CN116544275B - GaN HEMTs and method for reducing ohmic contact resistance of device - Google Patents
GaN HEMTs and method for reducing ohmic contact resistance of device Download PDFInfo
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- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
- 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
- H01L29/7782—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface with confinement of carriers by at least two heterojunctions, e.g. DHHEMT, quantum well HEMT, DHMODFET
- H01L29/7783—Field effect transistors with two-dimensional charge carrier gas channel, e.g. HEMT ; with two-dimensional charge-carrier layer formed at a heterojunction interface with confinement of carriers by at least two heterojunctions, e.g. DHHEMT, quantum well HEMT, DHMODFET using III-V semiconductor material
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- 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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Abstract
The invention relates to GaN HEMTs and a method for reducing ohmic contact resistance of a device, belonging to the technical field of microelectronic devices. The film layer structure comprises a substrate, a nucleation layer, a buffer layer, an insertion layer, a barrier layer and a cap layer from bottom to top in sequence; a metal electrode is grown on the barrier layer through the cap layer. The invention adds the step of the cycle treatment of Ashr+HCl before annealing, and realizes the etching technology with low damage, low etching rate and controllable etching thickness by combining oxygen plasma treatment and hydrochloric acid for multiple times. The results show a linear decrease in specific contact resistivity with increasing number of Asher treatments.
Description
Technical Field
The invention relates to GaN HEMTs and a method for reducing ohmic contact resistance of a device, belonging to the technical field of microelectronic devices.
Background
The GaN High Electron Mobility Transistor (HEMTs) has the characteristics of high electron mobility, high switching speed, large breakdown voltage and the like, and has wide application prospects in the fields of photoelectrons, radio frequency, power electronics and the like. In order to improve the performance of the GaN HEMTs device and exert the characteristic advantages of the GaN HEMTs device, the process exploration and optimization of the device are indispensable links. The source electrode and the drain electrode in the GaN HEMTs device are ohmic contacts, and the resistance value of the source electrode and the drain electrode directly influences the output current and the knee point voltage of the device, so that the reduction of the ohmic contact resistance value has important significance for improving the performance of the GaN HEMTs device. In the ohmic contact manufacturing process, metal is required to be diffused through the barrier layer by a high-temperature annealing method to achieve two-dimensional electron gas, so that the conduction of the source and drain ends of the device is achieved. If the barrier layer is too thick, it may cause difficulty in metal diffusion and thus cause an ohmic contact resistance to be too large. In order to reduce the ohmic contact resistance, the barrier layer needs to be thinned. The conventional barrier layer thinning method is ICP etching, but the ICP etching has the characteristics of high damage, high surface roughness and high etching rate, the barrier layer is easy to completely etch, the etching rate is difficult to control, the damage and the over etching easily cause the increase of the ohmic resistance value, and the aim of reducing the ohmic resistance value is difficult to realize. In addition, the damage caused by ICP etching can affect the stripping of photoresist and the yield of the subsequent process.
The Ashr device is characterized in that a certain amount of oxygen is introduced into a closed vacuum chamber, an alternating electric field is formed between parallel plate electrodes by utilizing radio frequency power, electrons excited by the electrode plates are accelerated by the electric field to ionize the oxygen to generate oxygen plasma, the oxygen plasma chemically reacts with organic matters such as photoresist on the surface of a material to generate waste such as carbon oxide gas and water, and the waste can be pumped away by a vacuum pump. Moreover, because Asher adopts low-power oxygen plasma for cleaning, the material has the characteristics of low oxidation rate and low damage to the material.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides GaN HEMTs and a method for reducing the ohmic contact resistance of a device, wherein the step of circulating treatment of Ashr and HCl is added before annealing, and the repeated circulating process is carried out by combining oxygen plasma treatment and hydrochloric acid, so that the etching technology with low damage, low etching rate and controllable etching thickness is realized. The results show a linear decrease in specific contact resistivity with increasing number of Asher treatments.
The invention adopts the following technical scheme:
The GaN HEMTs sequentially comprise a SiC substrate, an AlN nucleation layer, a GaN buffer layer (channel layer), an AlN insertion layer, an AlGaN barrier layer and a GaN cap layer from bottom to top, wherein the GaN buffer layer, the AlN insertion layer and the AlGaN barrier layer form a heterojunction;
The metal electrode is grown on the AlGaN barrier layer through the GaN cap layer.
Preferably, the thickness of the GaN buffer layer is 1-3 mu m, and the GaN buffer layer is not doped; the thickness of the AlN insertion layer is 0.5-1.5nm; the molar ratio of Al components in the AlGaN barrier layer is 0.15-0.25, and the thickness of the AlGaN barrier layer is 15-25nm; the thickness of the GaN cap layer is 1-3nm.
Preferably, the GaN buffer layer has a thickness of 1.5 μm; the thickness of the AlN intercalation layer is 1nm; the molar ratio of Al components in the AlGaN barrier layer is 0.2, and the thickness of the AlGaN barrier layer is 20nm; the thickness of the GaN cap layer is 2nm.
