WO2012063529A1 - 半導体装置およびその製造方法 - Google Patents
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- WO2012063529A1 WO2012063529A1 PCT/JP2011/066885 JP2011066885W WO2012063529A1 WO 2012063529 A1 WO2012063529 A1 WO 2012063529A1 JP 2011066885 W JP2011066885 W JP 2011066885W WO 2012063529 A1 WO2012063529 A1 WO 2012063529A1
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- H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
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- H01L29/0642—Isolation within the component, i.e. internal isolation
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- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
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- H01L29/772—Field effect transistors
- H01L29/78—Field effect transistors with field effect produced by an insulated gate
- H01L29/7801—DMOS transistors, i.e. MISFETs with a channel accommodating body or base region adjoining a drain drift region
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- H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
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- H01L29/40—Electrodes ; Multistep manufacturing processes therefor
- H01L29/41—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions
- H01L29/417—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions carrying the current to be rectified, amplified or switched
- H01L29/41725—Source or drain electrodes for field effect devices
- H01L29/41741—Source or drain electrodes for field effect devices for vertical or pseudo-vertical devices
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- H01L29/66409—Unipolar field-effect transistors
- H01L29/66446—Unipolar field-effect transistors with an active layer made of a group 13/15 material, e.g. group 13/15 velocity modulation transistor [VMT], group 13/15 negative resistance FET [NERFET]
- H01L29/66462—Unipolar field-effect transistors with an active layer made of a group 13/15 material, e.g. group 13/15 velocity modulation transistor [VMT], group 13/15 negative resistance FET [NERFET] with a heterojunction interface channel or gate, e.g. HFET, HIGFET, SISFET, HJFET, HEMT
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- H01L29/66227—Multistep manufacturing processes of devices having semiconductor bodies comprising group 14 or group 13/15 materials the devices being controllable only by the electric current supplied or the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched, e.g. three-terminal devices
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- H01L29/66477—Unipolar field-effect transistors with an insulated gate, i.e. MISFET
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- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
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- H01L29/76—Unipolar devices, e.g. field effect transistors
- H01L29/772—Field effect transistors
- H01L29/778—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface
- H01L29/7789—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface the two-dimensional charge carrier gas being at least partially not parallel to a main surface of the semiconductor body
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- H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
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- H01L29/40—Electrodes ; Multistep manufacturing processes therefor
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- H01L29/423—Electrodes ; Multistep manufacturing processes therefor characterised by their shape, relative sizes or dispositions not carrying the current to be rectified, amplified or switched
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- H01L29/42316—Gate electrodes for field effect devices for field-effect transistors
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Definitions
- the present invention relates to a semiconductor device used for high-power switching and a manufacturing method thereof, and more particularly to a semiconductor device using a GaN-based semiconductor among nitride semiconductors and a manufacturing method thereof.
- a high current switching element is required to have a high reverse breakdown voltage and a low on-resistance.
- a field effect transistor (FET: Field Effect Transistor) using a group III nitride semiconductor is excellent in terms of high breakdown voltage, high temperature operation and the like because of its large band gap.
- vertical transistors using GaN-based semiconductors are attracting attention as high-power control transistors.
- an opening is provided in a GaN-based semiconductor, and a mobility is increased by providing a regrowth layer including a channel of a two-dimensional electron gas (2DEG: 2 Dimensional Electron Gas) on the wall surface of the opening.
- 2DEG 2 Dimensional Electron Gas
- a low on-resistance and an excellent withstand voltage performance can be obtained.
- a vertical semiconductor device used as a high-current switching element needs to have good high-frequency characteristics as well as low on-resistance and high breakdown voltage performance.
- the drain electrode into which electrons from the channel located in the opening flow in and the source electrode located on the epitaxial surface so as to face the drain electrode have a large area in order to obtain a low on-resistance.
- the source electrode has a large area in order to obtain a low contact resistance with the epitaxial surface layer.
- the source electrode and the drain electrode constitute a parallel plate capacitor and act as a parasitic capacitance between the source and drain electrodes.
- An object of the present invention is to provide a semiconductor device capable of improving high-frequency characteristics in a vertical semiconductor device provided with an opening and having a channel in the opening, and a method for manufacturing the same.
- the semiconductor device of the present invention is a vertical semiconductor device including a GaN-based stacked body provided with an opening.
- the GaN-based stacked body has an n-type GaN-based drift layer / p-type GaN-based barrier layer / n-type GaN-based contact layer sequentially toward the surface layer side, and the opening portion extends from the surface layer to the n-type GaN.
- a regrowth layer including an electron transit layer and an electron supply layer, and an n-type GaN-based contact layer, a regrowth layer, and a p-type GaN-based barrier that reach the system drift layer and cover the opening
- a source electrode positioned around the opening so as to be in contact with the layer; a drain electrode positioned centered on the opening with the source electrode and the GaN-based laminate sandwiched therebetween; and a gate positioned on the regrowth layer An electrode.
- a capacitor in which a dielectric material is disposed between the source electrode as one electrode and the drain electrode as the other electrode is provided with a capacity reduction structure that reduces the capacity of the capacitor.
- the source electrode and the drain electrode face each other, and a capacitor having a predetermined capacity is formed by the GaN-based stacked body filled therebetween. This is a parasitic capacitance, which degrades the high frequency characteristics. Since the configuration of the present invention includes the capacitance reduction structure, the parasitic capacitance can be reduced. As a result, the frequency limit of current gain or power gain can be expanded.
- Capacitance C ( ⁇ ⁇ S) where the capacitor having the predetermined capacity is approximated as a parallel plate capacitor, and the dielectric constant of the material filled between the electrodes is ⁇ , the area of the electrodes is S, and the distance between the electrodes is d. / D.
- the capacitance reduction structure is a structure that reduces (K1) the dielectric constant ⁇ , or (K2) the area S of the electrode that overlaps in plan view with respect to the capacitance C.
- the GaN-based laminate is formed on a conductive GaN-based substrate, the drain electrode is positioned on the conductive GaN-based substrate, and the source electrode and the conductive GaN-based substrate are planarly arranged.
- the n-type GaN-based drift layer is limited to a region including the bottom of the opening, and the n-type GaN-based drift layer is surrounded around the limited n-type GaN-based drift layer.
- a low dielectric constant material having a dielectric constant lower than the dielectric constant can be filled. As a result, the parasitic capacitance is reduced and the high frequency characteristics can be improved.
- the dielectric constant ⁇ is the product ⁇ r ⁇ ⁇ o of the relative dielectric constant ⁇ r representing the ratio to the vacuum and the dielectric constant ⁇ o of the vacuum.
- the dielectric constant refers to the relative dielectric constant.
- the low dielectric constant material may be at least one of air, an insulating film, a non-doped GaN-based semiconductor, and a GaN-based wide gap semiconductor having a larger band gap than the n-type GaN-based drift layer.
- the capacity reduction structure is different from the above, and the GaN-based laminate is formed on a high-resistance (insulating) GaN-based substrate, and the drain electrode is limited to a region including the bottom of the opening in plan view. It is located in the high-resistance GaN-based substrate and can contact the n-type GaN-based drift layer.
- one source electrode is positioned around the opening, and the other drain electrode is limited to a region including the bottom of the opening and is positioned in the high-resistance GaN-based substrate.
- the electrodes of the parallel plate capacitor have no overlapping portion in plan view. For this reason, although the capacity does not become zero, it significantly decreases and the high frequency characteristics can be improved.
