Patent Application
20050040497
The present application claims priority from Japanese Application JP 2003-207831 filed on Aug. 19, 2003, the content of which is hereby incorporated by reference into this application.
1. Field of the Invention
The present invention relates to a semiconductor device using a substrate of a zinc blende single crystal semiconductor, for example, GaAs and InP, as group III-V compound semiconductor.
2. Related Art
In recent years, along with rapid increase of the demand for mobile communication systems or optical communication systems, research and development have been conducted vigorously for semiconductor devices used for the communication systems. For example, a heterojunction bipolar transistor (HBT) having a hole on the rear face of a GaAs substrate as group a III-V single crystal semiconductor as a power amplifier has been reported in Japanese Patent Laid-open No. 6-5620 (Paragraph No. 0016, FIG. 1) and a collector top heterojunction bipolar transistor (C-top HBT) has been reported in Japanese Patent Laid-open No. 10-41320 (Paragraph No. 0006, FIG. 1).
In fabricating a substrate of a group III-V single crystal semiconductor such as GaAs, dry etching using a gas, wet etching using a liquid or etching using both of them in combination is generally used. In dry etching, the lateral surface of a hole or step is always nearly perpendicular to the surface of a substrate, and thus, an inverted mesa shape is not formed. However, in a case where the shape of the lateral wall is nearly vertical, it is difficult to form an electrode material on the lateral surface. If a step is large, disconnection may be possibly caused at the step. Further, it is difficult to control the end of dry etching at an accuracy of about several nanometers. On the other hand, in a case of conducting etching only by wet etching, etching can be stopped automatically by utilizing a layer of a substance having different selectivity. For example, this is a method of wet etching a GaAs layer with an etching solution containing an acid and aqueous hydrogen peroxide and stopping etching with an InGaP layer.
However, the following is well-known: in a case of etching a GaAs substrate by using an etching solution of a composition known so far and a mask material, the etching is stopped at {111} A crystal plane in which inverted mesa and normal mesa shapes are developed, failing to obtain necessary shape and depth.
On the other hand, in the method of combining the dry etching and the wet etching, the wet etching is generally applied after the dry etching. Depending on the re-deposition place of reaction products between the dry etching gas and the etched substance, the reaction products act as a mask material to sometimes hinder the proceeding of the succeeding wet etching.
A technical subject of the present invention is to prevent occurrence of disconnection of electrodes caused by steps and bursting caused by residual air.
The present invention intends to provide a semiconductor device capable of overcoming a drawback due to the shape of a concave portion present in a zinc blende type compound semiconductor substrate in which the area of the bottom is larger than the surface in the cross-sectional shape, as well as a manufacturing method thereof.
According to the invention, a hole or step present in a semiconductor substrate constituting the semiconductor device is formed into a normal mesa shape irrespective of the orientation of the crystals on the surface of the semiconductor substrate. For this purpose, the invention also provides a novel method of manufacturing a semiconductor device using a new wet etching solution having an etching rate for a portion below the etching mask higher than that in the direction of the depth of the semiconductor substrate.
The basic constitution of the semiconductor device according to the invention is as described below. That is, the semiconductor device of the invention comprises at least a zinc blende type single crystal semiconductor substrate and a semiconductor active region formed in or on the zinc blende type single crystal semiconductor in which the zinc blende type single crystal semiconductor substrate is formed with a hole or a step in at least one surface thereof. Then, the hole or the step is shaped have a slope in which each angle formed at a corner between the surface left without forming a hole in a crystal plane formed with the hole or the step and the lateral surface of the hole is larger than 90°.
In a practical embodiment of the semiconductor device according to the invention, the lateral surfaces of the hole or the step in the etching direction parallel with the bottom of the zinc blende type single crystal semiconductor substrate include lateral surfaces crossing each other that are asymmetrical in shape.
In another practical embodiment of the semiconductor device according to the invention, the hole or the step is a rectangular shape, and the lateral surfaces in the etching direction parallel with the bottom of the zinc blende type single crystal semiconductor substrate include the lateral surfaces crossing each other that are asymmetrical in shape.
