JP2010034355A - Heterojunction bipolar transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heterojunction bipolar transistor that has a high-reliability electrode which hardly deteriorates even in self-heat-generating and high-current-density operation and feeds electricity stably up to high current density, and that has higher reliability. <P>SOLUTION: The heterojunction transistor has a sub-collector layer 2, a collector layer 3, a base layer 4, an emitter layer 5, and an emitter contact layer 6 laminated in order on a semiconductor substrate 1. In the heterojunction bipolar transistor, a barrier composite layer 13 having a barrier metal layer 13-2 (Fig.2) made of single metal or alloy having a melting point higher than that of Mo is interposed between the emitter contact layer 6 and emitter electrode 7. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to heterojunction bipolar transistors.

  Heterojunction bipolar transistors (HBTs) are excellent in ultra-high-speed operation, and are expected to be applied to electronic circuits for communication systems and measuring instruments. Like other devices, they have long-term stability in operation. Required. In particular, in the case of HBT, the current gain fluctuation deterioration in a long-term continuous operation has been a serious problem. This deterioration is caused by an increase in recombination current due to current injection. In order to suppress this deterioration, there is a method of forming a passivation structure (ledge structure) on the external base layer and depositing an insulating protective film on the ledge structure and the emitter mesa. Proposed.

FIG. 3 shows a cross-sectional view of a conventional HBT having a ledge structure and an insulating protective film (see Non-Patent Document 1 below). In the figure, 1 is a semiconductor substrate made of semi-insulating InP, 2 is a sub-collector layer made of n ⁺ -type InP, 3 is a collector layer made of n ⁻ -type InGaAs, 4 is a base layer made of p ⁺ -type InGaAs, 5 Is an emitter layer made of n-type InP, 6 is an emitter contact layer made of n ⁺ -type InGaAs, 7 is an emitter electrode, 8 is a collector electrode, 9 is a base electrode, 10 is an insulating protective film (silicon nitride film, etc.), 11 An interlayer insulating film (BCB film or the like) 12 is a metal wiring layer connected to the emitter electrode 7.

  FIG. 4 shows the structure of the emitter electrode 7 according to the prior art. The emitter electrode 7 is formed on the emitter contact layer 6 and has, for example, a 35 nm thick Ti layer (second intermediate metal layer 7-1), a 20 nm thick Pt layer (diffusion prevention metal layer 7-2), and a 180 nm thick. This is a patterned metal layer composed of an Au layer (conductive layer 7-3).

  As shown in FIG. 5 (see FIG. 1 of Non-Patent Document 3 below), the wiring layer 12 is formed by forming a contact hole after forming an interlayer insulating film 11 on the element surface including the emitter electrode 7. Formed by depositing a third intermediate metal layer 12-1 and a wiring metal layer 12-2, and possibly an upper metal layer, on the surface of the emitter electrode 7 exposed in the hole and the surface of the interlayer insulating film 11. Is done. Thereby, a structure in which the emitter electrode 7 and the metal wiring layer 12 are connected is obtained.

In such an HBT, when the emitter size is set to 1 × 4 μm ² , a high temperature energization accelerated deterioration test was conducted, and the current gain extrapolated to 125 ° C. under the energization conditions up to a collector current density of ² mA / μm ^2. It has been reported that the median life of deterioration is 1 × 10 ⁸ hours or more (see Non-Patent Document 1 below).
JP 2006-239525 A Fukai et al., IEICE Transactions-C, Vol. J89-C, No. 9, pp. 589-596 (2006). MR Rao, ST O'Neil and S. Tiku, "Metal particle effects on thin capacitors in high volume manufacturing", Proc. GaAs MANTECH Conference, (2003) pp. 73-76. K. Onodera, M. Hirano, I. Toyoda, M. Tokumitsu, and T. Tokumitsu, "Folded U-Shaped Microwire Technology for Ultra-Compact 3D MMICs", 1996 IEEE MTT-S Digest, pp. 1153-1156. N. Kashio et al., "Highly Reliable Submicron InP-based HBTs with over 300-GHz ft", IEICE TRANS. ELECTRON, VOL. E91-C, No. 7, 2008, pp. 1084-1090.

