FR3009129A1 - METHOD FOR MANUFACTURING GALLIUM NITRIDE ELECTRONIC COMPONENT - Google Patents

Abstract

The invention relates to a method for manufacturing an electronic component in and on a layer (3) of gallium nitride, comprising a step of thinning a portion of said layer (3) by ion milling.

Description

TECHNICAL FIELD The present application relates to the manufacture of electronic components, and in particular electronic power components, in and on gallium nitride layers. B12665 - 12-T0-0809 It relates more particularly to a method of etching a gallium nitride layer for the manufacture of a Schottky diode. DESCRIPTION OF THE PRIOR ART It has already been proposed to produce electronic power components, and in particular Schottky diodes, in and on gallium nitride layers. Indeed, gallium nitride is a semiconductor material whose use is likely to make components resistant to higher voltages than conventional silicon components. However, in practice, the manufacture of gallium nitride power components is not easy, especially because of the fact that gallium nitride is difficult to etch. To date, the manufacture of Schottky diodes gallium nitride on an industrial scale is problematic. Indeed, to manufacture such diodes, it is expected to thin a portion B12665 - 12-T0-0809 2 of a layer of gallium nitride to a thickness of several micrometers. The etching processes currently used to achieve such thinning have various disadvantages.

SUMMARY In order to overcome all or some of these disadvantages, one embodiment provides a method of manufacturing an electronic component in and on a layer of gallium nitride, comprising a step of thinning a portion of said layer by ion milling. . According to one embodiment, during the ionic machining step, the portion is bombarded by a beam of non-zero angle of incidence etching ions. According to one embodiment, the gallium nitride layer has a hexagonal crystalline structure oriented in a crystalline plane, and comprises dislocations, the angle of incidence being chosen so as to avoid the appearance on the surface of the portion. thinned, flatness defects related to the presence of dislocations.

According to one embodiment, the angle of incidence is in the range of 23 to 37 degrees. According to one embodiment, the angle of incidence is approximately 30 degrees. According to one embodiment, the etching ions are argon ions. According to one embodiment, the ionic machining step is carried out using a plasma free of species chemically reactive with gallium nitride. According to one embodiment, the portion is delimited by an etching mask formed on one face of said layer. According to one embodiment, the mask has a form of mesa with non-vertical sides. According to one embodiment, on at least a portion of the thickness of the mask, the tangent to the sidewall of the mask forms an angle greater than 120 degrees with the surface of said layer not covered by the mask . According to one embodiment, the pattern of the mask is defined by wet chemical etching using an etching solution etching the mask material isotropically. According to one embodiment, the mask is made of silicon oxide. BRIEF DESCRIPTION OF THE DRAWINGS These features and their advantages, as well as others, will be set forth in detail in the following description of particular embodiments in a non-limiting manner with reference to the accompanying figures in which: FIG. in section schematically showing an example of a Schottky diode formed in and on a layer of gallium nitride; and FIGS. 2A and 2B are sectional views schematically showing steps of a preferred embodiment of a method of etching a gallium nitride layer suitable for making a Schottky diode. DETAILED DESCRIPTION For the sake of clarity, the same elements have been designated by the same references in the various figures and, moreover, as is customary in the representation of the integrated circuits, the various figures are not drawn to scale. Furthermore, in the rest of the description, unless otherwise indicated, the terms "approximately", "substantially", "about" and "of the order of" mean "to within 10%", and directional references such as overlapping , 30 overlying, lateral, above, below, upper, lower, vertical, horizontal, etc. apply to oriented circuits as illustrated in the sectional views of the corresponding figures. FIG. 1 is a sectional view schematically showing an exemplary embodiment of a mesa-type Schottky diode 1 of B12665 - 12-T0-0809 4 formed by the junction between a lightly doped (N -), and an anode metallization 5 comprising for example nickel and / or gold, formed on the surface of the layer 3a.

In this example, the lightly doped gallium nitride layer 3a is formed on the surface of a layer 3b of more heavily doped gallium nitride (Nt). The assembly, or layer 3, constituted by the stack of layers 3a and 3b of gallium nitride rests on a support 7, for example a support of sapphire or silicon. In addition, a buffer layer 9, for example made of AlGaN, interfaces between the support 7 and the layer 3b. For example, the support 7 has a thickness of about 1 mm, and the stack 3 has a thickness of between 7 and 25 gm.