In another practical embodiment of the semiconductor device according to the invention, the lateral surfaces of the hole or the step in the etching direction parallel with the bottom of the zinc blende type single crystal semiconductor substrate are such that one of the lateral surfaces crossing to each other has an angle of 54° or less and the other of the lateral surfaces is more abrupt than the lateral surface of 54° or less. The angle of 54° or less is, more exactly, 54.7° as described above. The angle means herein a practical angle in the actual step. Accordingly, in another point of view, the angle means that the so-called normal mesa plane, in the present specification, is a plane shallower than the {111} A plane. This means that the angle formed between the hole or the normal mesa plane and the {111} A plane is 54° or less. The angular range described below has the same meanings.
In another practical embodiment of the semiconductor device according to the invention, a hole or a step is formed in a (100) plane of the zinc blende type single crystal semiconductor substrate, and the hole or the step has a normal mesa shape in which the average angle formed between the surface from the opening to the bottom of the hole or the step (that is, the lateral surface of the hole) and the left (100) plane is larger than 125.3°, in the cross section as viewed from a (011) plane vertical to the (100) plane or a plane parallel with the (011) plane, and a cross section as viewed from a (01-1) plane vertical to the (100) plane and the (011) plane or a plane parallel with the (01-1) plane.
A semiconductor device in another point of view of the invention has the following configuration. That is, the semiconductor device in another point of view of the invention comprises at least a zinc blende type single crystal semiconductor substrate, and a semiconductor element portion mounted on a first crystal plane of the semiconductor substrate. Then, the semiconductor substrate comprises a hole or a step penetrating the semiconductor substrate and including at least a portion of a region facing the semiconductor element portion of a second crystal plane facing the first crystal plane of the semiconductor substrate, and the hole or the step is shaped to have a slope in which each angle formed at a corner between the surface left without forming the hole in the crystal plane formed with the hole or the step and the lateral surface of the hole is larger than 90°. Then, a conductor layer is provided which is connected electrically by way of the hole penetrating the semiconductor substrate to the semiconductor element portion.
A typical example of the semiconductor element portion mounted on the first crystal plane of the semiconductor substrate is a heterojunction transistor. Further, the semiconductor element portion can use, depending on the demand, for example, power amplifiers, various semiconductor devices using FET, or optical semiconductor devices. Typical examples of the optical semiconductor devices include, for example, an APD (Avalanche Photo-Diode).
A method of manufacturing a semiconductor device according to the invention includes at least the steps of forming a resist film having an opening of a desired shape over a zinc blende type single crystal semiconductor substrate, and etching the thus prepared semiconductor substrate by use of an etching solution by impregnating the semiconductor substrate with the etching solution along the boundary between the resist film and the semiconductor substrate, thereby forming the cross section of the opening into a mesa shape in any etching direction in the opening.
Then, a typical example of the step of forming the cross section of the opening into the mesa shape is a step of applying etching by using an etching solution containing an acid, aqueous hydrogen peroxide and alcohols. Further, in another example, etching is applied by using an etching solution containing an acid, aqueous hydrogen peroxide and a surface active agent.
Before explaining concrete embodiments of the semiconductor device, descriptions are to be made of a method of forming a hole or a step in a normal mesa shape irrespective of the crystal orientation of a substrate crystal concerning the present invention.
In a case of using dry etching in the hole forming step, the angle is substantially vertical, that is, about 90°. Further, in a case of forming a hole by using the existent wet etching solution, etching is stopped at a {111} A plane. Accordingly, the normal mesa shape and the inverted mesa shape are developed depending on the crystal orientation and the angle is smaller than 90° when viewed at a certain cross-section.
