Method of forming metal contacts in the barrier layer of a group III-N HEMT

Abstract

Metal contact openings are etched in the barrier layer of a group III-N HEMT with a first gas combination that etches down into the barrier layer, and a second gas combination that etches further down into the barrier layer to a depth that lies above the top surface of a channel layer that touches and lies below the barrier layer.

Claims

1. A method of forming a high electron mobility transistor comprising: forming a layered structure including a barrier layer directly on a channel layer directly on a buffer layer directly on a substrate; forming metal contact openings in the layered structure by etching with a first gas combination, each of the metal contact openings having a first bottom surface that lies above and spaced apart from the channel layer; and etching the layered structure with a second gas combination to deepen each metal contact opening a distance to a second bottom surface, the second bottom surface lying above and spaced apart from the channel layer, wherein the second gas combination etches more of the barrier layer than does the first gas combination and wherein the first gas combination also etches through a cap layer on the barrier layer, and through a passivation layer on the cap layer, the cap layer including GaN, the passivation layer including silicon nitride and wherein the first gas combination includes boron trichloride (BCl.sub.3) and sulfur hexafluoride (SF.sub.6), and the second gas combination includes boron trichloride (BCl.sub.3) and chlorine (Cl.sub.2).

2. The method of claim 1, further comprising depositing a metal contact layer that contacts each second bottom surface and fills the metal contact openings.

3. The method of claim 1, wherein the channel layer comprises GaN.

4. The method of claim 1, wherein the barrier layer is AlGaN.

5. A method of forming a high electron mobility transistor comprising: forming a layered structure including a barrier layer directly on a channel layer directly on a buffer layer directly on a substrate; forming metal contact openings in the layered structure by etching with a first gas combination, each of the metal contact openings having a first bottom surface that lies above and spaced apart from the channel layer; and etching the layered structure with a second gas combination to deepen each metal contact opening a distance to a second bottom surface, the second bottom surface lying above and spaced apart from the channel layer, wherein the second gas combination etches more of the barrier layer than does the first gas combination and wherein the first gas combination also etches through a cap layer on the barrier layer, and through a passivation layer on the cap layer, the cap layer including GaN, the passivation layer including silicon nitride; depositing a metal contact layer that contacts each second bottom surface and fills the metal contact openings; and planarizing the metal contact layer to form a number of spaced-apart metal contacts that lie in the metal contact openings.

6. The method of claim 5, wherein the spaced-apart metal contacts consist of a source contact and a drain contact.

7. The method of claim 5, wherein the metal layer comprises a titanium layer, an aluminum copper layer over the titanium layer, and a titanium nitride layer over the aluminum copper layer.

8. A method of forming a high electron mobility transistor comprising: forming a layered structure including an AlGaN barrier layer directly on a GaN channel layer directly on a buffer layer directly on a substrate; forming metal contact openings in the layered structure by etching with a first gas combination, each of the metal contact openings extending into the AlGaN barrier layer and having a first bottom surface that lies above and spaced apart from the GaN channel layer; and etching the layered structure with a second gas combination to deepen each metal contact opening a distance to a second bottom surface, the second bottom surface lying above and spaced apart from the GaN channel layer, wherein the second gas combination etches more of the AlGaN barrier layer than does the first gas combination and wherein the first gas combination also etches through a cap layer on the AlGaN barrier layer, and through a passivation layer on the cap layer, the cap layer including GaN, the passivation layer including silicon nitride.

9. The method of claim 8, wherein the first gas combination includes boron trichloride (BCl.sub.3) and sulfur hexafluoride (SF.sub.6), and the second gas combination includes boron trichloride (BCl.sub.3) and chlorine (Cl.sub.2).

10. The method of claim 8, further comprising depositing a metal layer that contacts each second bottom surface and fills the metal contact openings.

11. The method of claim 10 and further comprising planarizing the metal layer to form a number of spaced-apart metal contacts that lie in the metal contact openings.

12. The method of claim 10, wherein the metal layer comprises a titanium layer, an aluminum copper layer over the titanium layer, and a titanium nitride layer over the aluminum copper layer.

