JP5089215B2 - Nitride compound semiconductor layer etching method and semiconductor device manufactured using the method - Google Patents

Description

  The present invention relates to a method for etching a nitride compound semiconductor layer and a semiconductor device manufactured using the method.

  As a semiconductor device using a nitride compound semiconductor typified by GaN, research on optical devices such as a semiconductor laser (LD) and a light emitting diode (LED) has been advanced. In addition, since nitride compound semiconductors have high dielectric strength, high thermal conductivity, and high saturation electron velocity, in recent years, high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HEMTs) using the same The practical application of electronic devices such as HBT) is expected.

  As an optical device using a nitride compound semiconductor, an n-type GaN layer, an active layer, and a p-type GaN layer are stacked on a substrate, and a ridge that is a stripe-like raised portion is formed on the p-type GaN layer. A blue-violet semiconductor laser is known. This ridge is for confining light and current, and is formed by etching a part of the p-type GaN layer.

  As an electronic device using a nitride compound semiconductor, a GaN / AlGaN heterostructure HEMT in which a GaN layer and an AlGaN layer are stacked on a substrate and a gate electrode is formed in a recess formed in the AlGaN layer. Is known. The recess portion of the AlGaN layer is formed by etching a part of the AlGaN layer.

  In the semiconductor laser as described above, the etching depth control is important because the distance between the active layer and the bottom of the ridge affects the optical output-current characteristics. In the HEMT as described above, since the threshold voltage is controlled by the etching depth, the etching depth control is extremely important as in the semiconductor laser.

  It is well known that wet etching is difficult because GaN-based materials are chemically stable. For this reason, a dry etching method is generally used as an etching method for the GaN layer and the AlGaN layer. However, in the conventional dry etching of the GaN layer and the AlGaN layer, a method of controlling the etching depth by controlling the time is employed, and there is a problem in the controllability and reproducibility of the etching depth.

  Thus, in the dry etching of the GaN layer and the AlGaN layer, it is difficult to accurately control the etching depth of the GaN layer and the AlGaN layer, and the etching can be stopped at a desired etching depth. Selective etching using an etching stop layer is desired.

As a selective etching technique for the GaN layer, the following Non-Patent Document 1 discloses that the GaN layer and the GaN layer are etched by ICP dry etching using three kinds of gases of Cl ₂ (40 sccm) / N ₂ (10 sccm) / O ₂ ( ₂ sccm). It is described that a selection ratio of about 60: 1 is achieved with the underlying AlGaN layer. In Non-Patent Document 1, an aluminum oxide film, that is, an etching resistance layer is formed on the surface of the AlGaN layer by adding a small amount of O ₂ gas to Cl ₂ / N ₂ gas. It is described that the etching rate of AlGaN thereafter decreases. Further, in Non-Patent Document 1, when the O ₂ gas is added to the Cl ₂ / N ₂ gas, the AlGaN layer is oxidized more rapidly than when the O ₂ gas is added to the Cl ₂ / Ar gas. It is also described.
Yanjun Han, et al. Jpn. J. Appl. Phys. Vol. 42 (2003) pp. L1139-L1141, Part 2, No. 10A, 1 October 2003

  According to the etching method described in Non-Patent Document 1, dry etching with relatively high selectivity of the GaN layer can be performed using the AlGaN layer as an etching resistance layer. However, as described above, three gases are used. Therefore, it is disadvantageous in terms of etching process stability, and the problems regarding the controllability and reproducibility of the etching depth during mass production cannot be solved. In addition, the etching selectivity of GaN to AlGaN is about 60 at the maximum, and the selectivity is not sufficient.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a nitride compound semiconductor layer etching method that is superior in etching depth controllability and reproducibility, and a semiconductor device manufactured using the etching method. It is in.

  According to a first aspect of the present invention for solving the above-described problem, a nitride compound semiconductor layer is formed on an Al-containing nitride compound semiconductor layer, and etching selection of the nitride compound layer with respect to the Al-containing compound semiconductor layer is performed. A method of etching a nitride compound semiconductor layer, wherein the nitride compound layer is dry-etched under a condition where the ratio is 100 or more.

  According to a second aspect of the present invention, in the method for etching a nitride compound semiconductor layer according to the first aspect, the Al-containing nitride compound semiconductor layer is an AlN layer or an AlGaN layer, and the nitride compound semiconductor layer Is an Al-containing nitride compound semiconductor layer having an Al content lower than that of the GaN layer or the Al-containing nitride compound semiconductor layer.