On the contrary, the inverted mesa shape is not developed for the hole in the invention when observed at any cross-section. According to this embodiment, in forming a semiconductor device on a group III-V single crystal semiconductor substrate, it is no more necessary to consider the difference of the hole shape depending on the crystal orientation, whereby the degree of freedom in the layout is improved greatly. Further, since all of the holes are in the normal mesa shape, disconnection does not occur in a case of forming an electrode or the like on the hole or the step. Further, worry of bursting caused by residual air in a case of filling a conductive substrate such as a silver paste in the hole for connection with a module substrate or the like is eliminated, improving reliability.
In the invention, it is important to form such a normal mesa shape irrespective of the crystal orientation. A typical example of the group III-V single semiconductor substrate served for a semiconductor device is a GaAs substrate or an InP substrate. Further, a semiconductor substrate in which a semiconductor epitaxial layer is formed on the surface of the GaAs substrate or the InP substrate may also be used depending on the purpose. In the present specification, single semiconductor substrates and semiconductor substrates formed with the epitaxial layers are collectively referred to as “semiconductor substrate”.
The group III-V compound semiconductor substrate crystal has a zinc blende type structure. In a case of forming an etching mask of a desired shape on the compound semiconductor crystal described above and forming a hole or a step by wet etching, the form of the invention can be attained by etching using an etching solution incorporated with alcohols and a surface active agent. The etching mask may be a resist used in the field of semiconductors. The resist is a photoresist comprising an organic polymeric resin material and both of positive or negative types may be used depending on the requirement. A mask made of an inorganic material such as WSi or SiO2 is not suitable for the etching mask. Since the inorganic mask bonds more firmly to the semiconductor substrate compared with the resist, etching proceeds mainly to the inside of the substrate. Accordingly, etching is stopped at the {111} plane as described above and both the normal mesa and inverted mesa shapes are developed.
An etching solution constituting a base for the etching solution of the invention may be a usual etching solution used for the compound semiconductor crystal. A typical example of the etching solution used customarily to the compound semiconductor crystals can include, for example, a mixed solution of an acid, for example, a hydrofluoric acid or sulfuric acid, hydrogen peroxide and water. The open hole of the normal mesa shape of the invention can be obtained by adding alcohols or a surface active agent to such a usual etching solution described above. Typical examples of the alcohols include isopropyl alcohol, ethanol and methanol. EMAL (trade name of product) is especially suitable as the surface active agent. The composition of the etching solution is determined depending on the conditions such as the composition and the thickness of the semiconductor crystal as an object of etching, and the depth of the hole, etc. If the alcohols or the surface active agent is added less than 20% by volume to the etching solution as a mixture of the acid and aqueous hydrogen peroxide, the effect intended in the invention cannot be obtained. On the other hand, if it exceeds 50%, the photoresist for use in the existent etching mask material is damaged undesirably. As has been described above, the addition amount is preferably from 35% to 40% by volume based on the etching solution although it depending on the conditions for the object to be etched. Circumstantial conditions such as etching temperature and etching time may be determined in accordance with usual conditions regarding the manufacture of the semiconductor devices, although it may depend on the conditions of fabrication.
It may be said that the etching method according to the invention described above is a method of conducting etching along the boundary between the etching mask and the compound semiconductor crystal while impregnating the compound semiconductor crystal with the etching solution along with the progress of etching and applying etching in the mesa shape in each of etching directions. That is, the manufacturing method of the invention utilizes the nature that side etching proceeds at a higher rate in the vicinity of the boundary between the substrate and the mask than the wet etching proceeds toward the inside of the substrate, by using the etching situation containing the alcohols and the resist mask. This will be described more specifically with reference to
However, in the etching according to the invention, the hole opening is extended more for the same bottom area and depth of the hole, as shown by a solid line 14. That is, in the case of using the etching solution containing the alcohols and the resist mask, since side etching proceeds in the vicinity of the mask boundary at etching power stronger than power for stopping the etching at the {111} A plane, the widening extent 38 of the hole is larger than the depth 39 of the hole, in which the inverted mesa shape does not develop but the normal mesa shape are developed at all of the planes. In addition, the average angle formed between the lateral surface of the hole and the surface left unetched below the mask is larger than 125.3° that represents the {111} A plane. According to this embodiment, all the holes can be formed into the normal mesa shape by using an easily available resist mask and merely adding alcohols to the existent etching solution containing acid and hydrogen peroxide.