In order to improve the operation speed of the HBT and increase the design margin of the HBT integrated circuit, long-term stability of the HBT operation in a high current density operation is required. In the HBT that realizes high-speed operation, the reduction of the emitter size has progressed, and at the present time, f _T and f _max > 300 GHz or more have been achieved with 0.5 μm wide emitters (see Non-Patent Document 4 above). The collector current density at this time is 5 mA / μm ² .

On the other hand, the thermal resistance of the device increases as the emitter size is reduced. For example, when the collector current density is 5 mA / μm ² with respect to the environmental temperature of 125 ° C., the junction temperature rises to 185 ° C. It has been confirmed by observation with a transmission electron microscope (TEM) of the device cross section that the deterioration of the HBT electrode is caused by the self-heating of the device. In particular, Ti shown in 7-1 in FIG. 4 diffuses into the semiconductor (emitter contact layer) depending on the deposition thickness at high temperatures. In addition, in Au shown by 7-3 in FIG. 4, atom movement occurs even at a high current density. For example, electromigration degradation is caused by the movement of Au atoms due to collision with a high-density, high-energy electron current when a high current density current of about 10 mA / μm ² is applied. It has been reported that Au was confirmed by TEM observation inside the semiconductor by the process.

  When these metal diffusion phenomena occur in the emitter electrode of the HBT, a defect occurs in the emitter contact layer due to the penetration of the metal into the emitter contact layer, and the resistance of the contact layer increases. If this takes a long time, there is a possibility that the characteristics of the HBT are deteriorated as the defect moves to the emitter layer or the base layer of the intrinsic part of the HBT.

  The main problem to be solved by the present invention is that it has a highly reliable electrode that is less likely to deteriorate even under the above-mentioned self-heating and high current density operation, and can be stably energized to a high current density, and has higher reliability. It is to provide a heterojunction bipolar transistor capable of achieving the above.

In the present invention, in order to solve the above problem, as described in claim 1,
In a heterojunction bipolar transistor in which a subcollector layer, a collector layer, a base layer, an emitter layer, and an emitter contact layer are stacked in this order on a semiconductor substrate, a melting point of Mo or higher is provided between the emitter contact layer and the emitter electrode. A heterojunction bipolar transistor is characterized in that a barrier metal layer made of a single metal or an alloy having a melting point of is interposed.

In the present invention, as described in claim 2,
2. The heterojunction bipolar transistor according to claim 1, wherein a first intermediate metal layer is interposed between the barrier metal layer and the emitter contact layer.

In the present invention, as described in claim 3,
3. The heterojunction bipolar transistor according to claim 2, wherein the first intermediate metal layer is made of Ti.

In the present invention, as described in claim 4,
2. The heterojunction bipolar transistor according to claim 1, wherein the barrier metal layer has a thickness of 10 nm to 300 nm.

In the present invention, as described in claim 5,
4. The heterojunction bipolar transistor according to claim 2, wherein the barrier metal layer has a thickness of 10 nm to 300 nm, and the first intermediate metal layer has a thickness of 6 nm to 10 nm. A heterojunction bipolar transistor is configured.

In the present invention, as described in claim 6,
6. The heterojunction bipolar transistor according to claim 1, wherein the emitter electrode has a conductive layer and a second intermediate metal layer or intermediate conductor layer in contact with the barrier metal layer. A heterojunction bipolar transistor is formed.

In the present invention, as described in claim 7,
7. The heterojunction bipolar transistor according to claim 6, wherein a diffusion preventing metal layer is interposed between the conductive layer and the second intermediate metal layer.