A method for reducing ohmic contact resistance of a device comprises the following steps:
(1) Sequentially growing an AlN nucleation layer, a GaN buffer layer (channel layer), an AlN insertion layer, an AlGaN barrier layer and a GaN cap layer on the surface of the SiC substrate;
(2) Performing mesa etching by adopting an Inductively Coupled Plasma (ICP) method to obtain a single mesa;
(3) Obtaining a pattern of a metal electrode to be grown on a single mesa by adopting a photoetching developing method;
(4) Sequentially adopting Ashr (oxygen plasma) and HCl to wash and perform multiple times of metal electrode growth pretreatment, in each treatment, firstly adopting Ashr to oxidize, then corroding oxide through HCl, and through multiple times of treatment, firstly corroding the GaN cap layer, then corroding part of AlGaN barrier layer, achieving the purpose of thinning the barrier layer, and effectively removing photoresist remained on the surface of the GaN HEMTs device;
(5) Growing a metal electrode by adopting a Physical Vapor Deposition (PVD) method;
(6) Stripping the metal grown by physical vapor deposition in acetone to obtain a metal electrode;
the stripping process is as follows: washing the surface of the sample in acetone solution by using a syringe until the metal above the photoresist is peeled off, wherein the purpose of peeling is to peel off the metal above the photoresist so as to form a metal electrode pattern;
(7) And (3) carrying out high-temperature rapid annealing in an N 2 atmosphere on the GaN HEMTs device in a rapid annealing furnace (RTP) to form ohmic contact.
The method for reducing ohmic contact resistance by using Ashr+HCl is characterized in that the Ashr is used for oxidizing the GaN surface, then the oxide is corroded by HCl (firstly, the GaN cap layer is corroded, and then the AlGaN barrier layer is corroded to achieve the purpose of reducing the barrier layer), and the method adopts a multiple-cycle process because the depth of surface oxidation by the Ashr is limited and the single treatment can not reach the preset depth;
In addition, during the preparation of GaN HEMTs, ohmic contact is usually performed by metal deposition and high-temperature rapid annealing techniques, and before metal deposition, a hydrochloric acid cleaning method is usually used to remove a natural oxide layer (GaO) on the surface of the GaN cap layer. However, the photoresist remained on the surface of the GaN cap layer after photoetching and development cannot be removed by the method, so that ohmic contact of the GaN HEMTs device is affected. In order to overcome the defects, the invention can effectively remove the photoresist remained on the surface of the GaN HEMTs device after photoetching development through combination of the Ashr and the HCl, and the etching effect of oxygen plasma ensures that the surface of the material is smoother, thereby being more beneficial to the deposition of a metal electrode, effectively carrying out the residual photoresist treatment and cleaning the surface, and realizing multiple effects.
Preferably, the specific process of performing mesa etching by Inductively Coupled Plasma (ICP) in the step (2) is as follows:
2.1, coating photoresist on the GaN cap layer;
2.2, exposing a region to be etched (the region is a non-mesa region) on the photoresist by utilizing a photoetching development technology;
2.3, etching the table top by using an inductively coupled plasma device, wherein the etching depth is 180-190nm, and extends to the GaN buffer layer, so that the effect of isolating devices on the substrate is achieved; the whole table top is divided into a plurality of single table tops for etching, and the single table tops form device isolation;
and 2.4, removing the coated photoresist.
Preferably, in the step (4), the metal electrode growth pretreatment process by using Ashr (oxygen plasma) comprises the following steps:
3.1, opening a chamber door of the oxygen plasma cleaning machine, putting a GaN HEMTs device to be annealed, and closing the chamber door;
3.2, vacuumizing until the vacuum degree in the chamber is lower than 1 multiplied by 10 -5 Torr;
3.3, introducing O 2,O2 into the chamber, and keeping the flow at 20-40 sccm, preferably 30sccm;
3.4, starting a microwave power supply, wherein the power is stabilized at 40-60W and kept for 80-120 s, and preferably, the power is stabilized at 50W and kept for 100s; the power is selected to be about 50W, so that the influence of physical bombardment on the surface damage and roughness of the material can be weakened while the oxidation rate is ensured;
And 3.5, taking out the GaN HEMTs device after the microwave power is turned off and the power is reduced to 0W and the O 2 flow is reduced to 0 sccm.
Preferably, in the step (4), the specific process of performing the metal electrode growth pretreatment by the hydrochloric acid cleaning method is as follows:
A. Preparing 30mL of dilute hydrochloric acid with the concentration of 15-25% in a culture dish, and preferably 20%;
B. Putting the GaN HEMTs subjected to the Ashr treatment into dilute hydrochloric acid for soaking for 4min;
C. The GaN HEMTs washed by dilute hydrochloric acid are put into deionized water to be washed for 1min, and the GaN HEMTs are dried by nitrogen after being washed.