  According to a third aspect of the present invention, in the method for etching a nitride compound semiconductor layer according to the first or second aspect, the dry etching uses chlorine gas as an etching gas and the etching pressure is 3 Pa to 8 Pa. It is performed.

  According to a fourth aspect of the present invention, in the method for etching a nitride compound semiconductor layer according to the third aspect, the dry etching further uses an oxygen gas as an etching gas.

  According to a fifth aspect of the present invention, in the method for etching a nitride compound semiconductor layer according to the fourth aspect, the addition amount of the oxygen gas is 20% or less with respect to the supply amount of the chlorine gas. Features.

  A sixth aspect of the present invention includes an etching stop layer made of an Al-containing nitride compound semiconductor, and a nitride compound semiconductor layer formed on at least a part of the surface of the etching stop layer. A semiconductor device manufactured using the nitride compound semiconductor layer etching method according to any one of the above aspects.

  According to the present invention, the nitride compound semiconductor layer is dry-etched under the condition that the etching selectivity of the nitride compound semiconductor layer with respect to the Al-containing nitride compound semiconductor layer of the underlayer is 100 or more. The semiconductor layer functions as an etching stop layer. Therefore, the nitride compound layer can be etched more selectively with respect to the underlying Al-containing nitride compound layer, and etching excellent in controllability and reproducibility of the etching depth is possible.

  In addition, by using chlorine gas as an etching gas and performing dry etching under conditions where the etching pressure is 3 Pa to 8 Pa, the balance between chlorine ions and chlorine radicals in the plasma is optimized, and the nitride compound semiconductor layer is more selective. Can be etched. In other words, in dry etching, etching is performed by generating ions and radicals by turning the source gas into plasma. (1) The atoms on the surface of the nitride compound semiconductor layer are jumped off by the kinetic energy of accelerated chlorine ions. Physical etching and (2) etching by chemical reaction between chlorine ions and chlorine radicals and atoms on the surface of the nitride compound semiconductor layer proceed. In this physical etching with ions, since selectivity to a substance is low, not only the nitride compound semiconductor layer but also the underlying Al-containing nitride compound semiconductor layer is scraped, and selective etching is difficult. On the other hand, in the chemical reaction etching using ions and radicals, although the etching rate is low, etching with high selectivity to a substance is possible. Therefore, by controlling the etching pressure within the above range, the balance between chlorine ions and chlorine radicals when chlorine gas is turned into plasma is optimized, and it becomes possible to selectively etch the nitride compound semiconductor layer. A selection ratio of 100 or more can be realized.

  Further, by using oxygen gas as the etching gas, an aluminum oxide film is formed when the surface of the underlying Al-containing nitride compound semiconductor layer is exposed, so that it can be used as a more reliable etching stop layer. .

  Further, if the etching method of the present invention is applied to, for example, a semiconductor laser ridge formation process or a HEMT recess formation process, the etching depth can be accurately controlled in these processes, and a highly reliable semiconductor device can be obtained. Can be obtained.

(First embodiment)
Hereinafter, a semiconductor device, a manufacturing method thereof, and an etching method of the present invention will be described with reference to the drawings.
FIG. 1 is a cross-sectional view showing a manufacturing process of a semiconductor laser element which is an embodiment of a semiconductor device of the present invention.

First, as shown in FIG. 1A, on an n ⁻ GaN substrate 1, an n ⁻ GaN layer 2, an n ⁻ AlGaN layer 3, an InGaN / GaN multiple quantum well active layer 4, a p ⁻ AlGaN layer 5 and a p − The GaN layer 6, the p-AlN etching stop layer 7, and the p-GaN layer 8 are epitaxially grown sequentially. The thickness of each layer is, for example, 2 μm for the n ⁻ GaN layer 2, 0.5 μm for the n ⁻ AlGaN layer 3, 50 nm for the InGaN / GaN active layer 4, 0.5 μm for the p ⁻ AlGaN layer 5, and the p-GaN layer 6. Is 1.7 μm, the p-AlN layer etching stop layer 7 is 3 nm, and the p-GaN layer 8 is 0.3 μm.

Next, as shown in FIG. 1B, an SiO ₂ layer is formed on the p-GaN layer 8 as an etching mask patterned in a stripe shape having a width of 1.5 nm. The thickness of the SiO ₂ layer 9 is, for example, 300 nm.