In the present invention, as described in claim 8,
8. The heterojunction bipolar transistor according to claim 6, wherein the conductive layer is made of Au, Al, Cu or Ag, the second intermediate metal layer is made of Ti or W, and the intermediate conductor layer is WSi or WSiN. A heterojunction bipolar transistor characterized by comprising:

In the present invention, as described in claim 9,
8. The heterojunction bipolar transistor according to claim 7, wherein when the second intermediate metal layer is made of Ti, the diffusion prevention metal layer is made of Pt.

In the present invention, as described in claim 10,
10. The heterojunction bipolar transistor according to claim 6, wherein the thickness of the second intermediate metal layer is 6 nm or more and 30 nm or less.

  By interposing a barrier metal layer between the emitter contact layer and the emitter electrode and suppressing metal diffusion, it does not easily deteriorate even under self-heating and high current density operation, and can be stably energized to a high current density. It is possible to provide a heterojunction bipolar transistor that has a highly reliable electrode and can achieve higher reliability. As a result, it is possible to increase the degree of freedom in power design of the semiconductor integrated circuit and the degree of freedom in use environment temperature.

  The present invention suppresses metal diffusion into the emitter contact layer by interposing a barrier metal layer made of a single metal or alloy having a melting point equal to or higher than the melting point of Mo between the emitter contact layer and the emitter electrode. It is characterized by.

  If the melting point of the metal increases, the temperature at which self-diffusion starts in the solid, that is, the Tamman temperature (see Patent Document 1 above) also increases, and if compared at the same temperature, the self-diffusion of metal atoms is less likely to occur. Accordingly, diffusion of other metal atoms is difficult to occur, so that a layer made of a single metal or an alloy having a melting point equal to or higher than the melting point of Mo including Mo effectively works as a barrier metal layer that suppresses diffusion of metal atoms. Examples of single metals having a melting point equal to or higher than the melting point of Mo include Mo, Ta, Re, and W.

  Hereinafter, embodiments of the present invention will be described by taking as an example the case where the barrier metal layer is made of Mo and the first intermediary metal layer made of Ti is provided, but the present invention is not limited to this. Absent.

FIG. 1 is a cross-sectional view showing an example of an HBT according to the present invention. In the figure, on a semiconductor substrate 1 made of semi-insulating InP, a sub-collector layer 2 made of n ⁺ -type InP and a collector layer 3 made of n-type InGaAs are sequentially grown by, for example, MBE (Molecular Beam Epitaxy). , A base layer 4 made of p ⁺ -type InGaAs, an emitter layer 5 made of n-type InP, and an emitter contact layer 6 made of n ⁺ -type InGaAs. A ledge structure is formed in the periphery of the emitter mesa so as to leave a part of the emitter layer so as to cover the external base layer.

  A barrier composite layer 13 is formed on the emitter contact layer 6, and as shown in FIG. 2, the barrier composite layer 13 includes a first intermediary metal layer 13-1 made of Ti that exhibits high adhesion to other metals, And a barrier metal layer 13-2 made of Mo which is a high heat-resistant metal.

  An emitter electrode 7 is formed on the barrier composite layer 13, and a metal wiring layer 12 is provided on the emitter electrode 7. The structure of the emitter electrode 7 is the same as the structure (Ti (7-1) / Pt (7-2) / Au (7-3)) of the conventional emitter electrode 7 shown in FIG. The structure is, for example, Ti / Pt / Au / Pt / Ti.

  Therefore, the overall configuration of the emitter section in this embodiment is a configuration in which the barrier composite layer 13 is inserted between the emitter electrode 7 and the emitter contact layer 6 in the heterojunction bipolar transistor according to the prior art shown in FIG. It corresponds to.

  Since Mo of the barrier metal layer 13-2 can be deposited by a vacuum deposition apparatus, the metal layers shown in FIGS. 2 and 4 can be collectively formed in the same deposition apparatus, and the process can be shortened. .