Preferably, in the step (5), the electrode grown by physical vapor deposition is 4 layers of metal, and the 4 layers of metal are sequentially Ti/Al/Ni/Au from bottom to top, wherein the thickness of Ti is 30nm, the thickness of Al is 150nm, the thickness of Ni is 50nm, and the thickness of Au is 50nm. The metal Ti has the function of generating solid-phase chemical reaction with N atoms in GaN in the annealing process to form semi-metal TiN with low resistance, and simultaneously generating a large number of nitrogen vacancies in the GaN to play a role of shallow donor so as to be beneficial to the formation of ohmic contact; the metal Al has the functions of promoting the solid-phase chemical reaction of N atoms and Ti as a catalyst and forming a compact alloy with low work function with the metal Ti; the metal Ni has the function of isolating the metal Al of the lower layer and the metal Au of the upper layer, and avoiding that the Au and the Al are mutually diffused to reach the surface of the GaN material to influence ohmic contact; the metal Au is used for protecting the lower layer of easily oxidized metal Ti and metal Al and preventing the lower layer of easily oxidized metal Ti and metal Al from forming insulating oxide or hydroxide after oxidation.
Preferably, the contact system formed by adopting the PVD-grown Ti/Al/Ni/Au and GaN materials cannot automatically form ohmic contact, and a high-temperature rapid annealing process is needed, so that on one hand, metal Ti and N atoms in GaN are subjected to solid-phase chemical reaction to form low-resistance semi-metal TiN, on the other hand, the metals are mutually diffused, and on the other hand, a series of low-resistance, low-work-function and thermally stable intermetallic alloys are formed through solid-phase interface reaction. In the RTP process, the temperature and time of annealing and atmosphere are key parameters affecting the ohmic contact performance, and in the step (7), the ohmic contact between the metal electrode and the AlGaN barrier layer is formed at high temperature by using a rapid annealing technology (RTP), and the specific steps are as follows:
7.1, opening a chamber door of the rapid annealing furnace, putting HEMT devices to be annealed, and closing the chamber door;
7.2, introducing N 2 into the cavity, and evacuating the air in the cavity;
7.3, after N 2 min is introduced into the chamber, heating the chamber to 830-870 ℃ within 10-20 seconds, and preserving heat for 20-50 seconds;
and 7.4, taking out the HEMT device after the chamber is quickly cooled to room temperature.
Preferably, in step 7.2, the flow rate of the gas is adjusted to 10SLM when N 2 is introduced;
In the step 7.3, the temperature is set to 850 ℃ during high-temperature annealing, the heating time is 17 seconds, the heating rate is 50 ℃/min, and the heat preservation time is 40 seconds.
The invention is not exhaustive and can be seen in the prior art.
The beneficial effects of the invention are as follows:
The invention improves the existing GaN HEMTs device ohmic contact preparation method, and utilizes an oxygen plasma cleaning (Ashr) combined hydrochloric acid cleaning method to pretreat a sample before metal electrode growth, on one hand, the potential barrier layer can be thinned, the Ashr is used for oxidizing the GaN surface, then the HCl is used for corroding the oxide (firstly corroding the GaN cap layer, and then the AlGaN potential barrier layer at the corroding part is corroded to achieve the purpose of thinning the potential barrier layer), and the adoption of a multiple cycle process mode is due to the limited depth of surface oxidation by the Ashr; the etching technology with low damage, low etching rate and controllable etching thickness is realized by combining oxygen plasma treatment with hydrochloric acid for a plurality of times;
On the other hand, the Ashr treatment can remove photoresist remained on the surface of GaN after photoetching development, and hydrochloric acid cleaning can remove natural oxide formed on the surface of the GaN cap layer, so that the specific contact resistivity of the GaN HEMTs device is effectively reduced, and ohmic contact is improved from two aspects.
By adopting the preparation method provided by the invention, the specific contact resistivity of the ohmic contact of the GaN HEMTs device subjected to the Ashr treatment and the hydrochloric acid treatment for 1 time is reduced by 12.8% compared with that of the GaN HEMTs device not subjected to the Ashr treatment and the hydrochloric acid treatment; the specific contact resistivity of ohmic contacts of GaN HEMTs device subjected to 2 times of Ashr treatment and hydrochloric acid treatment is reduced by 28.2% compared with that of the GaN HEMTs device not subjected to the Ashr treatment and the hydrochloric acid treatment; the specific contact resistivity of the ohmic contacts of the GaN HEMTs devices with 3 Asher treatments and hydrochloric acid treatments was reduced by 57.7% compared to devices without Asher treatments and hydrochloric acid treatments.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the description serve to explain the application.
Fig. 1 is a schematic cross-sectional view of an ohmic contact formed by an ash process-based method in a GaN HEMTs device provided by the invention.
FIG. 2 is a graph showing I-V curves for different contact pitches in a #1GaN HEMTs device with 0 Ashr treatments and hydrochloric acid treatments.