- the drain electrode that is located in the high-resistance GaN-based substrate is positioned so as to have a portion that is exposed on the back surface of the high-resistance GaN-based substrate, or has a portion that is exposed on the back surface of the high-resistance GaN-based substrate. Can be positioned so that there is no.
- external wiring can be conductively connected from the back surface side of the high-resistance GaN-based substrate, and the wiring of the semiconductor device can be configured compactly.
- external wiring is connected from the side of the GaN-based laminate, but it may be advantageous depending on the application.
- the method for manufacturing a semiconductor device of the present invention is a method for manufacturing a vertical semiconductor device including a GaN-based stacked body provided with an opening.
- the manufacturing method includes a step of forming a GaN-based laminate including an n-type GaN-based drift layer / a p-type GaN-based barrier layer / an n-type GaN-based contact layer on a conductive GaN-based substrate, and an n-type A step of forming an opening from the GaN-based contact layer into the n-type GaN-based drift layer, a step of forming a regrowth layer including an electron transit layer and an electron supply layer so as to cover the opening, And a step of forming a source electrode in contact with the n-type GaN-based contact layer, the regrowth layer, and the p-type GaN-based barrier layer.
- the n-type GaN-based drift layer of the GaN-based stacked body is formed only in the region including the bottom of the opening, and around the n-type GaN-based drift layer.
- a material having a dielectric constant lower than that of the n-type GaN-based drift layer is formed.
- an insulating layer is formed, and then an opening is provided in the insulating layer in the region including the bottom of the opening, and the n-type GaN-based drift is formed in the opening of the insulating layer.
- Layers can be selectively grown.
- a semiconductor device having a low parasitic capacitance can be easily manufactured using an existing method. That is, while forming an n-type GaN-based drift layer in the region below the opening through which electrons flow, an insulating layer is formed with an insulating film such as SiO 2 having a low dielectric constant to reduce parasitic capacitance. A semiconductor device can be easily manufactured.
- the n-type GaN-based drift layer In the step of forming the n-type GaN-based drift layer, (1) an i-type GaN-based semiconductor layer is formed, and then an n-type impurity is implanted into a region that includes the bottom of the opening, or (2 ) After forming an n-type GaN-based semiconductor layer, p-type impurities are then implanted into a region that will be around the region including the bottom of the opening so as to offset the n-type impurities in the n-type GaN-based semiconductor layer can do.
- p-type impurities are then implanted into a region that will be around the region including the bottom of the opening so as to offset the n-type impurities in the n-type GaN-based semiconductor layer can do.
- n-type GaN-based drift layer forming step (1) an n-type GaN-based semiconductor layer is formed, and then a resist pattern is formed in which the region including the bottom of the opening is masked and the other region is the opening. Then, the n-type GaN-based semiconductor layer in the resist pattern opening is removed by etching, and then the GaN-based semiconductor layer or i-type having a larger band gap than the n-type GaN-based drift layer is removed in the region removed by the etching.
- Forming a GaN-based semiconductor layer or (2) forming a GaN-based semiconductor layer or i-type GaN-based semiconductor layer having a band gap larger than that of the n-type GaN-based drift layer, and then forming a region including the bottom of the opening.
- the resist pattern is formed by masking the other areas removed and using the area including the bottom of the opening as the opening.
- the GaN-based semiconductor layer or i-type GaN-based semiconductor layer is larger band gap in the emission opening is removed by etching, then, it can be a region that is removed by the etching to form the n-type GaN-based drift layer.
- an insulating layer is formed, and then an opening of the insulating layer is provided in a region that includes the bottom of the opening, and the n-type GaN-based drift is formed in the opening of the insulating layer.
- an insulating protective film is formed on the regrowth layer, and then the insulating layer is formed from the back surface of the conductive GaN substrate or from the insulating protective film.
- a trench to be exposed can be formed, and the insulating layer can be removed by wet etching from the trench and filled with air.
- a vertical semiconductor device including a GaN-based stacked body provided with an opening is manufactured.
- This manufacturing method includes a step of preparing a high-resistance GaN-based substrate having a drain electrode provided in a limited region, and an n-type GaN-based drift layer / p-type GaN-based barrier layer sequentially on the high-resistance GaN-based substrate.
- Forming a regrowth layer including an electron transit layer and an electron supply layer and forming a source electrode in contact with the n-type GaN-based contact layer, regrowth layer, and p-type GaN-based barrier layer around the opening
- the drain electrode region is limited to a range including the bottom of the opening in plan view.
- FIG. 1 is a cross-sectional view showing a vertical GaN-based FET (semiconductor device) in Embodiment 1 of the present invention (cross-sectional view taken along the line II of FIG. 3). It is sectional drawing of the semiconductor device which shows the modification of vertical GaN-type FET of FIG.
- FIG. 2 is a plan view of the vertical GaN-based FET of FIG. 1.
- FIG. 4 is a plan view showing a vertical GaN-based FET having the same cross-sectional view as FIG. 1 and having a form different from that of FIG. 3.
- FIG. 2 shows a method for manufacturing the vertical GaN-based FET of FIG.
- FIG. 5B is a view showing a method of manufacturing the vertical GaN-based FET of FIG. 1 and a state in which an opening is provided by etching the insulating layer using a resist pattern as a mask after the state shown in FIG. 5A.
- FIG. 5B is a view showing a method of manufacturing the vertical GaN-based FET of FIG. 1, and showing a state in which an n ⁇ -type GaN drift layer is further epitaxially grown in the opening after the state shown in FIG. 5B.
- FIG. 5C It is a figure which shows the manufacturing method of the vertical GaN-type FET of FIG. 1, and shows the state after lift-off, further removing a resist pattern after the state shown to FIG. 5C. It is a figure which shows the state in which the p-type GaN barrier layer and the n + type GaN contact layer were formed. It is a figure which shows the state which provided the opening part by the etching. It is a figure which shows the step which provides the opening part by RIE, and has shown the state which has arrange
- FIG. 14 is a cross-sectional view of a semiconductor device belonging to a fourth embodiment of the present invention, showing a modification of the semiconductor device shown in FIG. 13. It is a figure which shows the influence which the thickness has on the power gain cut-off frequency and the current gain cut-off frequency when the thickness of the silicon oxide layer disposed around the n ⁇ -type GaN drift layer limited in the embodiment is changed.
- FIG. 1 is a cross-sectional view showing a vertical GaN-based FET (semiconductor device) 10 according to Embodiment 1 of the present invention.
- the vertical GaN-based FET 10 includes a conductive GaN substrate 1 and an n ⁇ -type GaN drift layer 4 / p-type GaN barrier layer 6 / n + -type GaN contact layer 7 grown epitaxially thereon.
- the n ⁇ -type GaN drift layer 4 is an n-type GaN-based drift layer
- the p-type GaN barrier layer 6 is a p-type GaN-based barrier layer
- the n + -type GaN contact layer 7 is an n-type GaN-based contact layer.
- the n ⁇ -type GaN drift layer 4 is almost limited to the range of the opening 28 in a plan view and does not spread over the whole.
- the n - outside the -type GaN drift layer 4, the n - -type GaN drift layer insulating layer 31 the dielectric constant ⁇ is lower than 4 is filled.
- the insulating layer 31 is disposed between the conductive GaN substrate 1 and the p-type GaN barrier layer 6 on the outside of the n ⁇ -type GaN drift layer 4 positioned so as to be centered on the bottom 28 b of the opening.
- the insulating layer 31 may be anything as long as it has a dielectric constant lower than that of the n ⁇ -type GaN drift layer 4.
- An example is silicon oxide SiO 2 .