  In the barrier composite layer 13, the thickness of the first intermediate metal layer 13-1 (Ti) was 6 nm, and the thickness of the barrier metal layer 13-2 (Mo) was 10 nm. The thickness of the first intermediary metal layer 13-1 is preferably 6 nm or more in order to ensure adhesion, and is preferably 10 nm or less in order to suppress diffusion of the metal itself. The thickness of the barrier metal layer 13-2 is desirably 10 nm or more in order to prevent the diffusion of the metal from the emitter electrode 7, and it is feared that stress is generated as the thickness increases. Is desirable. When Mo is used as the barrier metal layer 13-2, the thickness of Mo is limited due to the temperature rise during vapor deposition, but the thickness is 30 nm or more and 50 nm or less, which is an appropriate thickness considering the subsequent etching process.

  In the present embodiment, Ti is used for the first intermediary metal layer 13-1, but a metal having good adhesion to other metals other than Ti may be used, and a barrier metal layer. If there is no problem in the adhesion between 13-2 and the emitter contact layer 6, the first intermediate metal layer 13-1 may not be used.

  When forming the emitter electrode 7, first, a metal layer made of Ti / Pt / Au / Pt / Ti is formed, and Ti and Pt in the upper layer are removed by etching at the time of forming the contact hole, and Ti / Pt / Au It is set as this laminated film. The film thickness when the first metal layer was formed by vapor deposition was Ti (30 nm) / Pt (20 nm) / Au (350 nm) / Pt (30 nm) / Ti (70 nm). The thickness of the second intermediary metal layer 7-1 (Ti in this case) is 6 nm or more in order to ensure adhesion, and the effective film thickness for suppressing the insertion effect and diffusion of each metal is 6 nm or more. In order to suppress diffusion of the metal itself, it is desirable that the thickness be 30 nm or less. The thickness of Pt (7-2) is preferably 20 nm or less.

  As the material of the second intermediate metal layer 7-1 of the emitter electrode 7, Ti and W can be used. Instead of the second intermediate metal layer 7-1, an intermediate conductor layer made of WSi, WSiN or the like is used. Can be used. As a material of the conductive layer 7-3, Au, Al, Cu, or Ag can be used. In addition, when the material of the 2nd intermediate | middle metal layer 7-1 is other than Ti, the diffusion prevention metal layer 7-2 may become unnecessary.

A comparative energization test was performed between the HBT according to the present invention and the HBT according to the prior art. FIG. 6 shows the variation of the emitter resistance from the initial value due to energization. The vertical axis in the figure is the relative value obtained by dividing the emitter resistance by its initial value. The energization conditions are an environmental temperature of 195 ° C. and a collector current density of ² mA / μm ² . In the HBT according to the prior art, the increase in resistance starts in about 100 hours, whereas in the HBT according to the present invention, the increase in resistance is suppressed to about 1000 hours. By increasing the thickness of Ti (first intermediate metal layer 13-1) and suppressing the diffusion of Au (7-3) by the high heat-resistant metal Mo (barrier metal layer 13-2), an increase in resistance of the emitter contact layer 6 is suppressed. It was.

In order to quantitatively grasp the variation of the emitter resistance, the time when the variation from the initial value of the resistance becomes 3% was used as a judgment condition, and the dependence of the median value on the junction temperature during energization was examined. The result is shown in FIG. The vertical axis of the figure is a logarithmic scale. E + 03 represents 1 × 10 ³ , and the horizontal axis represents 1000 times the reciprocal number of the junction temperature during energization expressed in absolute temperature (Tj). The HBT according to the present invention (indicated by Ti / Mo / Ti / Pt / Au in the figure) has a longer lifetime with respect to the increase in emitter resistance than the HBT according to the prior art (indicated by Ti / Pt / Au in the figure). It can be seen that the emitter structure of the heterojunction bipolar transistor according to the present invention is useful for stabilizing the device.

