CN114784104A - Radio frequency device without gold ohmic contact based on two-step annealing and preparation method thereof - Google Patents

Abstract

The invention relates to a radio frequency device without gold ohmic contact based on two-step annealing and a preparation method thereof, comprising a first semiconductor lamination arranged on a substrate, wherein the first semiconductor lamination comprises a nitride channel layer; a source electrode, a second stack of semiconductor layers, and a drain electrode disposed on the same layer on the first stack of semiconductor layers, the second stack of semiconductor layers including an insertion layer disposed on the nitride channel layer and a nitride barrier layer disposed on the insertion layer; and a gate disposed on the nitride barrier layer; the source electrode and the drain electrode comprise a contact layer, a covering layer, a blocking layer and a cap layer which are sequentially stacked, the contact layer is formed by circularly stacking a low-work-function metal M sublayer and a metal Al sublayer, and the first sublayer and the last sublayer are metal M. The annealing alloy temperature is reduced, the problem of high on-resistance is solved, the cut-off frequency and the maximum oscillation frequency of the device are improved, and the process line is prevented from being polluted by impurities.

Description

Au-free ohmic contact radio frequency device based on two-step annealing and its preparation method

technical field

The invention relates to the field of radio frequency devices, in particular to a radio frequency device without gold ohmic contact based on two-step annealing and a preparation method thereof.

Background technique

One of the main problems of current RF power devices is the low dynamic on-resistance, which will reduce the maximum switching frequency of the device and increase the switching loss. Contact metallization schemes for source and drain electrodes have been researched and optimized for more than two decades. To ensure stable, reliable and low-resistance ohmic contact HEMT devices, the most commonly used metal stack scheme is Ti/Al/Ni/Au. From bottom to top are the contact layer, the cover layer, the barrier layer, and the cap layer. Ti is often used as a contact layer, which will react with N in AlGaN to form TiN, resulting in the generation of a large number of N vacancies, which makes electrons prone to tunneling effect. Al is often used as the cladding layer to prevent the Al and Ga elements in AlGaN from diffusing out, resulting in a decrease in the donor concentration. Ni is often used as an isolation layer to prevent the diffusion of Au to the surface of AlGaN. Au is often used as a cap layer to prevent Ti and Al from being oxidized to form a high-resistance oxide layer, resulting in an increase in contact resistance. On this basis, annealing in the temperature range of 800°C to 950°C after metal stacking promotes the formation of low-resistance contacts.

However, the mainstream semiconductor production line in the domestic market is HEMT devices produced by Si-CMOS process line. In the traditional Ti/Al/Ni/Au scheme, heavy metal Au will form deep-level impurities in Si, polluting the CMOS process line. . And the use of Au will not only cause high temperature annealing to form rough electrode surfaces and edges, but also lead to the appearance of a peak electric field, which reduces the breakdown characteristics of the device. Therefore, the use of a low-temperature gold-free ohmic process avoids the pollution problem caused by Au, thereby improving the performance reliability of the HEMT device, and on the other hand, enables large-scale manufacturing of Si-CMOS process lines.

For conventional W, Mo, Cu and other gold-free ohmic contact electrodes, during the high temperature annealing process, since the metal Al layer usually requires more than 100 nm, the thickness of Al will be much larger than the thickness of the base layer Ti, and a large amount of unreacted Al will agglomerate to form an island structure The Ti-Al alloys are incompatible, resulting in poor surface morphology and edge morphology of the source and drain electrodes.

SUMMARY OF THE INVENTION

The primary purpose of the present invention is to provide a gold-free ohmic contact radio frequency device based on two-step annealing and a preparation method thereof. Compared with the traditional Ti/Al/Ni/Au ohmic contact electrode metal system, the RF device inserts the Al thin layer into the low work function metal M layer, and the formed M/Al/MAl/M and other multi-layer metals are cross-stacked as the The contact layer, and then two-step annealing to form an ohmic contact, not only effectively increases the contact area between the metal M and Al, so that Ti and Al can fully react, but also reduces the requirements of the process annealing temperature, and also forms a surface Ohmic contact with low topographical roughness and low contact resistance value. Due to the excellent electrical conductivity, thermal conductivity and plasticity of Cu, and high compatibility with copper interconnect technology and assembly technology, the process complexity is reduced. The radio frequency device prepared on this basis effectively improves the problem of high on-resistance, and increases the cut-off frequency and the maximum oscillation frequency.