FIG. 3 is a graph showing I-V curves for different contact pitches in a #2GaN HEMTs device with 1 Ashr treatment and hydrochloric acid treatment.
FIG. 4 is a graph showing I-V curves for different contact pitches in a #3GaN HEMTs device with 2 Ashr treatments and hydrochloric acid treatments.
FIG. 5 is a graph showing the I-V curves of different contact pitches in a #4GaN HEMTs device with 3 Ashr treatments and hydrochloric acid treatments.
Fig. 6 is a plot of total resistance and a linear fit of total resistance to contact spacing for different contact spacing calculated for #1GaN HEMTs devices with 0 Asher treatments and hydrochloric acid treatments.
Fig. 7 is a graph of total resistance and a linear fit of total resistance to contact spacing for different contact spacing calculated for #2GaN HEMTs devices with 1 ash treatment and hydrochloric acid treatment.
Fig. 8 is a graph of total resistance and a linear fit of total resistance to contact spacing for different contact spacing calculated for #3GaN HEMTs devices with 2 Asher treatments and hydrochloric acid treatments.
Fig. 9 is a plot of total resistance and a linear fit of total resistance to contact spacing for different contact spacing calculated for #4GaN HEMTs devices with 3 Asher treatments and hydrochloric acid treatments.
FIG. 10 is a graph of specific contact resistivity versus Ashr times.
In the figure, 1, siC substrate, 2, alN nucleation layer, 3, gaN buffer layer, 4, alN insertion layer, 5, alGaN barrier layer, 6, gaN cap layer, 7, metallic Ti,8, metallic Al,9, metallic Ni,10, metallic Au.
The specific embodiment is as follows:
in order to better understand the technical solutions in the present specification, the following description will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the implementation of the present specification, but not limited thereto, and the present invention is not fully described and is according to the conventional technology in the art.
Example 1
As shown in FIG. 1, the GaN HEMTs sequentially comprise a SiC substrate 1, an AlN nucleation layer 2, a GaN buffer layer 3, an AlN insertion layer 4, an AlGaN barrier layer 5 and a GaN cap layer 6 from bottom to top, wherein the GaN buffer layer 3, the AlN insertion layer 4 and the AlGaN barrier layer 5 form a heterojunction;
a metal electrode is grown on the AlGaN barrier layer 5 through the GaN cap layer.
The thickness of the GaN buffer layer 3 was 1.5. Mu.m; the AlN insert layer 4 has a thickness of 1nm; the molar ratio of Al components in the AlGaN barrier layer 5 is 0.2, and the thickness of the AlGaN barrier layer 5 is 20nm; the thickness of the GaN cap layer 6 was 2nm.
Example 2
A method for reducing ohmic contact resistance of a device comprises the following steps:
(1) Sequentially growing an AlN nucleation layer, a GaN buffer layer (channel layer), an AlN insertion layer, an AlGaN barrier layer and a GaN cap layer on the surface of the SiC substrate;
(2) Performing mesa etching by adopting an Inductively Coupled Plasma (ICP) method to obtain a single mesa;
(3) Obtaining a pattern of a metal electrode to be grown on a single mesa by adopting a photoetching developing method;
(4) Sequentially adopting Ashr (oxygen plasma) and HCl to wash and perform multiple times of metal electrode growth pretreatment, in each treatment, firstly adopting Ashr to oxidize, then corroding oxide through HCl, and through multiple times of treatment, firstly corroding the GaN cap layer, then corroding part of AlGaN barrier layer, achieving the purpose of thinning the barrier layer, and effectively removing photoresist remained on the surface of the GaN HEMTs device;
(5) Growing a metal electrode by adopting a Physical Vapor Deposition (PVD) method;
(6) Stripping the metal grown by physical vapor deposition in acetone to obtain a metal electrode;
the stripping process is as follows: washing the surface of the sample in acetone solution by using a syringe until the metal above the photoresist is peeled off, wherein the purpose of peeling is to peel off the metal above the photoresist so as to form a metal electrode pattern;
(7) And (3) carrying out high-temperature rapid annealing in an N 2 atmosphere on the GaN HEMTs device in a rapid annealing furnace (RTP) to form ohmic contact.
The method for reducing ohmic contact resistance by using Ashr+HCl is characterized in that the Ashr is used for oxidizing the GaN surface, then the oxide is corroded by HCl (firstly, the GaN cap layer is corroded, and then the AlGaN barrier layer is corroded to achieve the purpose of reducing the barrier layer), and the method adopts a multiple-cycle process because the depth of surface oxidation by the Ashr is limited and the single treatment can not reach the preset depth;
In addition, during the preparation of GaN HEMTs, ohmic contact is usually performed by metal deposition and high-temperature rapid annealing techniques, and before metal deposition, a hydrochloric acid cleaning method is usually used to remove a natural oxide layer (GaO) on the surface of the GaN cap layer. However, the photoresist remained on the surface of the GaN cap layer after photoetching and development cannot be removed by the method, so that ohmic contact of the GaN HEMTs device is affected. In order to overcome the defects, the invention can effectively remove the photoresist remained on the surface of the GaN HEMTs device after photoetching development through combination of the Ashr and the HCl, and the etching effect of oxygen plasma ensures that the surface of the material is smoother, thereby being more beneficial to the deposition of a metal electrode, effectively carrying out the residual photoresist treatment and cleaning the surface, and realizing multiple effects.