- the relative dielectric constant (1 MHz) of silicon oxide SiO 2 is 3.8 while the relative dielectric constant of semiconductor GaN is 9.5. For this reason, the capacity of the capacitor having the source electrode S and the drain electrode D as two electrodes is almost halved. As a result, the high frequency characteristics are improved.
- the n ⁇ -type GaN drift layer 4 / p-type GaN barrier layer 6 / n + -type GaN contact layer 7 limited by the insulating layer 31 constitutes a GaN-based stacked body 15.
- a buffer layer made of an AlGaN layer or a GaN layer may be inserted between the GaN substrate 1 and the n ⁇ -type GaN drift layer 4.
- the GaN substrate 1 may be a so-called thick GaN substrate, or a substrate having a GaN layer in ohmic contact with the support base.
- GaN substrate or the like during the growth of the GaN-based laminate, and in the subsequent process, except for a predetermined thickness portion such as the GaN substrate, only a thin GaN layer base remains in the product state. There may be.
- These GaN substrates, substrates having a GaN layer in ohmic contact with the support base, and underlying GaN layers left thin on the product may be simply referred to as GaN substrates.
- the insulating substrate is described in Embodiment 4, there is a difference between the conductive and insulating forms of the GaN substrate, but the other substrate forms are the same as those of the conductive substrate.
- the drain electrode can be provided on the front or back surface of the thin GaN layer, depending on the manufacturing process and the structure of the product.
- the supporting base or the substrate is conductive.
- the drain electrode can be directly provided on the back surface (lower surface) or front surface (upper surface) of the supporting base or substrate.
- the p-type GaN-based barrier layer is the p-type GaN barrier layer 6 in the present embodiment, but a p-type AlGaN layer may be used.
- the GaN layer described above may be used as another GaN-based semiconductor layer depending on the case.
- the GaN-based layered body 15, through the n + -type GaN contact layer 7 to the p-type GaN barrier layer 6 n - opening 28 leading to the -type GaN drift layer 4 is provided.
- the opening portion 28 is formed by a wall surface (side surface) 28a and a bottom portion 28b.
- An epitaxially grown regrowth layer 27 is formed so as to cover the wall surface 28a and bottom 28b of the opening 28 and the surface layer (n + -type GaN contact layer 7) of the GaN-based stacked body 15.
- the regrowth layer 27 includes an i (intrinsic) type GaN electron transit layer 22 and an AlGaN electron supply layer 26.
- An intermediate layer such as AlN may be inserted between the i-type GaN electron transit layer 22 and the AlGaN electron supply layer 26.
- the source electrode S is electrically connected to the regrowth layer 27, the n + -type contact layer 7, and the p-type GaN barrier layer 6 on the GaN-based stacked body 15. In FIG. 1, the source electrode S extends downward and contacts the end surface of the regrowth layer 27 and the n + -type contact layer 7 on its side surface, and contacts the p-type GaN barrier layer 6 on its tip portion. You have an electrical connection.
- the drain electrode D is located on the back surface of the conductive GaN substrate 1.
- the p-type GaN barrier layer 6 is necessary for improving the withstand voltage performance at the off time and for improving the pinch-off characteristics.
- the p-type GaN barrier layer 6 can stably improve the above-mentioned breakdown voltage performance and pinch-off characteristics.
- Another advantage that the p-type GaN barrier layer 6 is electrically connected to the source electrode S occurs in a depletion layer that can form a pn junction between the p-type GaN barrier layer 6 and the n ⁇ -type drift layer 4 at the time of reverse bias. It can absorb holes. This prevents a decrease in breakdown voltage due to the remaining holes, and a good breakdown voltage performance can be continuously obtained over a long period of time.
- the insulating film 9 is located under the gate electrode G so as to cover the regrowth layer 27.
- the insulating film 9 is arranged to suppress a gate leakage current when a positive voltage is applied to the gate electrode, and a large current operation is facilitated. Further, since the threshold voltage can be shifted in the positive direction, it is easy to obtain normally-off. However, the insulating film 9 may be omitted and is not essential.
- a two-dimensional electron gas (2DEG: 2 Dimensional Electron Gas) is generated in the regrowth layer 27 at the interface on the AlGaN electron supply layer 26 side in the i-type GaN electron transit layer 22.
- a two-dimensional electron gas is generated at the interface on the AlGaN layer side in the i-type GaN electron transit layer 22 due to natural polarization, piezo polarization, or the like due to the difference in lattice constant.
- Electrons take a path from the source electrode S through the two-dimensional electron gas to the n ⁇ -type GaN drift layer 4 to the drain electrode D.
- the i-type GaN electron transit layer 22 and the AlGaN electron supply layer 26 in the regrowth layer 27 are continuously grown in the same growth tank, the density of impurity levels at the interface can be kept low. For this reason, it is possible to flow a large current (per area) with a low on-resistance while providing the opening 28 and flowing a large current in the thickness direction.
- parasitic capacitance is formed between the source electrode S and the drain electrode D or the conductive GaN substrate 1, and the high frequency characteristics are not good.
- the high frequency characteristics are determined by, for example, a limit frequency (power gain cutoff frequency) f maz at which the power gain Gu cannot be obtained and / or a limit frequency (current gain cutoff frequency) fT at which the current gain
- n ⁇ -type GaN drift layer 4 Since the n ⁇ -type GaN drift layer 4 is positioned so as to be centered on the opening 28 and include the bottom 28b, the electron flow is not hindered by the insulating layer 31 during the ON operation, and the n ⁇ -type GaN drift layer 4, the conductive GaN substrate 1 / the drain electrode D is reached. Then, n - an outer mold GaN drift layer 4, between the conductive GaN substrate 1 and the p-type GaN barrier layer 6, n - insulating layer 31 having a lower dielectric constant than the -type GaN drift layer 4 is Be placed. For this reason, the parasitic capacitance is reduced, and the high frequency characteristics can be improved.
- the n ⁇ -type GaN drift layer 4 has an n-type impurity concentration of, for example, 1 ⁇ 10 15 (1E15) cm ⁇ 3 to 1 ⁇ 10 17 (1E17) cm ⁇ 3 and a thickness of, for example, 1.0 ⁇ m to 10. It should be 0 ⁇ m or less.
- the p-type impurity concentration of the p-type GaN barrier layer 6 is preferably about 1 ⁇ 10 17 (1E17) cm ⁇ 3 to 1 ⁇ 10 19 (1E19) cm ⁇ 3 .
- an impurity that forms an acceptor in a GaN-based semiconductor such as Mg is used as the p-type impurity.
- the thickness of the p-type GaN barrier layer 6 varies depending on the thickness of the n ⁇ -type GaN drift layer and the like. For this reason, the thickness range cannot be determined unconditionally. However, a typical thickness can be about 0.3 ⁇ m to 1 ⁇ m in view of the thickness used in many specifications. If it is thinner than this, sufficient pressure resistance performance and pinch-off characteristics cannot be obtained, so it may be regarded as the lower limit of the thickness. Since the p-type GaN barrier layer 6 has a thickness of about 0.3 ⁇ m to 1 ⁇ m, if the Mg concentration is too high, the p-type GaN barrier layer 6 moves linearly toward the end face of the p-type GaN barrier layer 6.
- the n + -type GaN contact layer 7 preferably has an n-type impurity concentration of about 5 ⁇ 10 17 (5E17) cm ⁇ 3 to 5 ⁇ 10 19 (5E19) cm ⁇ 3 .
- the thickness is preferably about 0.1 ⁇ m to 0.6 ⁇ m, and the length is preferably 0.5 ⁇ m to 5 ⁇ m.