The radio frequency device includes a substrate; a first semiconductor stack arranged on the substrate, which includes a nitride channel layer; a source electrode, a second semiconductor stack and a leakage current arranged in the same layer on the first semiconductor stack a pole, the second semiconductor stack includes an insertion layer arranged on the nitride channel layer and a nitride barrier layer arranged on the insertion layer; and a gate arranged on the nitride barrier layer;

In the direction of the insertion layer pointing to the nitride barrier layer, the source electrode and the drain electrode include a contact layer, a cap layer, a barrier layer and a cap layer stacked in sequence, and the contact layer is composed of a low work function metal M sublayer and metal Al sublayers are formed by cyclic stacking, which comprises at least three sublayers, wherein the first sublayer and the last sublayer are metal M.

The thickness of the sub-layers is 5 nm to 15 nm, and the total thickness of the contact layer is 20 nm to 50 nm.

The metal M is selected from at least one of Ti, Ta, Mo and V, and the total thickness of the metal M sub-layer is not less than 20 nm.

The cover layer is selected from Al, and has a thickness of 30 nm to 50 nm.

The thickness of the sub-layer is that the thickness of metal Al and the total thickness of the cover layer are not greater than 150 nm.

The blocking layer is selected from Ni, Pt, Cr, Ti or Mo, and has a thickness of 30 nm to 50 nm; the cap layer is selected from Cu, and has a thickness of 50 nm to 80 nm.

After the contact layer and the cover layer are deposited, and before the barrier layer and the cap layer are deposited, a low temperature rapid thermal annealing treatment is performed, the annealing atmosphere is N ₂ , the annealing temperature is 500°C to 600°C, and the annealing time is 30s to 60s.

After the barrier layer and the cap layer are deposited, high temperature rapid thermal annealing is performed, the annealing atmosphere is N ₂ , the annealing temperature is 700°C to 850°C, and the annealing time is 60s to 10min.

The nitride barrier layer is an _{InxAlyGa1 -xyN layer} with a thickness of 5nm to 15nm, 0≤x≤1, 0≤y≤1.

The first semiconductor stack further includes a stress relief layer and a buffer layer sequentially stacked between the substrate and the nitride channel layer.

Compared with the prior art, the present invention at least has the following beneficial effects:

In the radio frequency device provided by the present invention, source-drain electrodes composed of a contact layer, a cover layer, a barrier layer and a cap layer are sequentially stacked on the nitride channel layer, and an insertion layer and a nitride barrier layer are arranged between the source-drain electrodes. The contact layer is composed of low work function metal M sublayers and metal Al sublayers cyclically stacked, and the cap layer is selected from Cu, thereby forming a radio frequency device without gold ohmic contact. The Al sublayer is inserted between the metal M layers, so that the metal M There is sufficient contact between the layer and the metal Al, which reduces the temperature of the annealing alloy, and the metal M and N elements fully react to generate N vacancies to increase the electron tunneling effect. The absence of Al preserves the high conductivity of 2DEG and reduces the contact resistance of the device. The manufacturing cost of the nitride-based HEMT device is effectively reduced, and the formation of impurities to contaminate the process line is avoided.

In the process of preparing the source-drain electrode of the present invention, the first low-temperature annealing treatment is performed after the deposition of the contact layer and the cover layer, and the second annealing treatment is performed after the deposition of the barrier layer and the cap layer, which is the same as the total thickness. Compared with the Ti/Al/Ni/Cu gold-free ohmic scheme, the present invention effectively reduces the annealing temperature for forming the ohmic contact and reduces the difficulty of the process.

Description of drawings

FIG. 1 is a schematic diagram of a metal structure in which a multi-layer Ti/Al overlapped HEMT ohmic contact electrode structure is Ti/Al/Ti/Al...Ti/Al/Ni/Cu according to an embodiment of the present invention.

FIG. 2 is a schematic structural diagram of a radio frequency device according to an embodiment of the present invention.

FIG. 3 is a schematic diagram of a TLM pattern of a lithography test piece in an embodiment of the present invention.

FIG. 4 is an I-V characteristic curve diagram of a TLM test according to an embodiment of the present invention.

FIG. 5 is an R-L characteristic curve diagram of a TLM test according to an embodiment of the present invention.