Next, as shown in FIG. 1C, the p-GaN layer 8 is dry etched with an ICP-RIE dry etching apparatus using the SiO ₂ layer 9 as an etching mask. At this time, chlorine gas is used as an etching gas, and dry etching is performed under the conditions of an etching pressure of 3 Pa to 8 Pa, so that the balance between chlorine ions and chlorine radicals in the plasma is optimized, and the p-AlN etching stop layer 7 is formed. On the other hand, the p-GaN layer 8 can be etched more selectively. For this reason, as shown in FIG. 1C, the etching is stopped at the p-AlN etching stop layer 7, and as a result, a ridge stripe in which the distance between the active layer and the ridge bottom is controlled as desired can be formed. it can.

Next, after removing the SiO ₂ layer 9 as an etching mask with buffered hydrofluoric acid, a new SiO ₂ layer 10 is formed to a thickness of, for example, 300 nm. Next, the SiO ₂ layer on the top of the ridge is removed by an etch back method using a resist. Thereafter, a p ⁻ GaN electrode 11 made of Ni (50 nm) / Au (300 nm) is formed on the substrate including the ridge, and further, n-GaN made of Ti (25 nm) / Al (500 nm) is formed on the back surface of the substrate. The electrode 12 is formed. Thereby, the semiconductor laser element is completed.

Table 1 and FIG. 2 show the experimental results of examining the relationship between the etching rate and the selection ratio of the GaN layer and the AlN layer when the etching pressure is changed. The experimental conditions are as follows.
Experimental conditions:
IPC power: 500 (W)
Bias: 75 (W)
Chlorine gas flow rate: 40 (sccm)
Etching pressure: 1, 2, 3, 4, 5, 6, 7, 8 (Pa)
Substrate temperature: 20 ° C

As is apparent from the experimental results in Table 1 and FIG. 2, when the etching pressure was less than 3 Pa, an etching selectivity of 100 or more could not be achieved. This is presumably because physical etching of not only the GaN layer but also the AlN layer proceeds by chlorine ions in the plasma.
On the other hand, when the etching pressure is in the range of 3 Pa to 8 Pa, it is understood that a selectivity of 100 or more is achieved, and etching with high selectivity is possible. In this range, it is estimated that the balance between chlorine ions and chlorine radicals is optimized.
When the etching pressure exceeds 8 Pa, an etching shape defect is likely to occur even if the selection ratio is 100 or more.

  As described above, according to the first embodiment, a highly selective GaN layer having a selectivity of 100 or more can be etched, and the etching depth can be accurately controlled. Therefore, a highly reliable semiconductor laser element can be obtained.

In addition, since selective etching with a selectivity of 100 or more can be performed, an AlN layer that is an etching stop layer can be thinly formed. For example, in the case of etching conditions with a selectivity ratio of 100, a 3 nm AlN layer may be formed for a 300 nm GaN layer, and for etching conditions with a selectivity ratio of 200, 3 nm for a 600 nm GaN layer. The AlN layer may be formed.
Thus, by forming a thin AlN layer having a large band gap, an increase in the series resistance of the semiconductor laser can be suppressed, and the characteristics of the entire semiconductor laser can be improved.

(Second Embodiment)
Next, a second embodiment of the present invention will be described.
The difference from the first embodiment is mainly that chlorine gas and oxygen gas are used as etching gases in the dry etching process of the p-GaN layer 8. By adding a small amount of oxygen gas to the chlorine gas as an etching gas, an aluminum oxide film is formed when the surface of the p-AlN etching stop layer 7 is exposed. Therefore, the etching is stopped at the p-AlN etching stop layer 7, and as a result, a ridge stripe in which the distance between the active layer and the bottom of the ridge is controlled as desired can be formed.

Table 2 and FIG. 3 show the experimental results of examining the relationship between the etching rate and the selection ratio of the GaN layer and the AlN layer when the addition amount of oxygen gas is changed. The experimental conditions are as follows.
Experimental conditions:
IPC power: 500 (W)
Bias: 75 (W)
Chlorine gas flow rate: 38 (sccm)
Oxygen addition amount: 0, 5, 10, 20, 25, 50, 75 (%)
Etching pressure: 5 (Pa)
Substrate temperature: 20 ° C

As shown in Table 2 and FIG. 3, it can be seen that by adding oxygen gas, the etching rate of the AlN layer is maintained at an extremely low level and functions as a reliable etching stop layer.
When the amount of oxygen gas added is in the range of 5% to 20%, a selectivity of 100 or more is achieved, and etching with high selectivity is possible.
On the other hand, when the amount of oxygen gas added exceeds 20%, the etching rate of the GaN layer also decreases, and the etching selectivity becomes less than 100, which is insufficient for good selective etching. This is presumably because when the oxygen partial pressure is high, the growth rate of the Ga oxide on the GaN surface becomes faster, the etching cannot catch up, and the etching rate of GaN decreases extremely.

