CA2009247C

CA2009247C - Multilayered intermetallic connection for semiconductor devices

Info

Publication number: CA2009247C
Application number: CA002009247A
Authority: CA
Inventors: Kenneth P. Rodbell; Paul A. Totta; James F. White
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1989-04-17
Filing date: 1990-02-05
Publication date: 1993-04-06
Anticipated expiration: 2010-02-02
Also published as: US5131709A; CA2009347A1; JPH0325150A

Abstract

ABSTRACT A sputtered low copper concentration multilayered, device interconnect metallurgy structure is disclosed herein. The interconnect metallization structure comprises a sputtered aluminum-copper (< 2) weight percent copper conductor. In the preferred embodiment, the AlCu conductor has formed on one or both of its surfaces a layer of an intermetallic compound formed from a Group IVA metal and aluminum. The Group IVA metal is deposited by sputtering. Optionally, onto said top intermetallic layer is further deposited a non-reflective, non-corrosive, etch-stop, capping layer.

Description

; 20092~7 MUL?ILAYERED INTERMETALLIC CONMECTION FOR
SEMICONDUCTOR DEVICES
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sac~qround of the Invention Field of the Invention This invention relates to the device structure for the interconnection of semiconductor devices on a substrate and more particularly to a multilayered sputtered interconnect metallurgy structure which structure includes a low percent copper content aluminum/copper conductor.

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Description of Related Art Thin narrow interconnections have been used for ;, some years for device interconnection purposes in the semiconductor integrated circuit industry. It is ! 15 predicted that performance of these devices in the ;~ future will be limited by the performance of the device interconnection at the submicron level. At the submicron level, various technical problems are known to occur. While aluminum has been the preferred interconnection metal, as device dimensions are scaled down and current density increases, pure aluminum has il been known to be susceptible to the problems of electromigration and hillock growth. To overcome the problems experienced with pure aluminum, aluminum has been alloyed with copper to form aluminum-copper.
However, high percentage aluminum-copper (> 2%) is known to be difficult to dry etch and corrodes easily.
, In an effort to improve on the use of aluminum-copper as the interconnection metallurgy, aluminum-copper has been taught to be layered with a refractory metal (i.e., U.S. Patent No. 4,017,890).
This patent teaches a method and resulting structure ! for forming narrow intermetallic stripes which carry " FI9-88-038 -l--' *;

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~' 2~9247 high currents on bodies such as semiconductors, integrated circuits, etc., wherein the conductive stripe includes aluminum or aluminum-copper with at least one transition metal. While the aluminum-copper and transition metal structure has been known to improve the electromigration problems associated with aluminum-copper, the problems of etching and corrosion, as well as, the complete elimination of hillocks have not been solved.
s 10 As known in the art, hillocks are known to result from the large differences between the thermal expansion coefficients of the metal interconnect lines and the substrate. To eliminate and minimi~e hillock ... .
formation, it has been known in the art to use a ~ 15 multilayered structure instead of a single layer of 'r the interconnect metallurgy. An effective reduction in hillock formation has been found to be achieved by using a multilayered structure of aluminum or aluminum intermetallic with a layer of refractory metal.
Wherefore, a typical interconnect metallurgy structure would comprise a layered structure of aluminum silicon compound onto which there has been deposited, a layer of refractory metal, such as, titanium (see article "Homogeneous and Layered Films of Aluminum/Silicon with Titanium For Multilevel Interconnects", 1988 IEEE, V-MIC Conference, June 25-26, 1985).
There have also been refinements to this layered i metal structure to provide a lower resistivity,hillock free, interconnect metallurgy. These refinements include the incorporation of a barrier ~, metal of, for example, titanium tungsten or titanium nitride under the aluminum silicon to prevent contact spiking and prevent the formation of ternary compounds in the aluminum silicon alloy (see article "Multilayerd Interconnections For VLSI" MRS Symposia ;~ Proceedings, Fall, 1987).
;~ In addition, in this area, there have also been :',`
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2 ~ O 9 2 Y7 other proposed device interconnect structures to reduce resistivity and provide a more planar and defect free interconnect structure.
For example, IBM Technical Disclosure Bulletin, Vol. 21, No. 11, April, 1979, pp. 4527-4528, teaches the enhancement of the metallurgy for the interconnection due to sputtered deposition.
~, r~oreover, the feature of using a capping layer to improve perrormance has been proposed in IBM TDB Vol.
17, No. lA, 1984 and TDB Vol. 21, No. 2, July 1978.
However, no structure has been discovered which can satisfy all performance criteria providing a low resi~tance, hillock free, corrosion resistant, etchable, interconnection metallurgy structure.
It is, therefore, an object of the present !, invention to provide sputtered low weight percent copper (< 2~) content aluminum/copper conductor for device interconnection on a substrate with superior electromigration characteristics.
It is a still further object of the present inventiGn to develop a multilayered interconnect metallurgy structure that is hillock free, dry etchable and corrosion resistant.
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It is another object of the present invention to provide a multilayered interconnect metallurgy structure which has a low resistivity.
,.,., ~, Summarv of the Invention A sputtered low-copper concentration multilayered, device interconnect metallurgy structure i 30 is disclosed herein. The interconnect metallization ~` structure comprises a sputtered a~uminum-copper (~ 2) weight percent copper conductor. In the preferred embodiment, the conductor layer is formed with a top and bottom layer of an intermetallic, said intermetallic also being sputtered and being of a * Registered trade mark ....