Detailed ways

Next, the technical solutions in the embodiments of the present invention will be clearly and completely described with reference to the accompanying drawings of the present invention, and the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, other embodiments obtained by those of ordinary skill in the art without creative work shall fall within the protection scope of the present invention. The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials can be obtained from open commercial sources unless otherwise specified.

Spatially relative terms such as "below," "below," "under," "over," "over," "over," and the like are used in this specification to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device other than those shown in the figures.

Additionally, the use of terms such as "first", "second", and the like, to describe various elements, layers, regions, sections, etc., is not intended to be limiting. The use of "having," "containing," "including," "including," and the like are open-ended terms indicating the presence of stated elements or features, but not the exclusion of additional elements or features. unless the context clearly dictates otherwise.

As shown in FIG. 1 and FIG. 2, an embodiment of the present invention provides a two-step annealing-based radio frequency device without gold ohmic contact, which includes a substrate 1, and the substrate can be a Si substrate, a SiC substrate or a sapphire substrate Bottom, preferably a SiC substrate. A stress release layer 2, a buffer layer 3 and a nitride channel layer 4 are sequentially stacked on the substrate 1. The stress release layer 2 is preferably an AlN layer with a thickness of 10 nm to 20 nm. The buffer layer 3 is made of GaN or AlGaN, and has a thickness of 1 μm to 2 μm. In this embodiment, a GaN buffer layer is preferred. The nitride channel layer 4 is selected from GaN, AlGaN or InGaN, and has a thickness of 20-50 nm. In this embodiment, a GaN channel layer is preferred. An insertion layer 5 and a nitride barrier layer 6 are stacked on the nitride channel layer 4 , and the insertion layer 5 is in contact with the nitride channel layer 4 . The insertion layer is an AlN insertion layer with a thickness of 1 nm to 2 nm. The nitride barrier layer 6 is selected from one of AlGaN, InAlAs, InAlN, and InAlGaN, and the thickness is 6-10 nm. Preferably, the nitride barrier layer 6 is selected from In _{x AlyGa 1-xy} N, 0≤x≤1 , 0≤y≤1, preferably, the nitride barrier layer 6 is selected from AlGaN.

The source electrode 8 and the drain electrode 7 are arranged on the surface of the nitride channel layer 4 in the same layer as the insertion layer 5. The source electrode 8 and the drain electrode 7 are composed of a contact layer, a cover layer, a barrier layer and a cap layer stacked in sequence. The contact layer in contact with the nitride channel layer. The contact layer is composed of a low work function metal M sublayer and a metal Al sublayer cyclically stacked, which includes at least three sublayers, wherein the first sublayer and the last sublayer are metal M, and the low work function metal M is selected from metals Ti and Ta. At least one of , Mo and V, preferably, the metal M is Ti. The metal M as the contact layer metal reacts with N in the nitride channel layer to generate MN, forming a large number of N vacancies, which is conducive to the electron tunneling effect. For example, metal Al is inserted between Ti layers, which can increase the contact area between Ti and Al to form a multilayer Ti/Al stack. In the environment of subsequent annealing, the Ti contact layer reacts with nitrogen in the nitride, and the reaction process The diffusion of the elements in the medium, especially the excessive diffusion of the Al element, will deteriorate the performance of the device. The multi-layer Ti/Al stack can suppress the diffusion of Al under the condition of ensuring the sufficient reaction of Ti and N. At the same time, the use of the stack will reduce the annealing temperature while maintaining the total thickness of Al, and the multi-layer Ti/Al stack will reduce the annealing temperature. For example, the contact layer is Ti/Al/Ti, Ti/Al/Ti/Al/Ti or Ti/Al/Ti/Al/Ti/Al/Ti. The thickness of each sublayer is 5nm to 15nm, the total thickness of the contact layer is 20nm to 50nm, the total thickness of the metal M sublayer is not less than 20nm, and there is a relationship between the thickness of metal M and the thickness of metal Al and the annealing temperature , when the thickness of the metal M is large, the optimal annealing temperature to form an ohmic contact is between 750°C and 950°C, and when the thickness of the Al layer is large, the optimal annealing temperature is around 550°C.