  In the above embodiment, a single barrier metal layer is used. However, a plurality of barrier metal layers may be used. In this case, the thickness of the barrier metal layer is the sum of the thicknesses of the barrier metal layers. Suppose that

It is a figure explaining the heterojunction bipolar transistor structure which concerns on this invention. It is a figure explaining the structure of the barrier composite layer of the heterojunction bipolar transistor which concerns on this invention. It is a figure explaining the heterojunction bipolar transistor structure by a prior art. It is a figure explaining the emitter electrode structure of the heterojunction bipolar transistor structure by a prior art. It is a figure explaining the metal wiring layer to the heterojunction bipolar transistor structure by a prior art. It is the figure which compared the time dependence of emitter resistance fluctuation | variation between the heterojunction bipolar transistor by a prior art, and the heterojunction bipolar transistor which concerns on this invention. It is the figure which compared the junction temperature dependence of the median time of 3% increase of emitter resistance between the heterojunction bipolar transistor by a prior art, and the heterojunction bipolar transistor which concerns on this invention.

Explanation of symbols

  1: semiconductor substrate, 2: subcollector layer, 3: collector layer, 4: base layer, 5: emitter layer, 6: emitter contact layer, 7: emitter electrode, 7-1: second intermediate metal layer, 7- 2: diffusion preventing metal layer, 7-3: conductive layer, 8: collector electrode, 9: base electrode, 10: insulating protective film, 11: interlayer insulating film, 12: metal wiring layer, 12-1: third mediator Metal layer, 12-2: wiring metal layer, 13: barrier composite layer, 13-1: first intermediate metal layer, 13-2: barrier metal layer.

Claims (10)

In a heterojunction bipolar transistor in which a subcollector layer, a collector layer, a base layer, an emitter layer, and an emitter contact layer are stacked in this order on a semiconductor substrate,
A heterojunction bipolar transistor characterized in that a barrier metal layer made of a single metal or alloy having a melting point equal to or higher than the melting point of Mo is interposed between the emitter contact layer and the emitter electrode. The heterojunction bipolar transistor according to claim 1,
A heterojunction bipolar transistor, wherein a first intermediate metal layer is interposed between the barrier metal layer and the emitter contact layer. The heterojunction bipolar transistor according to claim 2,
The heterojunction bipolar transistor, wherein the first intermediate metal layer is made of Ti. The heterojunction bipolar transistor according to claim 1,
A heterojunction bipolar transistor, wherein the barrier metal layer has a thickness of 10 nm to 300 nm. The heterojunction bipolar transistor according to claim 2 or 3,
The barrier metal layer has a thickness of 10 nm to 300 nm,
The heterojunction bipolar transistor, wherein the thickness of the first intermediate metal layer is 6 nm or more and 10 nm or less. The heterojunction bipolar transistor according to any one of claims 1 to 5,
The heterojunction bipolar transistor, wherein the emitter electrode has a conductive layer and a second intermediate metal layer or intermediate conductor layer in contact with the barrier metal layer. The heterojunction bipolar transistor according to claim 6,
A heterojunction bipolar transistor, wherein a diffusion preventing metal layer is interposed between the conductive layer and the second intermediate metal layer. The heterojunction bipolar transistor according to claim 6 or 7,
The heterojunction bipolar transistor, wherein the conductive layer is made of Au, Al, Cu or Ag, the second intermediate metal layer is made of Ti or W, and the intermediate conductor layer is made of WSi or WSiN. The heterojunction bipolar transistor according to claim 7,
The heterojunction bipolar transistor, wherein when the second intermediate metal layer is made of Ti, the diffusion preventing metal layer is made of Pt. The heterojunction bipolar transistor according to any one of claims 6 to 9,
The heterojunction bipolar transistor, wherein the thickness of the second intermediate metal layer is 6 nm or more and 30 nm or less.

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