RU2576412C1 - Method of selective reactive-ion etching for semiconductor heterostructure - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: method of selective reactive-ion etching for a semiconductor heterostructure having at least a sequence of layers GaAs/AlGaAs with preset characteristics includes the placement of the semiconductor heterostructure at a substrate holder in the reactor of a reactive-ion etching system with provided contact for the layer of gallium arsenide with process gas plasma, delivery of process gasses to the reactor and further selective reactive-ion etching at preset process parameters. In the method the semiconductor heterostructure is used with a layer of AlGaAs having the thickness of at least 10 nm, with the content of chemical elements Al_xGa_1-xAs at x equal or more than 0.22, the process gasses are represented by a mixture of boron trichloride and sulphur hexafluoride at a ratio of (2:1)-(9:1) respectively, selective reactive-ion etching is carried out at a pressure in the reactor equal to 2-7 Pa, power supplied to discharge of 15-50 W, temperature of the substrate holder of 21-23°C, total consumption rate of the process gases of 15-25 ml/min.

EFFECT: improved output of fit articles by increasing selectivity, controllability, reproducibility, anisotropy and reduced unevenness, density of defects and impurities at the surface of the semiconductor heterostructure.

2 cl, 9 dwg

Description

The invention relates to electronic microwave technology, in particular to methods of selective reactive ion etching of semiconductor heterostructures, and can be widely used in the manufacture of microwave electronic products, in particular microwave field effect transistors.

As is known, until recently, semiconductor structures of gallium arsenide (GaAs) were the main semiconductor structures for microwave field effect transistors.

The speed of such field effect transistors with submicron channel lengths is 10-12 GHz.

Significant progress in terms of improving performance has been achieved by the invention of the so-called high electron mobility transistors (HEMTs), the active region of which consists of doped wide-gap and undoped narrow-gap layers of a semiconductor heterostructure.

This provides a significant increase in the speed of such field-effect transistors (up to 100 GHz or more).

One of the main tasks in the manufacture of field effect transistors on semiconductor heterostructures is the process of selective etching of its individual layers.

A known method of manufacturing photodetector elements based on multilayer semiconductor GaAs / AlGaAs heterostructures, which consists in applying a sequence of layers to a substrate of semi-insulating gallium arsenide (GaAs): a conducting n ⁺ -GaAs layer, a multilayer periodic structure of GaAs / AlGaAs and a second conducting n ⁺ -CaAs -layer, followed by etching of the upper conductive n ⁺ -GaAs layer of a multilayer semiconductor heterostructure in an aqueous solution of hydrogen peroxide, in which, in order to increase the accuracy and precision of etching, In order to obtain suitable products, a thin stop layer of aluminum arsenide (AlAs) with a thickness of 2-5 nm is applied between the conductive n ⁺ -GaAs layer closest to the substrate and the periodic multilayer structure, and etching is carried out at a temperature of 20-22 ° C in solution, optionally containing organic acid.

The pickling solution as an organic acid contains tartaric acid in the following ratios of components, wt.:

Tartaric acid - 35-45;

Hydrogen peroxide - 5-10;

Water is the rest [1].

The disadvantage of this method is:

high unevenness of etching and reproducibility and, accordingly, low yield,

low anisotropy of etching, leading to an uncontrolled raster of the surface of the semiconductor heterostructure.

In particular, when forming the channel of a field-effect transistor with a Schottky barrier (PTSH), this drawback leads to a significant decrease in the electrical parameters of the PTSH or to a complete defect in this operation of its manufacture.

A known method of selective reactive ion etching of a GaAs / AlGaAs semiconductor heterostructure, comprising preparing a semiconductor structure, arranging it on a substrate holder in a reactor of a reactive ion etching system, ensuring that the GaAs layer is in contact with the plasma of the process gases, feeds the reactor with process gases — a mixture of silicon tetrachloride (SiCl ₄ ) and trifluoromethane (CHF ₃ ) and subsequent selective reactive ion etching at specified process conditions [2].

The advantage of the second analogue over the first is a significant reduction in unevenness, increased reproducibility, controllability of selective reactive ion etching, and, accordingly, increased yield.

However, the presence of a mixture of process gases, namely:

silicon tetrachloride - requires additional equipment for the conversion of SiCl ₄ from liquid to gaseous,

trifluoromethane - leads to the formation of polymer films on the surface of the semiconductor heterostructure, which can lead to deterioration of the electrical parameters of microwave products, the removal of which is a rather time-consuming process.