The cover layer is selected from Al, and the thickness is 30nm to 50nm. The thickness of the neutron layer in the contact layer is that the thickness of Al and the thickness of the cover layer add up to not more than 150 nm. The thickness relationship between the contact layer and the capping layer is the key to the fabrication of Au-free ohmic contact electrodes for nitride-based HEMTs. Under the condition of keeping the total thickness of the Al layer unchanged, the insertion of Al into the Ti layer can suppress the diffusion of Al under the condition that the sufficient reaction between Ti and N in the nitride is ensured. And under the condition that the total thickness of Al remains unchanged, the insertion of Al into the Ti layer will reduce the annealing temperature and ensure the reliability of the device. After the capping layer is deposited, a first low-temperature annealing treatment is performed, the annealing time is 30 seconds to 60 seconds, the annealing temperature is 500° C. to 600° C., and the annealing atmosphere is N ₂ .

The barrier layer is selected from Ni, Pt, Cr, Ti or Mo, and the thickness is 30nm to 50nm. Preferably, Ni is selected for the barrier layer. The cap layer is made of Cu with good process compatibility, and its thickness is 50nm to 80nm. The use of the barrier layer avoids the serious diffusion of Cu during the subsequent high temperature annealing process, avoids the generation of vacancy defects, and reduces the contact resistance.

After the cap layer is deposited, a second thermal annealing treatment is performed, the annealing temperature is 700° C. to 850° C., the annealing time is 60 seconds to 10 minutes, and the atmosphere is high-purity nitrogen.

The passivation layer 10 is disposed on the surface of the barrier layer between the source electrode 8 and the drain electrode 7 , and the passivation layer is selected from Si ₃ N ₄ , Al ₂ O ₃ or TiO ₂ . The gate opening is arranged in the passivation layer 10, and the gate 9 is arranged in the gate opening. The gate is preferably a T-shaped gate.

Based on the above radio frequency device, the present invention also discloses a preparation method of the radio frequency device, comprising the following steps:

First, a SiC substrate is selected, and an AlN stress release layer, a GaN buffer layer, a GaN channel layer, an AlN insertion layer and an AlGaN barrier layer are sequentially epitaxially grown on the substrate to obtain an epitaxial wafer.

Then, the epitaxial wafer was placed in acetone for 5 min, then placed in isopropanol for ₁₀ min, then rinsed with deionized water for more than ₅ times, and dried with nitrogen; The solution is used for electrodeless cleaning of the surface of the epitaxial wafer to remove oxides and impurities on the surface.

Next, the cleaned epitaxial wafers were uniformly glued, pre-baked at 105°C for 90s, exposed for 6s and developed for 60s to obtain a mask pattern, which was detected by a step meter. In this step, the cleaned epitaxial wafer is uniformly glued at the same time, and the process parameters are kept unchanged, and a test piece with a TLM electrode pattern is obtained after pre-baking, exposure, and development.

The first ICP etching, the buffer mesa is carved. At vacuum degree ~ 10-2 Torr, the gas flow rate of BCl ₃ gas is 10.0sccm, the flow rate of Cl ₂ gas is 90.0sccm, the etching depth is 210nm ~ 220nm, degumming, organic treatment, microscopic inspection.

Then, the glue was uniformed, pre-baked at 105°C for 90s, exposed for 6s, developed for 60s, and then continued to use the ICP etching process to etch the device area to form the source-drain ohmic contact electrode area. Keeping the ICP etching parameters unchanged, the ICP is etched to the surface of the channel layer, and the etching depth is 25-30 nm.

Next, the glue is uniformized, pre-baked at 105°C for 90s, exposed for 6s, developed for 60s, microscopically inspected, and detected by a step meter to redefine the source-drain ohmic contact electrode area.

The electron beam evaporation process was used to sequentially evaporate the contact layer metal Ti/Al/Ti with the thickness of 10nm/10nm/10nm and the cover layer metal Al with the thickness of 90nm. Metal stripping is then performed. Afterwards, the alloy was annealed at low temperature under _N2 atmosphere, the annealing temperature was 600 °C, and the annealing time was 30 s. In this step, electron beam evaporation is performed on the test piece to deposit the contact metal layer Ti/Al/Ti and the cover layer metal Al at the same time, and then the annealing parameters are kept unchanged, and the test piece is annealed.