To form a cathode contact, the stack 3 has been thinned in the peripheral region of the diode, until the surface of the layer 3b of gallium nitride with a higher doping is accessible. A contact metallization 11, forming an ohmic contact with the layer 3b, is provided on the surface of the thinned region. In this example, the metallization 11 has the shape of a ring surrounding the central portion of the diode. The realization of such a diode requires a thinning of the peripheral region of the layer 3 of gallium nitride to a relatively large thickness. By way of example, the lightly doped gallium nitride layer 3a has a thickness of between 5 and 12 gm. Thus, the thinning is for example a thickness slightly greater than 5 to 12 gm.

The upper face of the thinned zone must have excellent flatness. Indeed, this face being intended to support the cathode metallization 11, the presence of roughness or roughness would lead to a degradation of the ohmic contact performance, and therefore a decrease in the power of the diode.

B12665 - 12-T0-0809 By way of example, the case in which the layer 3 of gallium nitride is formed in one or more epitaxial growth stages, and has a hexagonal crystal structure of wurtzite type, the plan upper layer of gallium nitride layer 3 having a crystalline orientation (0001). This type of layer is indeed relatively easy to obtain insofar as such layers are commonly used for the manufacture of light emitting diodes (LEDs) or Schottky diodes gallium nitride.

The usual methods of wet chemical etching do not make it possible to etch the layer 3. This is particularly related to the crystalline orientation of this layer. In fact, the known chemical solutions capable of attacking gallium nitride, for example solutions based on phosphoric acid or potassium hydroxide, are ineffective in the crystalline plane which it is desired to etch, that is, ie the plane (0001) of the hexagonal structure. In the field of optics, especially with a view to producing light-emitting diodes (LEDs) based on gallium nitride, it has been proposed to use dry chemical etching or plasma etching processes. The plasmas used are generally mixtures based on chlorine. These methods are relatively effective, but they relate to very different engravings from that which one seeks to realize here. Indeed, in the field of LEDs, the layers to be etched are very thin layers, a few tens of nanometers thick. In addition, generally, in the etched regions, the entire thickness of the gallium nitride layer is eliminated. It is not, as in the case of a Schottky diode, a thinning of the gallium nitride layer with contact on the etched surface. In this case, a good flatness of the surface after etching is easier to obtain. As has been explained in detail in an earlier patent FR2959595 of the Applicant, such methods of chlorine-based plasma etching are not directly applicable to the type of etching that is to be performed. right here. Indeed, these processes lead, when they relate to thicknesses greater than one micrometer, to the formation, on the surface of the thinned regions, of hollow and / or protuberance flatness defects related to the presence of dislocations in the crystalline structure. of the layer 3. The previous work of the applicant have shown that with such plasma chlorine etching processes can, by adjusting the etching conditions, promote the formation of protuberances and avoid the formation of hollows, or, conversely, favor the formation of hollows and avoid the formation of protuberances, but that there is no compromise to avoid both the formation of protuberances and the formation of holes on the surface of the thinned region. In the aforementioned FR2959595 patent, it has been proposed to thin the gallium nitride layer in two steps: a first chlorine-based dry etching step under conditions which are likely to promote the appearance of flatness defects in form of protuberances and to avoid the appearance of hollow flatness defects; and a second wet etching step capable of removing the protuberances. In the second step, the protuberances can easily be etched wet because they are attacked by the etching solution in a different crystalline plane of the plane (0001) of the hexagonal structure. However, a problem that may arise is that during the first chlorine-based plasma etching step, the gallium nitride layer receives a large dose of ultraviolet radiation. Although the upper face of the portion of the layer 3 intended to receive the Schottky contact, or upper face of the mesa, is protected by a mask during this step, ultraviolet rays of the plasma are likely to pass through the mask and to create defects. on the B12665 - 12-T0-0809 7 upper side of the mesa. This can degrade the performance of the diode. Another problem is that chlorine-based plasma etching is a very strongly anisotropic etching. As a result, the side walls of the mesa formed in the gallium nitride layer are very steep (almost vertical). In addition, localized over-etching may occur resulting in the appearance at the foot of the mesa of a vertical trench (not shown) extending into the thinned region of the gallium nitride layer (a phenomenon commonly referred to in FIG. technique by the English term "trenching"). The very steep profile of the mesa and the possible presence of a trench surrounding the base of the mesa, make it difficult to form a possible passivation insulating layer (not shown) on the non-metallized surfaces of the gallium nitride layer. According to one aspect of an embodiment, provision is made to perform the etching operation of the layer 3 of gallium nitride by ion milling or ionic grinding, according to a method commonly referred to in the art as IBE, of the English "Ion Beam Etching" - ion beam etching. During such an etching, the surface to be etched is bombarded by a rare gas ion beam such as argon. Etching is a purely physical dry etching, i.e., unlike chlorine-based plasma etching processes of the aforementioned type, the etching ions do not react chemically with gallium nitride. The applicant has indeed found that the presence of chemically reactive species, such as chlorine, during the gallium nitride etching operation, does not allow to find etching conditions to avoid the formation of flatness defects shaped hollow on the surface of the thinned region. During etching, the substrate is preferably inclined relative to the direction of propagation of the etching ions, so that the etching beam reaches the gallium nitride layer with a non-zero angle of incidence. During etching, the substrate may be rotated about an axis approximately orthogonal to the gallium nitride layer, so that all unmasked regions of the gallium nitride layer are exposed. in the same way to the etching ions. An advantage of this embodiment is that the etching by ionic machining, although anisotropic, makes it possible to obtain a mesa having a less abrupt profile than the mesas obtained by chlorine-based chemical plasma etching processes. In other words, the slope of the peripheral flanks of the mesa is softer than when the thinning is performed by chemical plasma etching based on chlorine. In addition, the use of an etching by ion machining avoids the formation of a peripheral trench at the foot of the mesa. This facilitates, if necessary, the production of an insulating passivation layer on the non-metallized surfaces of the gallium nitride layer. Another advantage of this embodiment is that, during etching by ionic machining, the gallium nitride layer receives a much smaller dose of ultraviolet rays than during a plasma etching based on chlorine. This results notably from the fact that the argon-based etching plasma used during the ionic machining is generated in a chamber distinct from the etching chamber. This makes it possible to avoid the formation of defects on the surface of the gallium nitride layer intended to receive the Schottky contact. This results in improved performance of the diode. The tests carried out have shown that during the etching by ionic machining, there does not appear to be a protuberance-type flatness defect on the surface of the thinned region. The tests carried out have further shown that by appropriately choosing the angle of incidence of the etching ion beam on the gallium nitride layer, it is possible to avoid the formation of hollow type flatness defects on the surface of the thinned region.