In addition, this method of selective reactive ion etching is characterized by a sufficiently high etching rate of a gallium arsenide layer of a semiconductor heterostructure (of the order of 100 nm / min), which does not provide accurate control of selective reactive ion etching of thin GaAs layers (less than 50 nm).

There is also known a method for selective reactive ion etching of a GaAs / AlGaAs semiconductor heterostructure, comprising arranging a semiconductor heterostructure on a substrate holder in a reactor of a reactive ion etching system to ensure that the GaAs layer is contacted with the plasma of the process gases, feeding the reactor with a mixture of sulfur hexafluoride (SF ₆ ) and chlorine (Cl ₂₎ in a ratio of about 0.025, exch, unlike the process gas -. mixture (SiCl ₄₎ and (CHF ₃₎ of the second analogue and a subsequent selective reactive onnoe etching process under specified conditions [3] - prototype.

The absence of carbon atoms in the mixture of process gases ensures that the above-mentioned polymer films are not formed on the surface of the semiconductor heterostructure.

However, the use of pure chlorine in the process gas mixture is ineffective for etching the intrinsic oxides of the GaAs layer of the semiconductor heterostructure and, accordingly, increases the unevenness, reduces the reproducibility and controllability of selective reactive ion etching.

The technical result of the claimed method of selective reactive ion etching of a semiconductor heterostructure is to increase the yield by increasing selectivity, controllability, reproducibility, anisotropy and reducing unevenness, density of defects and contaminants on the surface of the semiconductor heterostructure.

The specified technical result is achieved by the claimed method of selective reactive ion etching of a semiconductor heterostructure having at least a sequence of GaAs / AlGaAs layers with predetermined characteristics, including the arrangement of the semiconductor heterostructure on a substrate holder in the reactor of the reactive ion etching system to ensure that the GaAs layer is in contact with the plasma of the process gases, the supply of technological gases to the reactor and subsequent selective reactive ion etching at the rear nnyh technological mode parameters. Wherein

using a semiconductor heterostructure having the mentioned AlGaAs layer with a thickness of at least 10 nm, with the content of chemical elements Al _x Ga _1-x As at x equal to or greater than 0.22,

as process gases, a mixture of boron trichloride (BCl ₃ ) and sulfur hexafluoride (SF ₆ ) is used with a ratio of (2: 1) - (9: 1), respectively,

selective reactive ion etching is carried out at a pressure in the reactor of 2-7 Pa, the power supplied to the discharge, 15-50 W, the temperature of the substrate holder 21-23 ° C, the total flow rate of the process gas 15-25 ml / min.

The end of selective reactive ion etching is judged by one of the known methods, for example, laser interferometry.

Disclosure of the claimed invention.

The use of a semiconductor heterostructure having the mentioned AlGaAs layer with a thickness of at least 10 nm, with a content of Al _x Ga _1-x As chemical elements at x equal to or greater than 0.22, and process gases - a mixture of boron trichloride (BCl ₃ ) and sulfur hexafluoride ( SF ₆ ) with a ratio of (2: 1) - (9: 1), respectively, provides:

firstly, the complete and high-quality etching of the GaAs layer of the semiconductor structure, the subsequent formation of an aluminum fluoride film on the surface of the AlGaAs layer, which acts as a stop layer and thereby ensures complete and high-quality termination of selective reactive ion etching of subsequent layers of the semiconductor heterostructure (starting from the AlGaAs layer ) and, as a result, achieving a high degree of selectivity,

secondly, the simultaneous removal of intrinsic oxides and thereby eliminating the need for additional processing of the GaAs layer for this purpose and, as a result, reducing the density of defects and contaminants on the surface of a semiconductor heterostructure,

thirdly, the exclusion of carbon compounds from technological gases and thereby the elimination of the formation of undesirable polymer films and, as a result, the reduction of contaminants on the surface of a semiconductor heterostructure.

fourthly, increased controllability and reproducibility.

And, as a consequence, this is an increase in the yield of semiconductor heterostructures.

The specified parameters of the technological regime of selective reactive ion etching individually provide:

pressure in the reactor 2-7 Pa - a high degree of anisotropy;

the power supplied to the discharge is 15-50 W — optimal conditions for bombarding the surface of a GaAs layer of a semiconductor heterostructure with process gas ions and thereby eliminating breakdown of the aforementioned aluminum fluoride film and eliminating further etching of the underlying layers;

substrate holder temperature 21-23 ° С:

firstly, the uniform distribution of temperature on the surface of the semiconductor heterostructure and thereby its uniform selective reactive ion etching,

secondly, the exclusion of the formation of structural defects due to a sufficiently low temperature;

a total consumption of process gases of 15-25 ml / min is optimal to ensure uniformity of selective reactive ion etching.