- the n ⁇ -type GaN drift layer 4 is preferably widely arranged on the bottom 28 b side of the opening 28. That is, the insulating layer 31 is preferably disposed on the substrate 1 side. However, the insulating layer 31 may be disposed on the side in contact with the p-type GaN barrier layer 6.
- FIG. 3 is a plan view of the vertical GaN-based semiconductor device 10 shown in FIG. 1, and FIG. 1 is a cross-sectional view taken along the line II in FIG.
- the opening 28 has a hexagonal shape, and the periphery of the gate electrode 12 per unit area is obtained by covering the periphery thereof with the source electrode S and avoiding the gate wiring 12 to form the closest packing (honeycomb structure). Take longer. The on-resistance can also be lowered from the surface of such a shape. Further, from the large area of the source electrode S that can be read from the plan view, it is possible to know the significance of the parasitic capacitance that has been described so far, with the source electrode S and the drain electrode D or the conductive GaN substrate as two electrodes.
- FIG. 4 is a plan view showing an opening array and an electrode structure of a vertical GaN-based FET according to the present invention, which has a cross-sectional structure similar to that of FIG. 1 but has a planar structure different from that shown in FIG.
- a vertical GaN-based FET having the electrode structure shown in FIG. 4 is also an embodiment of the present invention. Even if the openings 28 are formed in an elongated rectangular shape and the elongated rectangular openings 28 are arranged densely, the perimeter of the opening per area can be increased, and as a result, the current density can be improved. .
- the gate electrode G and the source electrode S have a shape that extends to the other side so as to be orthogonal to the gate pad 13 and the source pad 14 that are parallel in the longitudinal direction, and are interdigitated. .
- a configuration in which the gate electrode G and the source electrode S are arranged in a comb shape is more common.
- an insulating layer 31 a having a dielectric constant smaller than that of the n ⁇ -type GaN drift layer 4 is formed on the conductive GaN substrate 1.
- a resist pattern M1 is provided, and then an insulating layer opening 31h is provided in a range where the n ⁇ -type GaN drift layer 4 is to be formed by wet etching (FIG. 5B).
- the opening 31h of the insulating layer is preferably in a range including the bottom 28b of the opening 28.
- the n ⁇ -type GaN drift layer 4 is grown on the conductive GaN substrate and in the opening 31h of the insulating layer using the resist pattern M1.
- An n ⁇ -type GaN deposition layer 4f deposited on the resist pattern M1 on the same occasion is also formed.
- the n ⁇ -type GaN deposition layer 4f is lifted off when the resist pattern M1 is removed.
- the p-type GaN barrier layer 6 is formed under conditions (growth rate, etc.) in which the epitaxial growth spreads on the insulating layer 31 with the n ⁇ -type GaN drift layer 4 as the center. .
- the n + -type GaN contact layer 7 is grown to complete the stacked body 15.
- a GaN-based buffer layer (not shown) may be inserted between the GaN substrate 1 and the n ⁇ -type GaN drift layer 4.
- MOCVD metal organic chemical vapor deposition
- the GaN substrate 1 when a gallium nitride film is grown on the conductive substrate by the MOCVD method, trimethylgallium is used as a gallium source.
- High purity ammonia is used as the nitrogen raw material.
- Purified hydrogen is used as the carrier gas.
- the purity of high purity ammonia is 99.999% or more, and the purity of purified hydrogen is 99.999995% or more.
- the conductive substrate a conductive gallium nitride substrate having a diameter of 2 inches is used.
- the opening 28 is formed by RIE (reactive ion etching).
- RIE reactive ion etching
- FIGS. 8A and 8B after forming a resist pattern M2 on the surfaces of the epitaxial layers 4, 6, and 7, the opening is widened while the resist pattern M2 is etched back by RIE to widen the opening. .
- the slope of the opening 28, that is, the end face of the laminate 15 is damaged by being irradiated with ions. In the damaged portion, a dangling bond, a high density region of lattice defects, and the like are generated, and conductive impurities from the RIE apparatus or from a portion that cannot be specified reach the damaged portion to cause enrichment.
- the occurrence of the damaged portion causes an increase in drain leakage current and needs to be repaired.
- hydrogen and ammonia at a predetermined level, it is possible to obtain dangling bond repair, removal of impurities, and inactivation when the regrowth layer 27 described later is grown.
- the wafer is introduced into an MOCVD apparatus, and as shown in FIG. 9, an electron transit layer 22 made of undoped GaN and an electron supply layer 26 made of undoped AlGaN.
- a regrowth layer 27 containing GaN is grown.
- thermal cleaning is performed in an (NH 3 + H 2 ) atmosphere, and then an organometallic raw material is supplied while introducing (NH 3 + H 2 ).
- restoration of the damaged portion, removal of conductive impurities, and passivation are performed.
- the wafer is taken out of the MOCVD apparatus, and an insulating film 9 is grown as shown in FIG. Thereafter, again using photolithography and electron beam evaporation, the source electrode S is formed on the surface of the epitaxial layer and the drain electrode D is formed on the back surface of the GaN-based substrate 1 as shown in FIG.
- FIG. 11 is a sectional view showing a vertical GaN-based FET (semiconductor device) 10 according to the second embodiment of the present invention.
- the n ⁇ -type GaN drift layer 4 is limited to a range including the bottom 28 b of the opening 28, and the periphery of the limited n ⁇ -type GaN drift layer 4 is an air layer 30. Air can be seen to be almost the same, with a dielectric constant just a little higher than vacuum. Therefore, the relative dielectric constant is about 1, which is smaller than that of any material. Since the relative permittivity of the n ⁇ -type GaN drift layer 4 is roughly 10, the use of air can greatly reduce the parasitic capacitance.
- FIG. 1 shows that the entire thickness around the n ⁇ -type GaN drift layer 4 is replaced with an insulator having a lower dielectric constant
- FIG. 2 shows that the entire thickness may be partially replaced.
- the entire thickness around the n ⁇ -type GaN drift layer 4 shown in FIG. 11 is not the air layer 30, the thickness is partially set as the air layer 30, and the remaining thickness is set as the n ⁇ -type GaN drift layer 4. Also good.
- the manufacturing method of the semiconductor device of this embodiment is performed in the following steps. First, the process proceeds in common with the manufacturing method of the semiconductor device of the first embodiment up to the state shown in FIG.
- Intermittent trenches are provided in layer 7 / p-type GaN barrier layer 6. For this purpose, a resist pattern having an opening in the trench is formed, and then a trench reaching the insulating layer 31 is formed by dry or wet etching.
- the same resist pattern or a new resist pattern is formed, an etching solution is injected from the trench, and the insulating layer 31 is removed by wet etching.
- the insulating layer 31 is preferably left with a predetermined minute thickness on both the wall surface and the bottom without removing the entire thickness.
- the formation of the metal layer that closes the trench is preferably performed in an arrangement or posture in which the trench is shielded at an angle.
- FIG. 12 is a cross-sectional view showing a vertical GaN-based FET (semiconductor device) 10 according to the third embodiment of the present invention.
- the n ⁇ -type GaN drift layer 4 is limited to a range including the bottom portion 28 b of the opening 28, and the periphery of the limited n ⁇ -type GaN drift layer 4 is an impurity such as an i-type GaN layer.
- Intrinsic semiconductor layer 32 that does not contain is disposed over the entire thickness of n ⁇ -type GaN drift layer 4. By reducing the n-type impurity, the intrinsic semiconductor layer 32 has a low dielectric constant.