In a preferred embodiment, the angle of incidence of the etching beam is chosen so as to avoid the formation of depressions on the surface of the thinned region of the gallium nitride layer. B12665 - 12-T0-0809 For this, the angle of incidence of the etching beam on the gallium nitride layer, that is to say the angle between the main direction of the etching beam and the normal to the upper surface of the layer of etching. gallium nitride, can be in the range of 23 to 37 degrees, and is preferably about 30 degrees.

According to another aspect, as will be explained in more detail below with reference to FIGS. 2A and 2B, it may furthermore optionally be provided, during etching, to use, to protect the upper face of the mesa intended to receive the Schottky contact, a mask having lateral flanks with a relatively gentle slope. If the mask is made of a material that can be partially consumed during etching, for example silicon oxide, a part of the gently sloping profile of the mask is "projected" into the layer of gallium nitride during the engraving. This allows for a mesa having, at least in its lower part, a relatively smooth slope profile. This facilitates, where appropriate, the production of an insulating passivation layer on the non-metallized surfaces of gallium nitride. FIGS. 2A and 2B are sectional views schematically showing steps of a preferred embodiment of a method for etching a layer of gallium nitride, for example with a view to producing a Schottky diode of the type described in connection with FIG. 1. FIG. 2A represents the etching step itself, and FIG. 2B represents the profile of the mesa formed in the gallium nitride layer at the end of the etching step. FIG. 2A shows a portion of the gallium nitride layer 3 (constituted in this example by a stack of a lightly doped gallium nitride layer 3a and a layer of more heavily doped gallium nitride 3b) capable of B12665 - 12-T0-0809 10 of the type described with reference to FIG. 1. As in FIG. 1, the layer 3 rests, in this example, on a base consisting of a support 7 and a buffer layer 9.