Example 3
A method for reducing ohmic contact resistance of a device, as described in example 2, except that the specific process of performing mesa etching by Inductively Coupled Plasma (ICP) in step (2) is as follows:
2.1, coating photoresist on the GaN cap layer;
2.2, exposing a region to be etched (the region is a non-mesa region) on the photoresist by utilizing a photoetching development technology;
2.3, etching the table top by using an inductively coupled plasma device, wherein the etching depth is 180-190nm, and extends to the GaN buffer layer, so that the effect of isolating devices on the substrate is achieved; the whole table top is divided into a plurality of single table tops for etching, and the single table tops form device isolation;
and 2.4, removing the coated photoresist.
Example 4
A method for reducing ohmic contact resistance of a device as described in example 3, except that in the step (4), the metal electrode growth pretreatment using ash (oxygen plasma) is performed by:
3.1, opening a chamber door of the oxygen plasma cleaning machine, putting a GaN HEMTs device to be annealed, and closing the chamber door;
3.2, vacuumizing until the vacuum degree in the chamber is lower than 1 multiplied by 10 -5 Torr;
3.3, introducing O 2,O2 into the chamber, and keeping the flow at 20-40 sccm, preferably 30sccm;
3.4, starting a microwave power supply, wherein the power is stabilized at 40-60W and kept for 80-120 s, and preferably, the power is stabilized at 50W and kept for 100s; the power is selected to be about 50W, so that the influence of physical bombardment on the surface damage and roughness of the material can be weakened while the oxidation rate is ensured;
And 3.5, taking out the GaN HEMTs device after the microwave power is turned off and the power is reduced to 0W and the O 2 flow is reduced to 0 sccm.
Example 5
The method for reducing ohmic contact resistance of the device is as described in example 4, except that in the step (4), the specific process of performing the growth pretreatment of the metal electrode by the hydrochloric acid cleaning method is as follows:
A. Preparing 30mL of dilute hydrochloric acid with the concentration of 15-25% in a culture dish, and preferably 20%;
B. Putting the GaN HEMTs subjected to the Ashr treatment into dilute hydrochloric acid for soaking for 4min;
C. The GaN HEMTs washed by dilute hydrochloric acid are put into deionized water to be washed for 1min, and the GaN HEMTs are dried by nitrogen after being washed.
Example 6
In the method for reducing the ohmic contact resistance of the device, as shown in embodiment 5, the difference is that the electrode grown by physical vapor deposition in the step (5) is 4 layers of metals, and the 4 layers of metals are sequentially Ti/Al/Ni/Au from bottom to top, wherein the thickness of the metal Ti 7 is 30nm, the thickness of the metal Al 8 is 150nm, the thickness of the metal Ni 9 is 50nm, and the thickness of the metal Au 10 is 50nm. The metal Ti has the function of generating solid-phase chemical reaction with N atoms in GaN in the annealing process to form semi-metal TiN with low resistance, and simultaneously generating a large number of nitrogen vacancies in the GaN to play a role of shallow donor so as to be beneficial to the formation of ohmic contact; the metal Al has the functions of promoting the solid-phase chemical reaction of N atoms and Ti as a catalyst and forming a compact alloy with low work function with the metal Ti; the metal Ni has the function of isolating the metal Al of the lower layer and the metal Au of the upper layer, and avoiding that the Au and the Al are mutually diffused to reach the surface of the GaN material to influence ohmic contact; the metal Au is used for protecting the lower layer of easily oxidized metal Ti and metal Al and preventing the lower layer of easily oxidized metal Ti and metal Al from forming insulating oxide or hydroxide after oxidation.
Example 7
A method for reducing the ohmic contact resistance of a device, as described in example 6, except that the ohmic contact between the metal electrode and the AlGaN barrier layer is formed at a high temperature in step (7) using a rapid annealing technique (RTP), comprising the steps of:
7.1, opening a chamber door of the rapid annealing furnace, putting HEMT devices to be annealed, and closing the chamber door;
7.2, introducing N 2 into the chamber, and when introducing N 2, adjusting the flow rate of the gas to 10SLM, and evacuating the air in the chamber;
7.3, after N 2 min is introduced into the chamber, heating the chamber, wherein the temperature is required to be 850 ℃ during high-temperature annealing, the heating time is 17 seconds, the heating rate is 50 ℃/min, and the heat preservation time is 40s;
and 7.4, taking out the HEMT device after the chamber is quickly cooled to room temperature.