Next, the electron beam evaporation process is continued to deposit the barrier layer metal Ni and the cap layer metal Cu, the thickness of the metal Ni layer is 30 nm, the thickness of the metal Cu layer is 50 nm, and then metal stripping is performed. Afterwards, high temperature annealing and alloying were carried out in _N2 atmosphere, the annealing temperature was 700°C to 850°C, and the annealing time was 60s. In this step, electron beam evaporation was performed to deposit the barrier metal Ni and cap layer metal Cu on the test piece at the same time, and then annealed under the same conditions to obtain the TLM test structure of GaN-based HEMT without gold ohmic contact as shown in Figure 3 .

A passivation layer of Si ₃ N ₄ is deposited on the surface of the barrier layer between the source electrode and the drain electrode, and the passivation layer is etched to define the gate region. Electron beam evaporation is used to deposit gate metal in the gate area. Then annealed at 850°C for 30s to form a T-shaped gate. Refer to Figure 2 for its structure diagram.

The I-V test is performed on the TLM test structure obtained in this example. As shown in FIG. 3 , the TLM electrode spacings L are 5um, 10um, 15um, 20um, 25um, and 30um, respectively. The I-V characteristic curve at 850°C is shown in Figure 4, and it can be seen that a good ohmic contact characteristic is obtained. Its R-L curve is shown in Fig. 5. It is known from calculation that the resistance of the ohmic contact prepared in this example is 0.129Ω·mm, indicating that good ohmic contact performance is obtained.

The above-mentioned embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the above-mentioned embodiments, and any other changes, modifications, substitutions, combinations, The simplification should be equivalent replacement manners, which are all included in the protection scope of the present invention.

Claims (10)

1. A radio frequency device based on two-step annealing and without gold ohmic contact comprises,

a substrate;

a first semiconductor stack disposed on a substrate, including a nitride channel layer;

a source electrode, a second stack of semiconductor layers, and a drain electrode disposed on the same layer on the first stack of semiconductor layers, the second stack of semiconductor layers including an insertion layer disposed on a nitride channel layer and a nitride barrier layer disposed on the insertion layer;

and a gate disposed on the nitride barrier layer;

the source electrode and the drain electrode comprise a contact layer, a covering layer, a blocking layer and a cap layer which are sequentially stacked along the direction of the insertion layer pointing to the nitride barrier layer, the contact layer is formed by circularly stacking a low-work-function metal M sublayer and a metal Al sublayer and comprises at least three sublayers, wherein the first sublayer and the last sublayer are metal M.

2. The radio frequency device according to claim 1, characterized in that the thickness of the sub-layer is 5nm to 15nm and the total thickness of the contact layer is 20nm to 50 nm.

3. The rf device of claim 2, wherein the metal M is at least one of Ti, Ta, Mo and V, and the total thickness of the metal M sub-layer is not less than 20 nm.

4. A radio frequency device according to one of claims 1 to 3, characterized in that said cover layer is made of Al and has a thickness of 30nm to 50 nm.

5. The radio frequency device according to claim 4, characterized in that the thickness of the sublayer of metallic Al and the total thickness of the cover layer are not more than 150 nm.

6. The RF device of claim 4, wherein the barrier layer is selected from Ni, Pt, Cr, Ti or Mo and has a thickness of 30nm to 50 nm; the capping layer is made of Cu, and the thickness of the capping layer is 50nm to 80 nm.

7. The RF device according to claim 5 or 6, wherein after the deposition of the contact layer and the capping layer and before the deposition of the barrier layer and the cap layer, a low temperature rapid thermal annealing process is performed in an atmosphere of N₂The annealing temperature is 500 ℃ to 600 ℃, and the annealing time is 30s to 60 s.

8. The radio frequency device of claim 7, wherein the barrier layer and capAfter layer deposition, high-temperature rapid thermal annealing treatment is carried out, wherein the annealing atmosphere is N₂The annealing temperature is 700 ℃ to 850 ℃, and the annealing time is 60s to 10 min.

9. The RF device of claim 5 or 6, wherein the nitride barrier layer is selected to be In with a thickness of 5nm to 15nm_xAl_yGa_1-x-yAnd x is more than or equal to 0 and less than or equal to 1, and y is more than or equal to 0 and less than or equal to 1.

10. The radio frequency device according to claim 5 or 6, wherein the first semiconductor stack further comprises a stress relief layer and a buffer layer sequentially stacked between the substrate and the nitride channel layer.

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