On a part of the upper surface of the layer 3 intended to receive a Schottky contact electrode, provision is made to form a mask 13, for example made of silicon oxide. Since the selectivity of the ionic machining of gallium nitride with respect to the silicon oxide is relatively low, typically of the order of 1 to 2 to 1, the thickness of the mask 13 is preferably relatively large, for example the order of 0.5 to 1.5 times the thickness of gallium nitride that it is desired to burn. It will be noted, however, that the described embodiments are not limited to the case where the mask 13 is made of silicon oxide. If other materials are used to make the mask, we can adapt the thickness of the mask according to the selectivity of the ionic machining of gallium nitride with respect to these materials. The lateral flanks of the mask 13 preferably have a gently sloping profile, as shown in FIG. 2A. To obtain such a profile, it is possible, for example, to etch the pattern of the mask by wet etching, with the aid of an etching solution attacking the mask material isotropically (this is called an isotropic profile of the flanks of the mask). mask). The described embodiments are however not limited to this particular example. Any other known method for producing a mask 13 having a gentle slope profile may be used. By gently sloping profile is meant here that, at least in a lower portion of the flanks of the mask, the tangent or tangents to the blank of the mask form obtuse angles, for example greater than 120 degrees, with the surface of the nitride layer of gallium 3 not coated by the mask 13. After forming the mask 13, the regions of the gallium nitride layer not protected by the mask 13 are etched by ion milling. During etching, the assembly formed by the layer 3 and the mask 13 is bombarded by a beam of 15 ions of physical etching, for example argon ions. In this example, as shown in FIG. 2A, the ion beam arrives with a non-zero angle of incidence on the upper surfaces of the layer 3 and the mask 13. As indicated above, the angle of incidence beam 15 is preferably chosen so as to avoid the appearance of depressions on the thinned region of the gallium nitride layer. During etching, the carrier 7 may be rotated about an axis approximately orthogonal to the carrier 7. For example, the ion machining etching plasma is produced by an inductively coupled plasma source or ICP source ("Inductively Coupled Plasma") 15 powered by a radio frequency generator delivering a power of between 200 and 1500 W, for example a power of the order of 800 W, and comprises an argon flow of a few tens of hours. sccm (of the English standard cubic centimeter per minute), for example about 20 sccm.

The energy of the argon ion beam may be between 200 and 1000 eV, for example of the order of 500 eV. The pressure in the etching chamber is for example between 10 -2 and 10 -1 Pa, for example of the order of 5 * 10 -2 Pa, and the temperature of the etching medium is for example of the order 5 ° C. The described embodiments are however not limited to this particular example. FIG. 2B shows the profile of the mesa formed in the layer of gallium nitride 3 at the end of the etching step of FIG. 2A. As shown in FIG. 2B, the sides of the mesa are approximately vertical in an upper part of the mesa (this is called anisotropic profile of the mesa), then their slope gradually attenuates in the lower part of the mesa (we can then talk about isotropic profile of the mesa). The flanks of the lower portion of the mesa 35 follow a profile which is a function of the profile of a lower portion of the flanks of the mask 13, completely removed during etching. For example, the upper part (anisotropic) of the sides of the mesa extends over the upper half of the height of the mesa, and the lower part, slower (isotropic in this example), flanks of the mesa s extends on the lower half of the height of the mesa. However, the described embodiments are not limited to these particular proportions, which essentially depend on the thickness and the profile of the mask 13, as well as the selectivity of the ionic machining of the gallium nitride with respect to the mask material. Particular embodiments have been described. Various variations and modifications will be apparent to those skilled in the art. In particular, the described embodiments are not limited to the production of gallium nitride Schottky diodes. More generally, the proposed method may be used to manufacture any other electronic component whose embodiment comprises a thinning of a gallium nitride layer.

Claims (12)

REVENDICATIONS1. A method of manufacturing an electronic component in and on a layer (3) of gallium nitride, comprising a step of thinning a portion of said layer (3) by ion milling. 2. Method according to claim 1, wherein, during the ionic machining step, said portion is bombarded by a beam (15) of non-zero angle of incidence etching ions. The process according to claim 2, wherein the gallium nitride layer (3) has a hexagonal crystalline structure oriented in a crystalline plane (0001), and comprises dislocations, said angle of incidence being chosen so as to avoid appearance of flatness defects on the surface of the thinned portion due to the presence of dislocations. 4. The method of claim 2 or 3, wherein said angle of incidence is in the range of 23 to 37 degrees. The method of any one of claims 2 to 4, wherein said angle of incidence is about 30 degrees. 20 The method of any one of claims 2 to 5, wherein said etching ions are argon ions. 7. A process according to any one of claims 1 to 6, wherein the ion milling step is carried out by means of a plasma free of chemically reactive species with gallium nitride. 8. Method according to any one of claims 1 to 7, wherein said portion is defined by an etching mask (13) formed on one side of said layer (3). The method of claim 8, wherein the mask (13) is mesa-shaped with non-vertical flanks. The method of claim 8 or 9, wherein, on at least a portion of the thickness of the mask, the tangent to the sidewall of the mask forms an angle greater than 120 degrees with the surface of said layer (3) uncoated by the mask (13) .B12665 - 12-T0-0809 14 The method according to any one of claims 8 to 10, wherein the pattern of the mask (13) is defined by wet chemical etching using an etching solution etching the material of the mask (13). isotropic way. 12. A method according to any one of claims 8 to 11, wherein the mask (13) is silicon oxide.