Ohmic contact performance can be characterized by a specific contact resistivity ρ C, and the method employed by the present invention to measure specific contact resistivity ρ C is the Transmission Line Model (TLM). The TLM can also measure the sheet resistance R SH of the material at the same time as the specific contact resistivity ρ C, which is based on the following principle:
A table surface is formed on the surface of a material by a photoetching and etching method, a plurality of rectangular metal electrodes with the length of W C and the width of d are manufactured on the table surface, the metal electrodes are linearly arranged, and the distances l between every two adjacent metal electrodes are different. In the TLM test, assuming that the sheet resistance of the metal electrode is negligible compared to the sheet resistance of the semiconductor material, the total resistance R T between each two electrodes consists of two parts, namely the contact resistance between the electrode and the barrier layer and the bulk resistance of the material between the electrodes, expressed by the formula:
Where R C is the contact resistance between the electrode and the AlGaN barrier layer, the second term to the right of the equation is the bulk resistance of the material between the electrodes, where R SH is the sheet resistance of the material. The transport length L T is the average distance the carriers move in the semiconductor material under the metal electrode, and is defined according to the calculation of TLM theory The effective contact area between the metal electrode and the semiconductor material being L TWC(WC refers to the length of the metal electrode), the contact resistance being equal to the ratio of the specific contact resistance to the contact area,/>The contact resistance is formulated as:
the total resistance:
And respectively measuring the total resistance R T between two adjacent electrodes at different intervals, drawing a relation diagram of R T and L by taking L as an abscissa and R T as an ordinate, wherein the slope of a straight line in the diagram is R SH/WC, the intercept of the straight line and a y axis is 2R C, and the absolute value of the intercept of the straight line and an x axis is 2L T. From the slope of the line, the sheet resistance R SH of the material can be obtained, and from the transmission length definition, the specific contact resistivity ρ C can be obtained:
thus, the specific contact resistivity determined by the TLM method is commonly used in units of Ω -cm 2 or Ω -mm 2.
In the TLM test, the number of metal electrodes arranged linearly was 7; the length W C of the metal electrode is 100 μm, and the width d is 50 μm; the distance l between two adjacent metal electrodes is 5 μm, 10 μm, 15 μm, 20 μm, 30 μm, 40 μm in this order.
The method for cleaning the metal electrode by combining the Ashr treatment with the hydrochloric acid can effectively remove the photoresist remained on the GaN surface after photoetching development and the natural oxide layer on the GaN surface, thereby ensuring the direct contact between the metal electrode and the GaN and laying the foundation of forming good ohmic contact after annealing. According to the present invention, 4 sets of GaN HEMTs devices numbered #1, #2, #3, and #4 were preferably fabricated, respectively, wherein samples #1, #2, #3, and #4 were subjected to 0, 1, 2, and 3 Asher treatments, and hydrochloric acid treatments, respectively. The results show that the average specific contact resistivity of ohmic contacts of #1GaN HEMTs devices subjected to 0 Ashr treatments and hydrochloric acid treatments is 3.05X10 -5Ω·cm2; the average specific contact resistivity of ohmic contact of the #2GaN HEMTs device subjected to the Ashr treatment and the hydrochloric acid treatment for 1 time is 2.66 multiplied by 10 -5Ω·cm2, and is reduced by 12.8 percent compared with the #1 device; the average specific contact resistivity of the ohmic contact of the #3GaN HEMTs device subjected to 2 times of Ashr treatment and hydrochloric acid treatment is 2.19 multiplied by 10 -5Ω·cm2, and is reduced by 28.2 percent compared with that of the #1 device; the average specific contact resistivity of ohmic contacts of #4GaN HEMTs device with 3 Ashr treatments and hydrochloric acid treatments was 1.29×10 -5Ω·cm2, which was a 57.7% drop compared to #1 device. The result shows that the method based on the Ashr treatment can effectively reduce the ohmic contact of the GaN HEMTs device, increase the number of the Ashr treatment and linearly reduce the specific contact resistivity.
