silicon carbide comprises a heavily doped p-type layer at the interface to the platinum at a sufficiently high dopant concentration (>1·1019cm−3) to reduce the width of the depletion region at interface between the platinum and the heavily doped layer to allow carrier (electron) tunneling transport across the interface. The heavily doped layer may be an epitaxial layer or may be formed by high temperature ion implantation as, for example, described in U.S. Pat. No. 5,087,576. According to this method, during ion implantation, the silicon carbide is heated to a temperature of 600° C. or greater to prevent amorphization of, and minimize damage to, the silicon carbide crystal lattice during the ion bombardment. After the implantation step, the doped layer is annealed at a temperature of 1500° C. to encourage the implanted dopant ions to move to and occupy substitutional lattice sites thereby electrically activating the implanted dopant ions (activation step). The ion implantation step can either precede or follow step of depositing the platinum. Further, it is considered to be advantageous, after the deposition of the platinum layer on the silicon carbide, to perform an additional platinum annealing step at temperatures exemplarily as high as 900° C. to form platinum silicide. However, the equipment required for this ion implantation technique is specialized and expensive and also is platinum compared to aluminum.

In view of the above, a need for a method for forming an ohmic contact to a p-type silicon carbide layer that can reduce the thermal budget to the device still exists. Further, there is a need to a method for forming an ohmic contact to a p-type silicon carbide layer that is efficient and economic.

SUMMARY

Embodiments of the present invention relate to a method for forming an ohmic contact to a p-type silicon carbide layer. Particular embodiments relate to a method that allows formation of ohmic contacts to a p-type silicon carbide layer without annealing.

Embodiments of the invention can provide a method for forming an ohmic contact to a p-type silicon carbide layer that does not necessarily require an annealing step and which is efficient and economic. Further embodiments of the invention provide a semiconductor device manufactured by this method.

In a first step of the method according to the invention, the p-type silicon carbide layer having a first main side and a second main side opposite to the first main side is provided. In a second step first ions and second ions are implanted into the p-type silicon carbide layer from the first main side to form an implantation layer adjacent to the first main side. The step of implanting the ions into the p-type silicon carbide layer is performed by plasma immersion ion implantation (PIII), wherein the p-type silicon carbide layer is (at least partially) immersed in a plasma comprising the first ions and the second ions, the first ions being ionized aluminum (Al) atoms and the second ions being different from the first ions. The junction between the implantation layer and the p-type silicon carbide layer is the ohmic contact.

When ions are implanted into a p-type silicon carbide layer by plasma immersion ion implantation using a plasma comprising ionized aluminum atoms and second ions (e.g., ionized molecules comprising the element aluminum (Al), ionized compounds comprising the element Al, and/or ionized molecules not comprising the element Al), then the thus formed implantation layer forms an ohmic contact with the p-type silicon carbide layer, even when no high temperature annealing step is performed. This is surprising because when using the conventional ion beam implantation technique (e.g., the one described in U.S. Pat. No. 5,087,576 A) to implant aluminum ions into the p-type silicon carbide layer such a behavior could not be observed without performing a high temperature annealing step (activation step).

As it is known by the skilled person, the conventionally used ion beam implantation technique is a high-precision implantation tool to implant ion of a defined charge-to-mass ratio at an exact energy. In the conventional ion beam implantation technique, an energetic ion beam is obtained from an ion beam source by extracting ions from a plasma and accelerating them using the electric field across a single or multiple aperture grid system. To only implant ions of a specific kind (e.g., only ionized aluminum atoms), the extracted ions also undergo a magnetic ion charge and mass separation. Since the ion beam cross section is usually small compared to the target area to be implanted, the ion beam is raster-scanned across the surface area of the target to obtain large implantation areas with sufficient dose uniformity. Due to the line of sight nature of the technique, a manipulator stage is required which can support the target for rotation in the beam so that all sides of the target can be implanted. This adds cost and complexity, constrains the maximum size of the target which can be implanted, and increases the total time required to obtain satisfactory implantation of all target surfaces for relatively large targets.

Plasma immersion ion implantation (PIII) has been developed as an alternative implantation method with reduced complexity. In the plasma immersion ion implantation technique the target to be implanted is immersed in a plasma generated from a source, and negative high-voltage is applied so as to form a conformal electric sheath (space charge layer) between the target and the plasma. Ions located in or entering the sheath from the plasma side are accelerated by the sheath's electric field and implanted into the target when they impact the target. Important in view of the present invention and in contrast to the conventional ion beam implantation technique, no mass and/or charge separation is performed in PIII, with the consequence that all sorts of positively charged ions (e.g., the first ions and the second ions) comprised in the plasma and subjected to the electric field are implanted into the target. So far, the plasma immersion ion implantation technique is only known as a doping technique, but not for forming an ohmic contact to a p-type silicon carbide layer.

Although the inventors do not wish to be bound by any particular theory, it appears that an aluminum plasma (i.e., a plasma comprising aluminum ions) generated from a source in an plasma immersion ion implantation technique comprises in addition to ionized aluminum atoms (first ions) several other ion species (second ions, e.g., ionized compounds, atoms and/or molecules) which, when implanted together with the ionized aluminum atoms in the p-type silicon carbide layer, form an aluminum and silicon carbide based contact (Al/SiC contact) having ohmic properties. It is conceivable that this happens because, when implanted together, these first ions and these second ions form aluminum based compounds (e.g., silicides, carbides and/or ternary phases) with a more beneficial electron affinity, e.g., an electron affinity closer to the one of the p-type silicon carbide layer, thus lowering the Schottky barrier height. However, when only ions of a single type (e.g., ionized aluminum atoms) are implanted, which is the case in the conventional ion beam implantation technique due to their mass and/or charge based separation, such compounds are not formed and therefore a rectifying behavior is obtained.