- the semiconductor layer 32 having a reduced n-type impurity concentration such as i-type GaN may be a GaN-based semiconductor 32 (for example, AlInGaN) having a band gap larger than that of the n ⁇ -type GaN drift layer 4.
- the GaN-based semiconductor 32 having a band gap larger than that of the n ⁇ -type GaN drift layer 4 has a dielectric constant smaller than that of the n ⁇ -type GaN drift layer 4 as in the case of the intrinsic GaN-based semiconductor having a low impurity concentration.
- the GaN-based semiconductor 32 having a band gap larger than that of the n ⁇ -type GaN drift layer 4 may include an n-type impurity in the n ⁇ -type category. I-type GaN layer 32 or the band gap is low n-type impurity of the the n - -type GaN drift layer GaN-based semiconductor 32 is greater than 4, n - may not replace the entire thickness of the surrounding -type GaN drift layer 4 . As described above, it is only necessary to partially occupy the thickness around the n ⁇ -type GaN drift layer 4, and the n ⁇ -type GaN drift layer 4 may extend from the rest.
- FIG. 13 is a cross-sectional view showing a vertical GaN-based FET (semiconductor device) 10 according to the fourth embodiment of the present invention.
- the vertical GaN-based FET 10 includes an insulating GaN substrate 11 and an n ⁇ -type GaN drift layer 4 / p-type GaN barrier layer 6 / n + -type GaN contact layer 7 grown epitaxially thereon.
- the n ⁇ -type GaN drift layer 4 is not limited to the range including the bottom 28b of the opening 28.
- the present embodiment is characterized in that the drain electrode D is formed in the high-resistance GaN substrate or the insulating substrate 11.
- the drain electrode D is limited to a range facing the bottom 28 b of the opening 28 and penetrates the insulating GaN substrate 11. Due to the arrangement structure of the drain electrode D, there is no portion where the conductive portion such as the drain electrode D or the conductive substrate overlaps the source electrode S in a plan view. As a result, the image of a parallel plate capacitor is not established. As a result, the parasitic capacitance is greatly reduced and the high frequency characteristics are improved. Since the drain electrode D penetrates the insulating substrate 11 and is also exposed on the back surface of the insulating substrate 11, the wiring can be taken from the back surface of the insulating substrate 11. As a result, a compact wiring structure can be formed.
- the insulating GaN-based substrate 11 can be easily obtained by forming the drain electrode D in a predetermined region of the insulating GaN-based substrate 11 using existing equipment and a metal layer forming method. it can.
- FIG. 14 shows a modification of the semiconductor device of FIG. 13 (modification of the fourth embodiment), which is one embodiment of the present invention.
- the drain electrode D is formed in the high-resistance GaN substrate or the insulating substrate 11, and the drain electrode D is limited to the range facing the bottom portion 28 b of the opening 28.
- the drain electrode D is in contact with the GaN-based stacked body 15, but is not exposed on the back surface of the insulating GaN substrate 11. For this reason, the wiring connected to the drain electrode D needs to be accessed from the GaN-based laminate 15 side.
- the S (RF) parameter was simulated by changing the thickness of the insulating layer 31.
- the form of the semiconductor device 10 is as follows.
- n ⁇ -type GaN drift layer 4 The thickness of the n ⁇ -type GaN drift layer including the bottom 28b of the opening 28 is set to 5 ⁇ m, and the n-type impurity concentration is the same for all the specimens, 1 ⁇ 10 16 ( 1E16) cm ⁇ 3 .
- the insulating layer 31 disposed around the n ⁇ -type GaN drift layer 4 was made of SiO 2 (relative dielectric constant 3.8).
- the n - -type GaN drift layer 4 of lower thickness (substrate side) portion only to the upper (opening side) of the n - -type GaN drift layer 4 has a extending Embodiment (FIG. 2 reference).
- the thickness of the insulating layer 31 was changed from zero (same as a conventional semiconductor device) to a range of 3.5 ⁇ m. When the thickness is 3.5 ⁇ m, 70% of the total thickness of the n ⁇ -type GaN drift layer 4 is replaced with SiO 2 .
- 2 ) was determined by computer simulation, and the cutoff frequency at which the power gain and the current gain were 1 was determined.
- the results are shown in FIG. According to FIG. 15, the current gain cutoff frequency fT tends to hardly change or slightly decrease as the insulating layer 31 increases.
- the power cut-off frequency f max rises in a thicker range with the silicon oxide thickness of 1.5 ⁇ m as the bottom. In particular, when the thickness of silicon oxide is 3.5 ⁇ m, the highest power cutoff frequency f max is shown. In the example, silicon oxide is considered only up to a range that occupies 70% of the total thickness of the n ⁇ -type GaN drift layer 4.
- the power cutoff frequency f max is further expanded, and the current gain cutoff frequency fT is considered to show a balanced value.
- the thickness of the insulating layer 31 in FIG. 2 is increased to be the same as the thickness of the n ⁇ -type GaN drift layer 4 as shown in FIG. The frequency f max can be obtained.
- the periphery of the n ⁇ -type GaN drift layer located at the bottom of the opening between the source electrode and the drain electrode or the conductive substrate Is formed of an insulating layer, a semiconductor, or the like having a lower dielectric constant, thereby reducing parasitic capacitance and improving high-frequency characteristics.
- the drain electrode limited to the area
- GaN substrate 4 n ⁇ type GaN drift layer, 6 p type GaN barrier layer, 7 n + type GaN contact layer, 9 insulating film, 10 semiconductor device (vertical GaN-based FET), 11 insulating (high resistance) substrate , 12 gate wiring, 13 gate pad, 14 source pad, 15 GaN-based stacked body, 22 GaN electron transit layer, 26 AlGaN electron supply layer, 27 regrowth layer, 28 opening, 28a opening wall, 28b opening Bottom, 30 Air layer, 31 Insulating layer (SiO 2 ), 31h Insulating layer opening, 32 i-type GaN layer or GaN-based semiconductor whose band gap is larger than GaN, D drain electrode, G gate electrode, K Edge of opening Or corner, M1 resist pattern, S source electrode.