(1) Calculation of different contact pitch resistance between metal electrodes in TLM
In TLM, to calculate specific contact resistivity, firstly, different contact pitch resistances between metal electrodes which are linearly arranged are calculated, I-V test is carried out on the different contact pitch resistances, and the different contact pitch resistances R C are obtained by calculating the slope of an I-V curve. The source drain bias voltage V DS applied in the I-V test ranges from-1 to 1V, with a test step size of 0.01V. FIG. 2 is a schematic diagram of I-V curves of different contact pitches in a #1GaN HEMTs device subjected to 0 Ashr treatment and hydrochloric acid treatment, and the resistances of 40 Ω, 55 Ω, 70 Ω, 90 Ω, 127 Ω, 161 Ω of the contact pitches of 5 μm, 10 μm, 15 μm, 20 μm, 30 μm, and 40 μm can be obtained by extracting the slopes of the I-V curves; FIG. 3 is a schematic diagram of I-V curves for different contact pitches in a #2GaN HEMTs device subjected to 1 Ashr treatment and hydrochloric acid treatment, and the resistances of 38 Ω, 53 Ω, 71 Ω, 90 Ω, 127 Ω, 159 Ω, respectively, for contact pitches of 5 μm, 10 μm, 15 μm, 20 μm, 30 μm, and 40 μm can be obtained by extracting the slopes of the I-V curves; FIG. 4 is a schematic diagram of I-V curves for different contact pitches in a #3GaN HEMTs device subjected to 2 Ashr treatments and hydrochloric acid treatments, and the slopes of the I-V curves are extracted to obtain resistances of 32 Ω, 53 Ω, 70 Ω, 86 Ω, 123 Ω, 159 Ω for contact pitches of 5 μm, 10 μm, 15 μm, 20 μm, 30 μm, and 40 μm, respectively; FIG. 5 is a schematic diagram of I-V curves for different contact pitches in a #4GaN HEMTs device subjected to 3 Ashr treatments and hydrochloric acid treatments, and the resistances of 38 Ω, 55 Ω, 70 Ω, 87 Ω, 130 Ω, 178 Ω, respectively, for contact pitches of 5 μm, 10 μm, 15 μm, 20 μm, 30 μm, and 40 μm can be obtained by extracting the slopes of the I-V curves.
(2) Calculation of specific contact resistivity in TLM
And drawing the resistances with different contact pitches obtained in the I-V curve in a relation chart with the metal electrode pitch as an abscissa and the contact resistance as an ordinate, and obtaining the specific contact resistivity through linear fitting and TLM theoretical calculation. FIG. 6 is a graph of total resistance and a linear fit of total resistance to contact spacing (in the graph, the abscissa is the distance l between two adjacent metal electrodes, and the ordinate is the total resistance R T) calculated for different contact spacing in a #1GaN HEMTs device subjected to 0 Ashr treatment and hydrochloric acid treatment, and the contact resistance is 1.02Ω -mm, the sheet resistance is 351 Ω/≡, and the specific contact resistivity is 2.96X10: 10 -5Ω·cm2, calculated according to TLM theory; FIG. 7 is a graph of total resistance and linear fit of total resistance to contact spacing calculated for a #2GaN HEMTs device with 1 Ashr treatment and hydrochloric acid treatment, calculated according to TLM theory to have a contact resistance of 0.96 Ω mm, a sheet resistance of 350Ω/≡, and a specific contact resistivity of 2.63X10 -5Ω·cm2; FIG. 8 is a graph of total resistance and linear fit of total resistance to contact spacing calculated for a #3GaN HEMTs device with 2 Ashr and hydrochloric acid treatments, calculated according to TLM theory to have a contact resistance of 0.8Ω.mm, a sheet resistance of 357 Ω/≡, and a specific contact resistivity of 1.79X10 -5Ω·cm2; fig. 9 is a graph of total resistance and linear fit of total resistance to contact pitch calculated for #4GaN HEMTs devices with 2 Asher treatments and hydrochloric acid treatments, calculated according to TLM theory to have a contact resistance of 0.66 Ω·mm, a sheet resistance of 399 Ω/≡, and a specific contact resistivity of 1.09×10 -5Ω·cm2.
(3) Relation between specific contact resistivity and Ashr times
To compare the relationship between the specific contact resistivity and the number of Asher times, three TLM tests were performed on different regions of #1, #2, #3, #4 devices, respectively, wherein the average specific contact resistivity of the #1 device, which was subjected to 0 Asher treatments and hydrochloric acid treatments, was 3.05x10 -5Ω·cm2; the average specific contact resistivity of the #2 device, which was subjected to 1 ash treatment and hydrochloric acid treatment, was 2.66×10 -5Ω·cm2; the average specific contact resistivity of the #3 device, which was subjected to 2 Asher treatments and hydrochloric acid treatments, was 2.19x10 -5Ω·cm2; the average specific contact resistivity of the #4 device, which was subjected to 3 Asher treatments and hydrochloric acid treatments, was 1.29×10 -5Ω·cm2. As the number of Asher increases, the specific contact resistivity is linearly reduced, which indicates that the pre-growth pretreatment of the metal electrode based on the Asher treatment provided by the invention can effectively reduce the ohmic contact of the GaN HEMTs device. The specific contact resistivity and the Ashr number are linear in the experiment, and the specific contact resistivity and the Ashr number are determined by comprehensively considering the thickness of the barrier layer of the device and the experiment cost by adopting a plurality of Ashr times.
While the foregoing is directed to the preferred embodiments of the present invention, it will be appreciated by those skilled in the art that various modifications and adaptations can be made without departing from the principles of the present invention, and such modifications and adaptations are intended to be comprehended within the scope of the present invention.