The combination of features of these parameters of the technological mode of selective reactive ion etching in combination with these other features of the invention will provide:

a) achieving a high selectivity of reactive ion etching of the semiconductor heterostructure (GaAs layer relative to the Al _x Ga _1-x As layer) of about 200: 1, as well as controllability, reproducibility, anisotropy,

b) reducing the unevenness, density of defects and surface contamination of the semiconductor heterostructure.

And, as a consequence, both of these are an increase in the yield of suitable semiconductor heterostructures.

So, the set of essential features of the claimed method of selective reactive ion etching of a semiconductor heterostructure will fully provide the claimed technical result, namely, increased yield.

The use of a semiconductor heterostructure having:

said AlGaAs layer with a thickness of less than 10 nm is undesirable because of its possible breakdown,

with the content of chemical elements Al _x Ga _1-x As at x less _than 0.22, is unacceptable due to the imbalance in the rates of formation and etching of the aluminum fluoride film.

The use as a process gas of a mixture of boron trichloride and sulfur hexafluoride at a ratio different from (2: 1) to (9: 1), respectively, is not desirable due to the deterioration - reduction of the results of selective reactive ion etching (selectivity, controllability, reproducibility, anisotropy, increasing unevenness, density of defects and pollution).

The implementation of selective reactive ion etching with the parameters of the technological regime:

pressure in the reactor of less than 2 Pa and more than 7 Pa is undesirable, in the first case due to a significant increase in the level of ion bombardment, leading to a decrease in selectivity, in the second - due to an increase in the level of the chemical component of reactive ion etching, leading to a decrease in anisotropy;

less than 15 W and more than 50 W of power supplied to the discharge is undesirable, in the first case due to a decrease in the etching rate, in the second because of a decrease in controllability and reproducibility, as well as an increase in the etching unevenness;

a temperature of the substrate holder of less than 21 ° C and more than 23 ° C is undesirable due to increased unevenness of reactive ion etching;

the total consumption of process gases of less than 15 ml / min and more than 25 ml / min is undesirable in the first case - due to a decrease in the rate of selective reactive ion etching, an increase in the unevenness of etching and the density of defects and contaminants, in the second - due to a decrease in the rate of reactive ion etching due to the fact that the active particles of the process gases do not have time to react with the surface of the substrate and are removed from the reactor.

The invention is illustrated by drawings.

In FIG. Figure 1 shows the dependence of the etching anisotropy of a semiconductor heterostructure on the pressure in the reactor.

In FIG. Figure ₂ shows the dependence of the etching non-uniformity of the semiconductor heterostructure on the ratio of the components of the process gas mixture BCl ₃ / SF ₆ .

In FIG. Figure 3 shows the dependence of the etching selectivity of the semiconductor GaAs / AlGaAs heterostructure on the pressure in the reactor.

In FIG. Figure 4 shows the dependence of the etching rate of the GaAs layer on the pressure in the reactor.

In FIG. Figure 5 shows the dependence of the etching rate of GaAs on the power supplied to the discharge.

In FIG. Figure ₆ shows the dependence of the etching rate of GaAs on the ratio of the components of the process gas mixture BCl ₃ / SF ₆ .

In FIG. 7 shows the dependence of the etching rates of the GaAs and AlGaAs layers on the ratio of the components of the process gas mixture SF ₆ / [SiCl ₄ + SF ₆ ] - prototype.

In FIG. 8 is an image obtained by a scanning electron microscope etched by the claimed method of a channel of a microwave field-effect transistor made on said semiconductor heterostructure.

In FIG. 9 is a matrix of the distribution of the values of the saturation currents of the drain-source channel of the microwave field-effect transistor etched by the claimed method.

Examples of specific implementations of the claimed method of selective reactive ion etching of a semiconductor heterostructure.

Example 1

Let us consider selective reactive ion etching of a semiconductor heterostructure having GaAs layers 50 nm thick and Al _x Ga _1-x As 10 nm thick with a content of Al _x Ga _1-x As chemical elements at x equal to 0.22.

The method includes the following sequence of technological operations.

Preliminarily, windows are formed on the surface of the semiconductor heterostructure by photolithography for subsequent selective reactive ion etching.

The semiconductor heterostructure is placed on a substrate holder in the reactor of the Corial 200RL reactive ion etching system, so that the GaAs layer of the semiconductor heterostructure is in contact with the plasma of the process gases.