(Third embodiment)
FIG. 4 is a cross-sectional view of a HEMT having a GaN / AlGaN heterostructure that is an embodiment of the semiconductor device of the present invention.

  As shown in the figure, on a substrate 41 made of sapphire, a buffer layer 42 made of GaN, a GaN layer 43 serving as a channel layer, a first n-type AlGaN layer 44 serving as a carrier supply layer, and an AlN layer 45 serving as an etching stop layer. In addition, a second n-type AlGaN layer 46 having an Al content lower than that of the AlN layer 45 is formed. The Al content of the second AlGaN layer 46 is, for example, about 1/5 of the Al content of the AlN layer 45. Further, a gate electrode 47 is formed inside the recess where a part of the AlN layer 45 is exposed by etching the second AlGaN layer 46. Further, a source electrode 48 and a drain electrode 49 are formed on the second AlGaN layer 26 so as to sandwich the gate electrode 47.

  In the above configuration, the thicknesses of the first n-type AlGaN layer 44, the AlN layer 45, and the second n-type AlGaN layer 46 are, for example, 20 nm, 2 nm, and 10 nm, respectively.

Next, a method for manufacturing HEMT will be described.
First, trimethyl gallium (TMGa) and ammonia (NH ₃ ) as raw materials for GaN are introduced into the MOCVD apparatus having the sapphire substrate 1 at flow rates of 14 μmol / min and 12 l / min, respectively, and a growth temperature of 550 ° C. Then, the low-temperature buffer layer 42 made of GaN having a layer thickness of 40 nm is epitaxially grown on the substrate 1. Next, TMGa and NH ₃ are introduced at flow rates of 58 μmol / min and 12 l / min, respectively, and a high temperature buffer layer 43 made of GaN having a layer thickness of 3000 nm is epitaxially grown on the low temperature buffer layer 42 at a growth temperature of 1050 ° C. .

Next, TMGa and NH ₃ are introduced at a flow rate of 19 μmol / min and 12 l / min, respectively, and an electron transit layer 44 made of GaN having a layer thickness of 50 nm is epitaxially grown on the buffer layer at a growth temperature of 1050 ° C. Further, trimethylaluminum (TMAl), TMGa, and NH ₃ were introduced at flow rates of 100 μmol / min, 19 μmol / min, and 12 l / min, respectively, and the growth temperature was 1050 ° C. and the layer thickness of Al _0.3 Ga _0. The first electron supply layer 45 made of ₇ N is epitaxially grown on the electron transit layer 44.

Next, TMAl and NH ₃ are introduced, and an etching stop layer 46 made of AlN having a thickness of 2 nm is epitaxially grown on the first AlGaN electron supply layer 45. Further, trimethylaluminum (TMAl), TMGa, and NH ₃ are introduced, and a second electron supply layer 47 made of Al _0.2 Ga _0.8 N having a layer thickness of 10 nm is epitaxially grown on the AlN etching stop layer 46.
In addition, 100% hydrogen is used as a carrier gas in the introduction of TMAl, TMGa, and NH ₃ in the formation process of these layers.

Next, a mask made of a SiO ₂ film is formed on the second electron supply layer 47 by patterning using photolithography, and openings corresponding to the electrode shapes are formed in regions where the source electrode and the drain electrode are to be formed. Form. Then, Al, Ti and Au are vapor-deposited in this order in this opening to form the source electrode 49 and the drain electrode 50.

Next, the mask on the second electron supply layer 47 is temporarily removed, and a mask made of an SiO ₂ film is formed on the electron supply layer 47 again. Using this mask, the second electron supply layer made of AlGaN is formed. A recess for arranging the gate electrode 48 is formed by etching 47. In the step of forming the recess, the etching method of the present invention is used.
Specifically, dry etching was performed with an ICP-RIE dry etching apparatus using chlorine gas and oxygen as etching gases under the following conditions.
PC power: 500 (W)
Bias: 75 (W)
Chlorine gas flow rate: 38 (sccm)
Oxygen addition amount: 2 (sccm)
Etching pressure: 5 (Pa)
Substrate temperature: 20 ° C

  Next, Pt and Au are deposited in this order on the formed recess to form the gate electrode 48. Thereby, the HEMT shown in FIG. 4 is completed.