- SiO 2 Insulating layer
- 31h Insulating layer opening 32 i-type GaN layer or GaN-based semiconductor whose band gap is larger than GaN
- D drain electrode D drain electrode
- G gate electrode K Edge of opening Or corner
- M1 resist pattern S
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Abstract
Description
しかし、高電流のスイッチング素子として用いられる縦型半導体装置には、低いオン抵抗および高耐圧性能とともに、良好な高周波特性を備えることが必要である。
上記開口部に位置するチャネルからの電子が流入するドレイン電極と、そのドレイン電極に対向するようにエピタキシャル表層に位置するソース電極とは、低いオン抵抗を得るために、大きな面積をとる。とくにソース電極は、エピタキシャル表層と低い接触抵抗を得るために広い面積となる。この結果、ソース電極とドレイン電極とは平行平板コンデンサを構成し、ソース・ドレイン電極間の寄生容量として作用する。この寄生容量は、電力利得などの周波数限界を小さくするなどして、高周波特性を劣化させる。
本発明は、開口部が設けられ、当該開口部にチャネルを備える縦型半導体装置において、高周波特性を向上することができる半導体装置およびその製造方法を提供することを目的とする。
本発明の構成では容量低下構造を備えるので、寄生容量を低下させることができる。この結果、電流利得または電力利得の周波数限界を拡大することができる。
上記の所定容量のコンデンサを平行平板コンデンサと近似して、電極間に充填された材料の誘電率をε、電極の面積をS、電極間の距離をdとして、容量C=(ε・S)/d、と見積もる。上記の容量低下構造は、この容量Cについて、(K1)誘電率εを低下させる構造、か、または(K2)平面的にみて重複する部分の電極の面積Sを減少する構造である。
これによって、寄生容量は低下して、高周波特性を向上させることができる。なお、誘電率εは、真空に対する比を表す比誘電率εrおよび真空の誘電率εoの積εr・εoであり、材料間の誘電率の比較をする場合、比誘電率で説明すれば十分である。以後の説明では、とくに断らない限り、誘電率といえば比誘電率をさしている。
これによって、約9.5の比誘電率を有するGaN系ドリフト層を、空気(比誘電率約1)、SiO2(比誘電率3.5~4.0)などに置き換えることで、容量を低下させることができる。
この構成によれば、一方のソース電極は開口部の周囲に位置し、他方のドレイン電極は開口部の底部を含む領域に限定されて、高抵抗GaN系基板内に位置する。この配置では、平行平板コンデンサの電極は平面的にみて重複部分がない。このため、容量はゼロにはならないが大幅に低下し、高周波特性を向上させることができる。
基板の裏面に露出する部分を持つドレイン電極の場合、高抵抗GaN系基板の裏面側から外部配線を導電接続することができ、半導体装置の配線をコンパクトに構成することができる。基板の裏面に露出する部分を持たない場合、GaN系積層体の側から外部配線を接続することになるが、用途によっては好都合の場合もある。
この方法によって、寄生容量を低下させた半導体装置を既存の製造装置を用いて簡単に製造することができる。
この方法によって、既存の方法を用いて簡単に寄生容量の低い半導体装置を製造することができる。すなわち、電子が流れる開口部の下方領域にn型GaN系ドリフト層を形成しながら、その周囲を誘電率の低いSiO2などの絶縁膜で絶縁層を形成して、寄生容量の低減をはかった半導体装置を簡単に製造することができる。
この方法で、寄生容量の小さい半導体装置を簡単に得ることができる。
上記の方法によって、寄生容量の小さい半導体装置を比較的簡単に得ることができる。
上記の方法によって、比誘電率が小さい(約1)の空気層をn型GaN系ドリフト層の周りに配置した半導体装置を簡単に製造することができる。
この方法によって、ともに導電性部分であるソース電極とドレイン電極とが平面的にみて重複しないか、または重複部分が非常に小さい、半導体装置を比較的容易に製造することができる。
図1は、本発明の実施の形態1における縦型GaN系FET(半導体装置)10を示す断面図である。縦型GaN系FET10は、導電性のGaN基板1と、その上にエピタキシャル成長した、n-型GaNドリフト層4/p型GaNバリア層6/n+型GaNコンタクト層7、を備える。ここで、n-型GaNドリフト層4がn型GaN系ドリフト層であり、p型GaNバリア層6がp型GaN系バリア層、n+型GaNコンタクト層7がn型GaN系コンタクト層である。n-型GaNドリフト層4は、平面的に見てほぼ開口部28の範囲に限定されていて、全体に広がっていない。n-型GaNドリフト層4の外側には、そのn-型GaNドリフト層4よりも誘電率εが小さい絶縁層31が充填されている。すなわち開口部の底部28bに中心を合わせて位置するn-型GaNドリフト層4の外側であって、導電性GaN基板1とp型GaNバリア層6との間に、絶縁層31が配置される。絶縁層31は、n-型GaNドリフト層4よりも誘電率が低ければ何でもよい。
たとえば酸化珪素SiO2を挙げることができる。半導体GaNの比誘電率が9.5であるのに対して、酸化珪素SiO2の比誘電率(1MHz)は3.8である。このため、ソース電極Sとドレイン電極Dとを二電極とするコンデンサの容量は、ほぼ半減する。この結果、高周波特性は向上する。
なお、GaN基板1は、導電性であれば、いわゆる一体物の厚手のGaN基板でも、または支持基体上にオーミック接触するGaN層を有する基板であってもよい。さらに、GaN系積層体の成長時にGaN基板等の上に形成して、その後の工程で、GaN基板等の所定厚み部分を除いて、製品の状態では薄いGaN層下地のみが残っているものであってもよい。これら、GaN基板、支持基体上にオーミック接触するGaN層を有する基板、製品に薄く残された下地のGaN層などを、単にGaN基板と略称する場合もある。絶縁性基板は、実施の形態4において触れるが、GaN基板の形態として導電性かまたは絶縁性かの違いはあるが、その他の基板の形態としては、上記導電性基板の場合と変わりはない。
上記の薄い下地の導電性GaN層の場合、ドレイン電極は、製造工程および製品の構造によるが、薄いGaN層の表面または裏面に設けることができる。本実施の形態では、GaN基板または支持基体等が製品に残る場合、当該支持基体または基板は、導電性とする。導電性の場合は、ドレイン電極は、その支持基体または基板の裏面(下面)またはおもて面(上面)に直接設けることができる。
また、p型GaN系バリア層は、本実施の形態ではp型GaNバリア層6としているが、p型AlGaN層を用いてもよい。積層体15を構成するその他の層についても、場合に応じて、上記に示したGaN層を他のGaN系半導体層としてよい。
i型GaN電子走行層22とAlGaN電子供給層26との間にAlN等の中間層を挿入してもよい。ソース電極Sは、GaN系積層体15上において、再成長層27、n+型コンタクト層7、およびp型GaNバリア層6に電気的に接続する。図1では、ソース電極Sは、下方に延在して、その側面で再成長層27の端面およびn+型コンタクト層7に接触し、その先端部でp型GaNバリア層6に接触して電気的接続を得ている。ドレイン電極Dは導電性GaN基板1の裏面に位置する。
開口部28に中心を合わせ、底部28bを含むようにn-型GaNドリフト層4が位置することで、オン動作時、電子流は絶縁層31に妨げられることなく、そのn-型GaNドリフト層4を通って、導電性GaN基板1/ドレイン電極Dに到達する。そして、n-型GaNドリフト層4の外側であって、導電性GaN基板1とp型GaNバリア層6との間には、n-型GaNドリフト層4よりも誘電率が低い絶縁層31が配置される。このため寄生容量は小さくなり、高周波特性を向上することができる。