However, the angle at the lateral surface of the hole is not always constant but the angle near the hole opening sometimes larger than that near the bottom of the hole.
Further,
Then, the lateral extension of the normal mesa shape to the substrate surface described above shows that the etching solution is impregnated as etching proceeds along the boundary between the etching mask and the compound semiconductor crystal. Accordingly, while the paired cross-sectional shapes in both directions have the same normal mesa shape, they are different from each other in an actual detailed shape. That is, a pair of lateral surfaces in the etching direction is asymmetrical to each other. Generally, the paired lateral surfaces in the etching direction have an angle of 54° or less, whereas the other paired lateral surfaces form a more acute angle.
Then, a description is to be made of an embodiment of a semiconductor device using a collector top HBT according to the invention.
A collector top HBT is formed on a (100) plane of a semi-insulative GaAs substrate 9 as a group III-V single crystal semiconductor, while a hole 3 of a normal mesa shape in which an angle formed at the corner between the substrate surface and the lateral surface of the hole is larger than 90° like in the first embodiment is formed on a substrate below the HBT. On a semi-insulative GaAs substrate 9, are formed an InGaP buffer layer (InP molar ratio increasing gradually from 0.5 to 1.0, undoped, layer thickness of 1.5 μm) 18, highly doped n-type InGaAs sub-emitter layer (InAs molar ratio of 0.5, Si concentration at 4×1019 cm−3, layer thickness of 0.6 μm) 19, an n-type InAlAs emitter layer (InAs molar ratio of 0.5, Si concentration at 5×1017 cm−3, layer thickness of 0.2 μm) 20, a p-type GaAsSb base layer (GaAs molar ratio of 0.5, C concentration at 3×1019 cm−3, layer thickness of 70 nm) 21, an n-type InP collector layer (Si concentration at 3×1016 cm−3, layer thickness of 0.8 μm) 22, and n-type InGaAs cap layer (InAs molar ratio of 0.5, Si concentration at 4×1019 cm−3, layer thickness of 0.2 μm) 23, in which a collector electrode 24 and a base electrode 27 are formed in a non self-alignment manner. The collector electrode 24 and the base electrode 27 may of course be formed with the so-called self-alignment manner. In the transistor parasitic region in the emitter layer and the sub-emitter layer 19 (a region other than the HBT intrinsic area just below the collector electrode 24), a high resistance InAlAs parasitic emitter region 25 and an n-type AlAs parasitic emitter region 26 implanted with boron ions are formed, which minimizes the base current flowing through the parasitic emitter base junction. The ions to be implanted may be helium, oxygen, fluorine, or a combination thereof in addition to boron, by which the high resistance region 25 and n-type region 26 are formed in the same manner.
The GaAs substrate 9 and the InGaP buffer layer 18 just below the HBT and including intrinsic region are removed, and an emitter electrode 30 is formed, just below the HBT, in contact with the highly doped n-type InGaAs sub-emitter layer 19.