Claims (4)
1. A method for reducing ohmic contact resistance of a device, comprising the steps of:
(1) Sequentially growing an AlN nucleation layer, a GaN buffer layer, an AlN insertion layer, an AlGaN barrier layer and a GaN cap layer on the surface of the SiC substrate;
(2) Performing mesa etching by adopting an inductively coupled plasma etching method to obtain a single mesa;
(3) Obtaining a pattern of a metal electrode to be grown on a single mesa by adopting a photoetching developing method;
(4) The method comprises the steps of sequentially adopting low-power oxygen plasma cleaning and HCl cleaning to perform multiple metal electrode growth pretreatment, in each treatment, firstly adopting the low-power oxygen plasma cleaning to oxidize, then corroding oxide through HCl, corroding a GaN cap layer firstly through multiple treatments, corroding part of AlGaN barrier layer secondly, achieving the purpose of thinning the barrier layer, and removing photoresist remained on the surface of the GaN HEMTs device;
(5) Growing a metal electrode by adopting a physical vapor deposition method;
(6) Stripping the metal grown by physical vapor deposition in acetone to obtain a metal electrode;
(7) Carrying out high-temperature rapid annealing on the GaN HEMTs device in an N 2 atmosphere in a rapid annealing furnace to form ohmic contact;
The GaN HEMTs comprises a SiC substrate, an AlN nucleation layer, a GaN buffer layer, an AlN insertion layer, an AlGaN barrier layer and a GaN cap layer from bottom to top in sequence, wherein the GaN buffer layer, the AlN insertion layer and the AlGaN barrier layer form a heterojunction;
the metal electrode grows on the AlGaN barrier layer through the GaN cap layer;
The thickness of the GaN buffer layer is 1-3 mu m, and the GaN buffer layer is not doped; the thickness of the AlN insertion layer is 0.5-1.5 nm; the molar ratio of Al components in the AlGaN barrier layer is 0.15-0.25, and the thickness of the AlGaN barrier layer is 15-25 nm; the thickness of the GaN cap layer is 1-3 nm;
The process of performing metal electrode growth pretreatment by adopting low-power oxygen plasma cleaning comprises the following steps:
3.1, opening a chamber door of the oxygen plasma cleaning machine, putting a GaN HEMTs device to be annealed, and closing the chamber door;
3.2, vacuumizing until the vacuum degree in the chamber is lower than 1 multiplied by 10 -5 Torr;
3.3, introducing O 2,O2 into the cavity, and keeping the flow at 30 sccm;
3.4, starting a microwave power supply, and stabilizing the power at 50W and keeping the power for 100s;
3.5, taking out the GaN HEMTs device after the microwave power is turned off and the power is reduced to 0W and the O 2 flow is reduced to 0 sccm;
In the step (7), ohmic contact between the metal electrode and the AlGaN barrier layer is formed at high temperature by using a rapid annealing technology, and the specific steps are as follows:
7.1, opening a chamber door of the rapid annealing furnace, putting a GaN HEMTs device to be annealed, and closing the chamber door;
7.2, introducing N 2 into the cavity, and evacuating the air in the cavity;
7.3, after N 2 and min are introduced into the chamber, heating the chamber to 850 ℃ within 17 seconds, and preserving heat for 40 seconds;
7.4, taking out the GaN HEMTs device after the chamber is rapidly cooled to room temperature;
In step 7.2, the flow rate of the gas is adjusted to 10 SLM when N 2 is introduced.
2. The method for reducing ohmic contact resistance of a device according to claim 1, wherein the specific process of performing mesa etching by inductively coupled plasma etching in step (2) is as follows:
2.1, coating photoresist on the GaN cap layer;
2.2, exposing a region to be etched on the photoresist by utilizing a photoetching development technology;
2.3, etching the table top by using an inductively coupled plasma device, wherein the etching depth is 180-190 nm, and extends to the GaN buffer layer, so that the effect of isolating devices on the substrate is achieved;
and 2.4, removing the coated photoresist.
3. The method for reducing ohmic contact resistance of a device according to claim 2, wherein in the step (4), the specific process of performing the metal electrode growth pretreatment by the hydrochloric acid cleaning method is as follows:
A. preparing dilute hydrochloric acid with the concentration of 15-25% in a culture dish of 30 mL%;
B. Putting the GaN HEMTs subjected to low-power oxygen plasma cleaning treatment into dilute hydrochloric acid for soaking 4 min;
C. the GaN HEMTs washed by dilute hydrochloric acid are put into deionized water for washing 1 min, and the GaN HEMTs are dried by nitrogen after washing.
4. The method of claim 3, wherein the electrode grown by physical vapor deposition in the step (5) is 4 layers of metal, and the 4 layers of metal are sequentially Ti/Al/Ni/Au from bottom to top, wherein the thickness of Ti is 30 nm, the thickness of Al is 150 nm, the thickness of Ni is 50 nm, and the thickness of Au is 50 nm.
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