p型GaNバリア層6のp型不純物濃度は、1×1017(1E17)cm-3~1×1019(1E19)cm-3程度とするのがよい。p型不純物には、MgなどのGaN系半導体中にアクセプタを形成する不純物が用いられる。また、p型GaNバリア層6の厚みは、n-型GaNドリフト層の厚み等によって変わる。このため、厚み範囲は一概に決めることはできない。しかし、代表的な厚みについては、多くの仕様において用いられる厚みという点から、0.3μm~1μm程度をあげることができる。これより薄いと、十分な耐圧性能やピンチオフ特性を得られないので、厚みの下限とみてもよい。このp型GaNバリア層6は、この0.3μm~1μm程度の厚みを持つことから、あまり高濃度のMg濃度を含有させると、p型GaNバリア層6の端面に向かって直線的に移動してチャネルに悪影響(オン抵抗の増大)を及ぼす。また、チャネルOFF時のn-型GaNドリフト層とのpn接合での逆方向特性(耐圧性能)を劣化させる。
n+型GaNコンタクト層7のn型不純物濃度は5×1017(5E17)cm-3~5×1019(5E19)cm-3程度とするのがよい。また厚みは、0.1μm~0.6μm程度とし、長さは、0.5μm以上5μm以下とするのがよい。
電流は、ソース電極Sから、直接に、またはn+型GaNコンタクト層7を経由して、再成長層27内のチャネル(電子走行層22)に入り、GaNドリフト層4を経て、ドレイン電極Dへと流れる。ソース電極Sおよびその配線と、ゲート電極G、ゲート配線12およびゲートパッド13から構成されるゲート構成体とが、相互に干渉しないために、ソース配線は、図示しない層間絶縁膜上に設けられる。層間絶縁膜にはビアホールが設けられ、そのビアホールに充填された導電部を含むソース電極Sは、層間絶縁膜上のソース導電層(図示せず)と導電接続される。このような構造によって、ソース電極Sを含むソース構成体は、大電力用の素子に好適な、低い電気抵抗および高い移動度、を持つことができる。
図4は、図1と同じような断面構造を有しながら、平面構造は図3に示すものと異なる、本発明の縦型GaN系FETの開口部配列および電極構造を示す平面図である。この図4に示す電極構造等を有する縦型GaN系FETも本発明の実施の形態例である。開口部28を細長い矩形状にして、その細長い矩形状の開口部28を密に配置することでも、上記の面積当たりの開口部周囲長を大きくでき、この結果、電流密度を向上させることができる。この場合、ゲート電極Gとソース電極Sとは、長手方向を並行させたゲートパッド13とソースパッド14とに直交するように相手側へ延び出て、相互に櫛歯状に入り組む形状を呈する。高周波用の縦型GaN系FETとしては、図4に示すように、ゲート電極Gとソース電極Sとが櫛歯状に配置される形態のほうが普通である。
上記の層の形成は、MOCVD(有機金属化学気相成長)法などを用いるのがよい。たとえばMOCVD法で成長することで、結晶性の良好な積層体15を形成できる。GaN基板1の形成において、導電性基板上に窒化ガリウム膜をMOCVD法によって成長させる場合、ガリウム原料として、トリメチルガリウムを用いる。窒素原料としては高純度アンモニアを用いる。キャリアガスとしては純化水素を用いる。高純度アンモニアの純度は99.999%以上、純化水素の純度は99.999995%以上である。n型ドーパント(ドナー)のSi原料には水素ベースのシランを用い、p型ドーパント(アクセプタ)のMg原料にはシクロペンタジエニルマグネシウムを用いるのがよい。
導電性基板としては、直径2インチの導電性窒化ガリウム基板を用いる。温度1030℃、圧力100Torrで、アンモニアおよび水素の雰囲気中で、基板クリーニングを実施する。その後、1050℃に昇温して、圧力200Torr、窒素原料とガリウム原料の比率であるV/III比=1500で窒化ガリウム層を成長させる。
次いで、上記ウエハをMOCVD装置から取り出し、図10に示すように、絶縁膜9を成長させる。その後、再びフォトリソグラフィと電子ビーム蒸着法を用いて、図1に示すように、ソース電極Sをエピタキシャル層表面に、ドレイン電極DをGaN系基板1の裏面に形成する。
図11は、本発明の実施の形態2における縦型GaN系FET(半導体装置)10を示す断面図である。本実施の形態では、n-型GaNドリフト層4は、開口部28の底部28bを含む範囲に限定され、その限定されたn-型GaNドリフト層4の周囲は空気層30とされている。空気は、誘電率が真空より少し高いだけで、ほぼ同じとみることができる。したがって比誘電率は約1であり、どのような物質よりも小さい誘電率である。n-型GaNドリフト層4の比誘電率は、大雑把に10であるので、空気とすることで、寄生容量を大幅に減少させることができる。
図12は、本発明の実施の形態3における縦型GaN系FET(半導体装置)10を示す断面図である。本実施の形態では、n-型GaNドリフト層4は、開口部28の底部28bを含む範囲に限定され、その限定されたn-型GaNドリフト層4の周囲は、i型GaN層などの不純物を含まない真性半導体層32がn-型GaNドリフト層4の全厚みに配置されている。n型不純物を低くすることで、真性半導体層32は、その誘電率が低くなる。その結果、寄生容量を低くでき、高周波特性を向上させることができる。
上記のi型GaNなどのn型不純物濃度を低減された半導体層32は、バンドギャップがn-型GaNドリフト層4よりも大きいGaN系半導体32(たとえば、AlInGaN)であってもよい。バンドギャップがn-型GaNドリフト層4よりも大きいGaN系半導体32は、不純物濃度が低い真性GaN系半導体と同様に、誘電率がn-型GaNドリフト層4よりも小さい。バンドギャップがn-型GaNドリフト層4よりも大きいGaN系半導体32を周囲に配置することで、寄生容量を低下させることができ、高周波特性を向上することができる。バンドギャップがn-型GaNドリフト層4よりも大きいGaN系半導体32は、n-型の範疇でn型不純物を含んでもよい。
上記のn型不純物が低いi型GaN層32またはバンドギャップがn-型GaNドリフト層4よりも大きいGaN系半導体32は、n-型GaNドリフト層4の周囲の全厚みを置き換えなくてもよい。上述のように、n-型GaNドリフト層4の周囲において、厚みを部分的に占めるだけでよく、残りはn-型GaNドリフト層4が延在してもよい。
図13は、本発明の実施の形態4における縦型GaN系FET(半導体装置)10を示す断面図である。縦型GaN系FET10は、絶縁性GaN基板11と、その上にエピタキシャル成長した、n-型GaNドリフト層4/p型GaNバリア層6/n+型GaNコンタクト層7、を備える。実施の形態1~3における半導体装置と異なり、本実施の形態では、n-型GaNドリフト層4は開口部28の底部28bを含む範囲に限定されない。
本実施の形態では、高抵抗GaN基板または絶縁性基板11内にドレイン電極Dが形成されている点に特徴を有する。とくにドレイン電極Dは、開口部28の底部28bに面する範囲に限定され、絶縁性GaN基板11を貫通している。このドレイン電極Dの配置構造によって、平面的にみてドレイン電極Dや導電性基板などの導電性部分が、ソース電極Sと重なる部分はなくなる。この結果、平行平板コンデンサという描像は成立しなくなる。この結果、寄生容量は大きく低下して、高周波特性は向上する。
上記のドレイン電極Dは、絶縁性基板11を貫通しており、絶縁性基板11の裏面にも露出するので、配線を絶縁性基板11の裏面からとることができる。この結果、コンパクトな配線構造を形成することができる。
上記の絶縁性GaN系基板11は、既存の設備および金属層形成方法を用いて、ドレイン電極Dを絶縁性GaN系基板11の所定領域内に限定して形成することで、容易に得ることができる。
しかし、本変形例では、ドレイン電極Dは、GaN系積層体15に接しているが、絶縁性GaN基板11の裏面に露出していない。このため、ドレイン電極Dに接続する配線は、GaN系積層体15の側からアクセスする必要がある。