  Also in this embodiment, by adding a small amount of oxygen gas to chlorine gas as an etching gas, an aluminum oxide film is formed on the surface of the underlying AlN layer 45, and the AlN layer 45 functions as an etching stop layer. In the present embodiment, since the second AlGaN layer 46 to be etched is also a mixed crystal containing Al, Al exposed to the surface of the second AlGaN layer 46 reacts with oxygen, and the second AlGaN layer 46 reacts. An aluminum oxide film is also formed on the surface of the AlGaN layer 46. For this reason, the etching rate is unavoidably reduced as compared with the case where the etching target is a GaN layer not containing Al. However, as described above, since the second AlGaN layer 46 has a lower Al content than the AlN layer 45, the degree of formation of the aluminum oxide film is also reduced. By utilizing such a difference in the degree of formation of the oxide film due to the difference in Al content, etching with high selectivity with a selectivity of 100 or more is possible also in this embodiment.

In the HEMT formed as described above, a two-dimensional electron gas is formed in the vicinity of the heterointerface between the GaN layer 43 and the first n-type AlGaN layer 44 by applying a predetermined voltage to the gate electrode 47. The The concentration of the two-dimensional electron gas is determined by the distance between the gate electrode 27 and the hetero interface where the two-dimensional electron gas is formed. As a result, the threshold voltage of the FET is determined.
According to the present embodiment, etching with high selectivity with a selectivity of 100 or more is possible, so that the etching depth can be accurately controlled, which is extremely advantageous for controlling the threshold voltage.

  In the present embodiment, as in the second embodiment, an etching method in which a small amount of oxygen gas is added to the chlorine gas as an etching gas is applied. However, as in the first embodiment, an etching gas is used. As an etching method, only chlorine gas may be used to control the etching pressure.

FIG. 1 is a cross-sectional view showing a manufacturing process of a semiconductor laser device which is a semiconductor device according to an embodiment of the present invention. FIG. 2 is a graph showing the relationship between the etching rate and the selection ratio of the GaN layer and the AlN layer when the etching pressure is changed. FIG. 3 is a graph showing the relationship between the etching rate and the selection ratio of the GaN layer and the AlN layer when the amount of oxygen gas added is changed. FIG. 4 is a cross-sectional view showing an HFET that is a semiconductor device according to an embodiment of the present invention.

Explanation of symbols

1: n ^- GaN substrate 2: n ^- GaN layer 3: n ^- AlGaN layer 4: InGaN / GaN multiple quantum well active layer 5: p ^- AlGaN layer 6: p-GaN layer 7: p-AlN etching stop layer 8: p-GaN layer 8
9: SiO ₂ layer 10: SiO ₂ layer 11: p ^- GaN electrode 12: n-GaN electrode

Claims (6)

Forming a nitride compound semiconductor layer on the Al-containing nitride compound semiconductor layer, the Al the nitride compound etching selectivity of the semiconductor layer is the nitride under the conditions of 100 or more for containing nitride compound semiconductor layer compound semiconductor layer Dry etching ,
The dry etching is performed using chlorine gas or a gas in which oxygen gas of 20% or less is added to chlorine gas as an etching gas, and the etching pressure is 3 Pa to 8 Pa . Etching method of compound semiconductor layer.   The Al-containing nitride compound semiconductor layer is an AlN layer or an AlGaN layer, and the nitride compound semiconductor layer is an Al-containing nitride compound semiconductor having a lower Al content than the GaN layer or the Al-containing nitride compound semiconductor layer. The method for etching a nitride compound semiconductor layer according to claim 1, wherein the method is a layer. The method for etching a nitride compound semiconductor layer according to claim 2, wherein the Al-containing nitride compound semiconductor layer is an AlN layer, and the nitride compound semiconductor layer is a GaN layer . 4. The method of etching a nitride compound semiconductor layer according to claim 1 , wherein the etching gas is a gas in which oxygen gas of 5% or less is added to chlorine gas. 5 . 5. The method for etching a nitride compound semiconductor layer according to claim 1, wherein the Al-containing nitride compound semiconductor layer is formed with a thickness of 3 nm or less . An etching stop layer made of an Al-containing nitride compound semiconductor, and a nitride compound semiconductor layer formed on at least a part of the surface of the etching stop layer,
A semiconductor device manufactured using the nitride compound semiconductor layer etching method according to claim 1.

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