13. The method of claim 11, wherein the spaced-apart metal contacts consist of a source contact and a drain contact.

14. A method of forming a high electron mobility transistor comprising: forming a layered structure including an AlGaN barrier layer directly on a GaN channel layer directly on a buffer layer directly on a substrate, wherein the buffer layer comprises a AlN layer, a AlGaN layer, and a GaN layer; forming metal contact openings in the layered structure by etching with a first gas combination, each of the metal contact openings extending into the AlGaN barrier layer and having a first bottom surface that lies above and spaced apart from the GaN channel layer, wherein the first gas combination includes boron trichloride (BCl.sub.3) and sulfur hexafluoride (SF.sub.6); and etching the layered structure with a second gas combination to deepen each metal contact opening a distance to a second bottom surface, the second bottom surface lying above and spaced apart from the GaN channel layer, wherein the second gas combination includes boron trichloride (BCl.sub.3) and chlorine (Cl.sub.2) and wherein the first gas combination also etches through a cap layer on the AlGaN barrier layer, and through a passivation layer on the cap layer, the cap layer including GaN, the passivation layer including silicon nitride; depositing a metal layer that contacts each second bottom surface and fills the metal contact openings; and planarizing the metal layer to form a number of spaced-apart metal contacts that lie in the metal contact openings.

15. The method of claim 14, wherein the metal layer comprises a titanium layer, an aluminum copper layer over the titanium layer, and a titanium nitride layer over the aluminum copper layer.

16. The method of claim 14, wherein the spaced-apart metal contacts consist of a source contact and a drain contact.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1-5 are a series of cross-sectional views illustrating an example of a method 100 of forming a group III-N HEMT in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) FIGS. 1-5 show a series of cross-sectional views that illustrate an example of a method 100 of forming a group III-N HEMT in accordance with the present invention. As described in greater detail below, the method of the present invention utilizes a two-step etch process to form metal contact openings in a group III-N HEMT with depths that are easily controlled and do not extend into the channel layer.

(3) As shown in FIG. 1, method 100 utilizes a conventionally-formed group III-N HEMT 108. HEMT 108, in turn, includes a single-crystal, lightly-doped, p-type silicon semiconductor substrate 110 (e.g., <111>), and a layered structure 112 that is formed on the top surface of substrate 110.

(4) Layered structure 112, in turn, includes a buffer layer 114 that touches substrate 110, a channel layer 116 that touches buffer layer 114, and a barrier layer 118 that touches channel layer 116. Further, layered structure 112 can optionally include a cap layer 120 that lies over barrier layer 118.

(5) Buffer layer 114 provides a transition layer between substrate 100 and channel layer 116 as a result of lattice mismatches. Channel layer 116 and barrier layer 118 have different polarization properties and band gaps that induce the formation of a two-dimensional electron gas (2DEG) that lies at the top of channel layer 116. Cap layer 120 provides enhanced reliability.

(6) Each of the layers in layered structure 112 can be implemented with one or more sequential group-III nitride layers, with the group-III including one or more of In, Ga, and Al. For example, buffer layer 114 can be implemented with sequential layers of AlN (a thermally-stable material), AlGaN, and GaN. In addition, channel layer 116 can be formed from GaN, while barrier layer 118 can be formed from AlGaN. Further, cap layer 120 can be formed from GaN.

(7) Further, HEMT 108 includes a passivation layer 122 that touches the top surface of layered structure 112 (on the top surface of cap layer 120 if present, or the top surface of barrier layer 118 when cap layer 120 is not present). Passivation layer 122 can be implemented with, for example, a silicon nitride layer.

(8) As further shown in FIG. 1, method 100 begins by forming a patterned photoresist layer 124 on passivation layer 122. Patterned photoresist layer 124 is formed in conventional manner, which includes depositing a layer of photoresist, projecting a light through a patterned black/clear glass plate known as a mask to form a patterned image on the layer of photoresist to soften the photoresist regions exposed by the light, and removing the softened photoresist regions.

(9) As shown in FIG. 2, after patterned photoresist layer 124 has been formed, the exposed regions of passivation layer 122, the underlying portions of cap layer 120 (when present), and the underlying portions of barrier layer 118 are dry etched using a gas combination that includes boron trichloride (BCl.sub.3) and sulfur hexafluoride (SF.sub.6) to form source and drain metal contact openings 132.

(10) Each metal contact opening 132 has a bottom surface 136 that lies above and spaced apart from the top surface of channel layer 116. In the present example, the following etch conditions are used:

(11) Pressure: 19 mT-21 mT (preferably 20 mT);

(12) TCP RF: 200 W-400 W (preferably 300 W);

(13) Bias RF: 47.5 W-52.5 W (preferably 50 W);

(14) BCl.sub.3: 20 ccm-30 ccm (preferably 25 ccm);

(15) SF.sub.6: 45 ccm-65 ccm (preferably 55 ccm);

(16) He Clamp: 5T-10T (preferably 6T); and

(17) Temp: 45 degC-65 degC (preferably 55 C).