n-型GaNドリフト層4:開口部28の底部28bを含むn-型GaNドリフト層の厚みを5μmとし、そのn型不純物濃度は、すべての試験体に対して同一の、1×1016(1E16)cm-3とした。
n-型GaNドリフト層4の周囲に配置する絶縁層31は、SiO2(比誘電率3.8)とした。また、その配置は、n-型GaNドリフト層4の厚みの下側(基板側)部分のみとして、上側(開口部側)はn-型GaNドリフト層4が延在する形態とした(図2参照)。絶縁層31の厚みは、ゼロ(従来の半導体装置と同じ)から3.5μmの範囲に変化させた。厚み3.5μmの場合、n-型GaNドリフト層4の全厚みの70%をSiO2で置き換えたことになる。
結果を、図15に示す。図15によれば、電流利得遮断周波数fTは、絶縁層31の増大につれてほとんど変わらないか、または少し低下する傾向がみられる。しかし、電力遮断周波数fmaxは、酸化ケイ素の厚み1.5μmを底にして、より厚い範囲で上昇する。
とくに酸化ケイ素の厚み3.5μmでは、最高の電力遮断周波数fmaxを示す。
実施例では、酸化ケイ素は、n-型GaNドリフト層4の全厚みの70%を占める範囲までしか検討していない。酸化ケイ素の比率をこれより高めることで、より一層、電力遮断周波数fmaxは拡大し、電流利得遮断周波数fTもこれに均衡のとれた値を示すものと考えられる。製造上の制御は難しいが、図2における絶縁層31の厚みをより厚くして、たとえば図1に示すようにn-型GaNドリフト層4の厚みと同じにすることで、より一層高い電力遮断周波数fmaxを得ることができる。
Claims (11)
- 開口部が設けられたGaN系積層体を備える縦型の半導体装置であって、
前記GaN系積層体は、表層側へと順次、n型GaN系ドリフト層/p型GaN系バリア層/n型GaN系コンタクト層、を有し、前記開口部は表層から前記n型GaN系ドリフト層内にまで届いており、
該開口部を覆うように位置する、電子走行層および電子供給層を含む再成長層と、
前記n型GaN系コンタクト層、前記再成長層および前記p型GaN系バリア層に接するように、前記開口部の周囲に位置するソース電極と、
前記ソース電極と前記GaN系積層体を挟んで、前記開口部に中心を合わせて位置するドレイン電極と、
前記再成長層上に位置するゲート電極とを備え、
前記ソース電極を一方の電極とし、かつ前記ドレイン電極を他方の電極としてその間に誘電材料が配置されたコンデンサを構成するとみて、該コンデンサの容量を低下させる構造である容量低下構造を備えることを特徴とする、半導体装置。 - 前記容量低下構造であって、前記GaN系積層体は導電性GaN系基板上に形成され、前記ドレイン電極は該導電性GaN系基板に位置し、前記ソース電極と前記導電性GaN系基板とは、平面的に見て重複していて、前記n型GaN系ドリフト層は、前記開口部の底部を含む領域に限定されており、その限定されたn型GaN系ドリフト層の周りに、該n型GaN系ドリフト層の誘電率よりも低い誘電率を有する低誘電率材料が充填されていることを特徴とする、請求項1に記載の半導体装置。
- 前記低誘電率材料が、空気、絶縁膜、ノンドープGaN系半導体、および前記n型GaN系ドリフト層よりも大きいバンドギャップを有するGaN系ワイドギャップ半導体のうちの、少なくとも1つであることを特徴とする、請求項2に記載の半導体装置。
- 前記容量低下構造であって、前記GaN系積層体は高抵抗GaN系基板上に形成され、前記ドレイン電極は、平面的にみて前記開口部の底部を含む領域に限定されるように前記高抵抗GaN系基板内に位置し、前記n型GaN系ドリフト層に接することを特徴とする、請求項1に記載の半導体装置。
- 前記ドレイン電極が、前記高抵抗GaN系基板の裏面に露出する部分を持つように位置するか、または、前記高抵抗GaN系基板の裏面に露出する部分を持たないように位置することを特徴とする、請求項4に記載の半導体装置。
- 開口部が設けられたGaN系積層体を備える縦型の半導体装置の製造方法であって、
導電性GaN系基板上に、順次、n型GaN系ドリフト層/p型GaN系バリア層/n型GaN系コンタクト層、を含む前記GaN系積層体を形成する工程と、
前記n型GaN系コンタクト層から前記n型GaN系ドリフト層内に届く前記開口部を形成する工程と、
前記開口部を覆うように、電子走行層および電子供給層を含む再成長層を形成する工程と、
前記開口部の周囲に、前記n型GaN系コンタクト層、前記再成長層および前記p型GaN系バリア層に接するようにソース電極を形成する工程とを備え、
前記GaN系積層体のn型GaN系ドリフト層の形成工程では、前記開口部の底部を含む領域に限定して該n型GaN系ドリフト層を形成し、当該n型GaN系ドリフト層の周囲に、そのn型GaN系ドリフト層の誘電率よりも低い誘電率を有する材料を形成することを特徴とする、半導体装置の製造方法。 - 前記n型GaN系ドリフト層の形成工程では、絶縁層を形成し、次いで、前記開口部の底部を含むことになる領域の絶縁層に開口部を設け、その絶縁層の開口部内に前記n型GaN系ドリフト層を選択成長させることを特徴とする、請求項6に記載の半導体装置の製造方法。
- 前記n型GaN系ドリフト層の形成工程では、(1)i型GaN系半導体層を形成し、次いで、前記開口部の底部を含むことになる領域にn型不純物を注入するか、または(2)n型GaN系半導体層を形成し、次いで、前記開口部の底部を含む領域の周りになる領域にp型不純物を、前記n型GaN系半導体層におけるn型不純物を相殺するように注入することを特徴とする、請求項6に記載の半導体装置の製造方法。
- 前記n型GaN系ドリフト層の形成工程では、(1)n型GaN系半導体層を形成し、次いで前記開口部の底部を含む領域をマスクしてその他の領域を開口部とするレジストパターンを形成し、そのレジストパターン開口部におけるn型GaN系半導体層をエッチングによって除去し、次いで、そのエッチングによって除去された領域に、前記n型GaN系ドリフト層よりもバンドギャップが大きいGaN系半導体層もしくはi型GaN系半導体層を形成する、か、または(2)前記n型GaN系ドリフト層よりもバンドギャップが大きいGaN系半導体層もしくはi型GaN系半導体層を形成し、次いで前記開口部の底部を含む領域を除いたその他の領域をマスクして前記開口部の底部を含む領域を開口部とするレジストパターンを形成し、前記レジストパターン開口部における前記バンドギャップが大きいGaN系半導体層もしくは前記i型GaN系半導体層をエッチングによって除去し、次いで、そのエッチングによって除去された領域に、前記n型GaN系ドリフト層を形成することを特徴とする、請求項6に記載の半導体装置の製造方法。
- 前記n型GaN系ドリフト層の形成工程では、絶縁層を形成し、次いで、前記開口部の底部を含むことになる領域に絶縁層の開口部を設け、該絶縁層の開口部内に前記n型GaN系ドリフト層を選択成長させ、その後、前記再成長層を形成したあと、該再成長層上に絶縁性保護膜を形成し、次いで、前記導電性GaN基板の裏面から、または前記絶縁性保護膜から、前記絶縁層を露出させるトレンチを形成し、該トレンチからの湿式エッチングにより前記絶縁層を除去して空気を充填させることを特徴とする、請求項6に記載の半導体装置の製造方法。
- 開口部が設けられたGaN系積層体を備える縦型の半導体装置の製造方法であって、
ドレイン電極が領域を限定して設けられた高抵抗GaN系基板を準備する工程と、
前記高抵抗GaN系基板上に、順次、n型GaN系ドリフト層/p型GaN系バリア層/n型GaN系コンタクト層、を含むGaN系積層体を形成する工程と、
前記n型GaN系コンタクト層から前記n型GaN系ドリフト層内に届く開口部を形成する工程と、
前記開口部を覆うように、電子走行層および電子供給層を含む再成長層を形成する工程と、
前記開口部の周囲に、前記n型GaN系コンタクト層、前記再成長層および前記p型GaN系バリア層に接するようにソース電極を形成する工程とを備え、
前記ドレイン電極の領域は、平面的にみて前記開口部の底部を含む範囲に限定されていることを特徴とする、半導体装置の製造方法。
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