In view of the above, in an exemplary embodiment the step of implanting the ions into the carbide layer by plasma immersion ion implantation comprises: a step of positioning the p-type silicon carbide layer in an enclosing chamber; a step of generating from a source comprising aluminum the plasma comprising the first ions and the second ions in which the p-type silicon carbide layer is immersed; independently from the step of generating the plasma, a step of applying a negative voltage between the p-type silicon carbide layer and a ground potential to accelerate the first ions and the second ions in the plasma towards the p-type silicon carbide layer from the plasma surrounding the p-type silicon carbide layer at an ion energy sufficient to implant the first ions and the second ions into the p-type silicon carbide layer.

In an exemplary embodiment, the second ions further comprise at least one of carbon and hydrogen, and a concentration of the at least one of carbon and hydrogen in the plasma is the same or higher than a concentration of aluminum in the plasma.

Here, and in what follows, a concentration of a specific element (e.g., Al, H or C) refers to the number of atoms of this element (including ionized atoms of this element) in a defined volume divided by the defined volume.

In an exemplary embodiment, the source is a gas and comprises at least aluminum and carbon. The gas may also comprise hydrogen. For example, the gas comprises one of trimethylaluminum, triethylaluminum, tri-1-butylaluminum and trimethyl(tri-sec-butoxy)aluminum or any mixture thereof.

According to an exemplary embodiment of the invention, the plasma comprises nitrogen (N) and chlorine (Cl), a concentration of the nitrogen is 10% of the concentration of aluminum in the plasma or less, and a concentration of chlorine in the plasma is less than the concentration of aluminum in the plasma. When nitrogen and/or chlorine is implanted into the p-type silicon carbide layer, compensation effects may be induced. By keeping the concentration of nitrogen and chlorine in the plasma below the above values, these compensation effects can be prevented or at least alleviated.

According to an exemplary embodiment of the invention, the accelerated ions hit onto the first main side with an energy of less than 10 keV.

According to an exemplary embodiment of the invention, the applied voltage is between 1 kV and 10 kV, for example between 3 kV and 7 kV.

According to an exemplary embodiment of the invention, the implantation layer is formed to have a minimal aluminum concentration of at least 1·1018cm−3, or at least 1·1019cm−3. When the implantation layer has an aluminum concentration of at least 1·1018cm−3the electrical properties of the thus formed ohmic contact can be improved.

According to an exemplary embodiment of the invention, after the ion implantation step, there is a step of forming a metal layer on the first main side in direct contact with the implantation layer. The metal layer comprises one of the contact metals known in the art, for example, aluminum (Al), platinum (Pt), titanium (Ti), copper (Cu), gold (Au), nickel (Ni), etc., or mixtures or alloys thereof. In contrast to the prior art, the metal layer is not necessarily required to achieve the ohmic behavior of the contact but may, for example, serve as a connecting interface, e.g., for facilitating the connection of a second device. Therefore, in contrast to the prior art, the metal layer also does not necessarily need to be annealed and thus can, for example, be removably connected to the implantation layer, e.g., it can be a metal plate or a wire pressed against the implantation layer without annealing. If desired, however, the metal layer can also be irremovably connected to the implantation layer, e.g., deposited on the implantation layer or bonded to the implantation layer, e.g., by wire bonding through copper (Cu). This flexibility offered by the present invention offers significant advantages in device design and manufacture.

As mentioned above, according to the method of the invention, no high temperature annealing step is required after the ion implantation step to activate the implanted ions. Surprisingly, as the inventors found out, such a high temperature activation step may even worsen the ohmic behavior of the contact, or completely prevent it. Although the inventors do not wish to be bound by any particular theory, it appears that this may be due to the fact that the compounds formed in the implantation layer during the plasma immersion ion implantation step (for example aluminum carbides) have a low thermal stability such that they break up at the high temperatures used in the activation step.

Therefore, according to an aspect of the invention, during and after the implantation step and before the metal layer forming step a temperature of the p-type silicon carbide layer is kept below 1300° C., or below 1000° C. In other words, no activation step is performed. According to an exemplary embodiment, during and after the ion implantation step a temperature of the p-type silicon carbide layer is permanently kept below 1300° C., or below 1000° C.

As mentioned above, in contrast to the prior art, when forming the ohmic contact, the metal layer also does not necessarily need to be annealed to obtain the ohmic behavior. Also, since the ion energies are lower than in the conventional ion beam implantation technique, the p-type silicon carbide layer does not need to be heated to an elevated temperature to prevent amorphization of and minimize damage to the crystal lattice of the p-type silicon carbide layer during ion implantation. Therefore, according to an aspect of the invention, a temperature of the p-type silicon carbide layer is kept below 500° C., or below 100° C., during the ion implantation step and below 800° C., or below 100° C., after the metal layer forming step. According to an exemplarily embodiment, during and after the ion implantation step, a cumulative duration during which the temperature of the p-type silicon carbide layer exceeds 500° C., or 100° C., is less than two minutes.

According to another exemplary embodiment, the concentration of aluminum in the metal layer is less than 1·1013cm−3, a concentration of titanium is less than 2·1011cm−3and a concentration of nickel is less than 1·1011cm−3, or the metal layer is void of aluminum, titanium or nickel.

According to an aspect of the invention the method is used for manufacturing an ohmic contact in a power semiconductor device, for example a p-i-n diode, a Schottky diode, a junction barrier Schottky (JBS) diode, an insulated gate bipolar transistor (IGBT), a bipolar junction transistor (BJT) or a thyristor.

According to an aspect of the invention, the invention pertains to a power semiconductor device comprising an ohmic contact manufactured according to the method of the invention.