Then carry out selective reactive ion etching of the semiconductor heterostructure with the following parameters of the technological mode:

the pressure in the reactor is 2.3 Pa,

power supplied to the discharge, 40.0 W,

substrate temperature 22.0 ° C,

the total gas flow rate is 20.0 ml / min, while the gas flow rate of boron trichloride (BCl ₃ ) is 18.0 ml / min, and sulfur hexafluoride (SF ₆ ) is 2.0 ml / min, which corresponds to a ratio of 9 : 1 respectively.

Examples 2-5.

Analogously to example 1, selective reactive ion etching is carried out for a semiconductor heterostructure having the same GaAs / Al _x Ga _1-x As layers, but with different process conditions specified in the claims (examples 2-4), and beyond (examples 4-5).

Example 6 corresponds to the sample prototype. Data on the prototype sample are not available, with the exception of the speed and selectivity of etching.

The fabricated samples of the semiconductor heterostructure (examples 1-5) were investigated for:

1. Anisotropy by measuring the geometric dimensions of the etched windows on a scanning electron microscope Carl Zeiss Merlin (Fig. 1).

2. Irregularities by measuring the depth of etching using a Dektak 150 profilometer (FIG. 2).

3. Selectivity by comparing the etching rate of the GaAs layer relative to the AlGaAs layer (Fig. 3).

4. The etching rate of the GaAs layer by measuring the ratio of the etching depth to the etching time (Fig. 4-6).

As can be seen from FIG. 1, a satisfactory level of anisotropy (over 30) is achieved in the pressure range in the reactor 2-7 Pa.

As can be seen from FIG. 2, a low level of etching unevenness (less than 3%) is achieved when the ratio of the components of the process gas mixture BCl ₃ / SF _{6 is} in the range from 2: 1 to 9: 1.

As can be seen from FIG. 3, a high level of etching selectivity (over 200) is achieved in the pressure range in the reactor 2-7 Pa.

As can be seen from FIG. 4-6, the etching rate of the GaAs layer varies widely depending on the input parameters of the process, which ensures high controllability of the process. Therefore, the speed indicator does not impose additional restrictions on the input parameters of the process.

As can be seen from FIG. 7:

- the etching rate of the GaAs layer in the technological process of the prototype cannot be lower than 1000 Ǻ / min, which accordingly does not allow to control the process with high accuracy,

- the selectivity of etching GaAs relative to AlGaAs in the technological process of the prototype does not exceed 100, which is significantly lower than the selectivity of etching by the claimed method.

As can be seen from FIG. 8, the channel surface of the field-effect transistor etched by the claimed method in the optimum mode does not contain any impurities or polymer films, which also illustrates the achievement of a high yield.

So, samples of a semiconductor heterostructure, selective reactive ion etching of which was carried out under the parameters of the technological mode according to the claimed claims (examples 1-3) have a yield of not less than 97 percent, due to the selectivity of the etching of the GaAs layer relative to the AlGaAs layer of at least 200: 1, unevenness no more than 3%, anisotropy no less than 30.

Thus, the claimed method of selective reactive ion etching of a semiconductor heterostructure fully provides a technical result, namely an increase in yield of not less than 97 percent.

Information sources

1. RF patent No. 2065644 IPC H01L 21/00, priority 06/14/1994, publ. 08/20/1996.

1. Patent US 5837617 A.

2. Patent EP 0492951 A1 - prototype.

Claims (2)

1. A method of selective reactive ion etching of a semiconductor heterostructure having at least a sequence of GaAs / AlGaAs layers with predetermined characteristics, comprising arranging a semiconductor heterostructure on a substrate holder in a reactor of a reactive ion etching system to ensure that the gallium arsenide layer is contacted with the process gas plasma, fed to process gas reactor and subsequent selective reactive ion etching at specified process conditions, o Leach in that the semiconductor heterostructure AlGaAs layer having said thickness is not less than 10 nm, the content of the chemical elements Al _x Ga _1-x As with x equal to or greater than 0.22, as process gases, a mixture of boron trichloride and sulfur hexafluoride when the ratio (2: 1) - (9: 1), respectively, selective reactive ion etching is carried out at a pressure in the reactor of 2-7 Pa, the power supplied to the discharge, 15-50 W, the temperature of the substrate holder 21-23 ° C, the total consumption process gases 15-25 ml / min. 2. The method of selective reactive ion etching of a semiconductor heterostructure according to claim 1, characterized in that the end of reactive ion selective etching is judged by one of the known methods, for example laser interferometry.

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