A method of manufacturing a collector top HBT having the heat dissipation hole shown in
Then, the n-InGaAs cap layer 23 is wet etched using a mixed solution of phosphoric acid, aqueous hydrogen peroxide and water using the region of the collector electrode 14 as a mask to form an undercut of 0.3 μm (
The n-type InP collector 22 is then removed by wet etching using an aqueous solution of hydrochloric acid (
Then, the SiO2 sidewall 31 is removed using an aqueous solution of hydrofluoric acid, and a base electrode Pt (20 nm)/Ti (50 nm)/Pt (50 nm)/Au (200 nm)/Mo (20 nm) 27 is formed by a lift-off method using electron beam vapor deposition (
Then, an SiO2 film (film thickness of 0.5 μm) 28 is deposited at 250° C. by a plasma excited chemical vapor deposition method, base contact holes for connecting base electrode and interconnection are formed, Mo (film thickness of 0.15 μm)/Au (film thickness of 0.8 μm)/Mo (film thickness of 0.15 μm) is entirely deposited as a first layer interconnection metal, and base interconnection by photolithography and argon ion milling are conducted (since the base interconnection is perpendicular to the surface of the drawing sheet and present in the HBT parasitic region, it is not illustrated) . Then, SiO2 film (film thickness 0.5 μm) is deposited at 250° C. by a plasma excite chemical vapor phase deposition method again and collector contact holes 34 for connecting the collector electrode 24 and the interconnections are formed (
Then, an adhesive 35 is coated over the entire wafer surface, and appended on a glass substrate 36 (
Successively, the substrate at a region including the HBT intrinsic region is removed such that the angle formed at the corner between the substrate surface and the lateral surface of the hole was larger than 90° by photolithography and dry etching or wet etching, or the etching as a combination thereof to form a heat dissipation hole of a normal mesa shape (
Then, the InGaP buffer layer 18 is removed by using an aqueous solution of hydrochloric acid to expose the lower surface of the InGaAs sub-emitter layer 19 (
According to this embodiment, since all the heat dissipation holes formed are in the normal mesa shape, when the emitter electrode is formed so as to cover the holes, disconnection does not occur. Further, also in a case of filling a conductive substance such as a silver paste in the heat dissipation hole and bonding to the module substrate or the like for emitter grounding, worry of bursting caused by the residual air can be eliminated to provide an effect of improving reliability. While a heat dissipation hole is formed for several collective HBTs in the figures, one heat dissipation hole may be formed below each HBT to produce the same effect. Further, while the collector top HBT has been referred to in this embodiment, the emitter top HBT may be employed to produce the same effect.
Further, while the GaAs substrate is used, it is applicable also to the HBT using an InP substrate.
Also in a semiconductor device using the group III-V single crystal semiconductor such as FET or APD, an advantageous effect of improving the degree of freedom in the layout can be obtained in a case where normal mesa shapes are always developed irrespective of the crystal orientation.
According to the invention, worry of electrode disconnection caused by the step can be eliminated irrespective of the crystal orientation. Further, in a case of filling the conductive substance such as a silver paste in the hole and bonding to the module substrate, bursting caused by residual air no more occurs.
According to a first aspect of the invention, it is possible to provide a semiconductor device capable of overcoming the drawback due to the shape of the concave portion present in the zinc blende type compound semiconductor substrate in which the area of the bottom is larger than that of the surface in view of the cross sectional shape.
According to a second aspect of the invention, it is possible to provide a method of manufacturing a semiconductor device comprising a step capable of forming a concave portion of a shape in which the area of the surface is larger than that of the bottom in view of the cross sectional shape in a zinc blende type compound semiconductor irrespective of the crystal orientation.
Description of Reference Numerals
1: group III-V single crystal semiconductor substrate,
2: substrate surface, 3: hole, 4: lateral surface of hole at the opening, 5: angle formed between the substrate surface and the lateral surface of hole, 6: (100) plane, 7: (01-1) plane, 8: (011) plane, 9: GaAs substrate, 10: {111} plane A, 11: angle formed between (100) plane and {111} plane A as viewed from cross section (01-1), 13: angle between (100) plane and {111} plane A as viewed from cross section (011), 14: lateral surface of hole, 15: angle formed between (100) plane and the lateral surface of hole as viewed from cross section (01-1), 17: angle formed between (100) plane and the lateral surface of hole as viewed from cross section (011), 18: buffer layer 19: sub-emitter layer, 20: emitter layer, 21: base layer 22: collector layer, 23: cap layer, 24: collector electrode, 25: high resistance parasitic emitter region, 26: parasitic sub-emitter region, 27: base electrode, 28: insulative film, 29: interconnection, 30: rear face emitter electrode, 31: insulative film sidewall, 32: boron ion, 33: inter-device isolation region, 34: collector contact hole, 35: adhesive, 36: glass substrate, 37: resist mask, 38: extension of hole, 39: depth of hole.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-207831 | Aug 2003 | JP | national |