(18) The BCl.sub.3 and SF.sub.6 gas combination under the above conditions etches down into barrier layer 118 for a period of time, but then etches substantially no deeper into barrier layer 118 after the period of time. For example, the BCl.sub.3 and SF.sub.6 gas combination under the above preferred conditions etches down into an AlGaN barrier layer 118 a distance of approximately 43 Å during an etch time of 65 seconds.

(19) However, from 65 seconds to 200 seconds, the BCl.sub.3 and SF.sub.6 gas combination etches substantially no deeper into the AlGaN barrier layer 118. Thus, barrier layer 118 is etched with the BCl.sub.3 and SF.sub.6 gas combination for a predefined time that is equal to or greater than the period of time.

(20) As shown in FIG. 3, after the BCl.sub.3 and SF.sub.6 etch, the gas is changed and the regions of barrier layer 118 exposed by the metal contact openings 132 are dry etched for a predetermined period of time using a gas combination that includes BCl.sub.3 and CL.sub.2 to deepen each bottom surface 136 to a lower bottom surface 140. In the present example, the BCl.sub.3 and CL.sub.2 gas combination etches more of barrier layer 118 than does the BCl.sub.3 and SF.sub.6 gas combination.

(21) Each lower bottom surface 140 lies above and spaced apart from the top surface of channel layer 116 by a separation distance D. After the etch, patterned photoresist layer 124 is removed in a conventional manner, such as with an ash process. In the present example, the following etch conditions are used:

(22) Pressure: 14 mT-16 mT (preferably 15 mT);

(23) TCP RF: 200 W-400 W (preferably 300 W);

(24) Bias RF: 8 W-12 W (preferably 10 W);

(25) BCl.sub.3: 70 ccm-90 ccm (preferably 80 ccm);

(26) Cl.sub.2: 10 ccm-30 ccm (preferably 20 ccm);

(27) He Clamp: 5T-10T (preferably 6T); and

(28) Temp: 45 degC-65 degC (preferably 55 C).

(29) The BCl.sub.3 and CL.sub.2 gas combination under the above conditions further etches down into barrier layer 118 at a (slow) rate of approximately 1.05 Å/s. Since the initial depths of the metal contact openings 132 in barrier layer 118 are each approximately 43 Å, and since the BCl.sub.3 and CL.sub.2 gas etches down into barrier layer 118 at a rate of approximately 1.05 Å/s, the final depths of the metal contact openings 132 can be easily controlled.

(30) For example, if barrier layer 118 is 180 Å thick and 43 Å of barrier layer 118 have been removed by the BCl.sub.3 and SF.sub.6 etch, then the BCl.sub.3 and CL.sub.2 etch requires approximately 101.9 seconds at a rate of approximately 1.05 Å/s to extend each metal contact opening 132 down another 107 Å into barrier layer 118, thereby forming the lower bottom surfaces 140 to be 150 Å deep in barrier layer 118 and leaving a 30 Å separation distance D.

(31) An approximate etch time of 101.9 seconds is substantially longer than the few etch seconds available in the prior art, thereby allowing easy control of the depths of the metal contact openings 132. As a result, one of the advantages of the present invention is that the depths of the source and drain metal contact openings 132 can be easily controlled and prevented from exposing or extending into channel layer 116.

(32) As shown in FIG. 4, after the source and drain metal contact openings 132 have been deepened to the lower bottom surfaces 140, a metal layer 144 is deposited to touch the top surface of passivation layer 122 and fill up the metal contact openings 132 in barrier layer 118, cap layer 120, and passivation layer 122. Metal layer 144 is free of gold, and can include, for example, a titanium layer, an aluminum copper layer (0.5% Cu) that touches and lies over the titanium layer, and a titanium nitride cap that touches and lies over the aluminum copper layer.

(33) As shown in FIG. 5, after metal layer 144 has been formed, metal layer 144 is planarized in a conventional manner, such as with chemical-mechanical polishing, to expose the top surface of passivation layer 122. The planarization forms source and drain metal contacts 150 in the source and drain metal contact openings 132, respectively. The planarization also forms a group III-N HEMT structure 152. The metal contacts 150, which are free of gold, make ohmic connections to barrier layer 118. Method 100 then continues with conventional steps to complete the formation of a packaged group III-N HEMT.

(34) It should be understood that the above descriptions are examples of the present invention, and that various alternatives of the invention described herein may be employed in practicing the invention. For example, group III-N HEMTs are conventionally formed as depletion-mode devices, but can also be formed as enhancement-mode devices.

(35) The present invention applies equally well to enhancement-mode devices as the substrate and buffer layer structures of these devices are the same. Therefore, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.