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~ :' 2~Q~247 thickness of approximately 700A. Onto said ; intermetallic layer is further deposited an etch stop, and non-corrosive, protective capping layer. ;~
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Brief Description of the Drawinqs ' S FIG. l is a cross-sectional view of the preferred , embodiment of an interconnect metallurgy according to -~, the subject irvention.
,j FIGS. 2 through 8 are cross-sectional views of !'' the process for building the preferred interconnect ,~ 10 metallurgv of the subject invention in a step-by-step fashion.
FIG. 9 is a graph of the lifetime (hours) versus weight percent copper for interconnect metalluries of i the subject invention as compared to prior art ', 15 interconnect metallizations.

3 FIG. l0 is a graph of the resistivity versus weight percent copper for various alternative embodiment metallurgies of the subject invention before and after anneal~

Description of the Preferred Embodiments FIG. l is a cross-sectional view of the preferred 3 embodiment of an interconnect metallurgy structure according to the subject invention. FIG. l represents the interconnect structure after being processed through final annealing.
Referring to FIG. l, the interconnect metallurgy is seen to comprise a four-layer structure over an '~ interplanar stud connection l0 surrounded by an ~ insulator 8 to make connection to a device substrate -~ 30 6. The four-layer structure consists of a bottom-l sputtered layer 13 of an intermetallic formed by the ;l! reaction between the conductor layer 14 and ', pre-annealed surface layer 12. The layer 13 is ~, '~ FI9-88-038 -4-,~ .

20~9247 typically 700~ thick and in a preferred embodiment would comprise TiA13. Onto said sputtered intermetallic layer 13 is a sputter-deposited, low percent (< 2) weight percent copper, aluminum-copper, conductor layer 14. After annealing , the layer 14 is typically 8,500A thick and consists of a composition of 99.5~ aluminum and 0.5~ copper (aluminum-.5% copper hereafterj. On layer 14 is a second intermetallic layer 15 of the same thickness and composition as the layer 13. A layer 18 of aluminum-.5% copper or pure aluminum of approximately 100A to 500A thick is then sputter deposited to cap the structure. While this completes the structure for a single interconnect layer according to the subject invention, it should be lS recognized by those skilled in the art that said -, layers can then be repeated in a multiple level ! sequence to complete the interconnect circuit for the devices.
Referring now to FIG. 2, FIG. 2 shows a planar ~ 20 insulator 8 and contact stud 10 ~Jith a Group IVA metal --~ layer 12 sputter deposited thereon. The layer 12 is deposited by the following process. After formation of the device contact metallization 10, the semiconductor wafer would be loaded into a sputtering tool which has been pumped to a low pressure. An in-,; situ sputter clean is then performed to remove any oxide from the contact metal 10 formation on the wafer at this time. This in-situ sputter clean typically is a mild sputter clean, run, for example, at about five minutes at low power (approximately 1,000 watt) in a ~ hiah-pressure argon ambient.
; Following the sputter cleaning, the first level of metallization 12 is then deposited. This first level metallization 12 is comprised of a Group IVA
metal, preferably titanium, deposited on the device contact metallization 10 of the wafer in a blanket formation. This layer 12 is deposited at low power in . . .

-: , , 20092~7 a high pressure, high purity, argon plasma from an ultra-pure titanium target at a rate of about 60A per minute. Preferably, the titanium is deposited to a thickness of approximately 250A.
~ 5 Referring now to FIG. 3, folloS;~ing the deposition i of the layer 12, the interconnect metallization layer ~ 14 is next blanket deposited. The interconnect 3 metallization 14 is preferably aluminum-0.5% copper , (approximately 9,500A thic~). The aluminum-copper is ;~ 10 deposited at high power using a direct current ~, magnetron in a high puritv argon plasma from an ;~ ultra-pure pre-alloyed target typically aluminum-0.5 ~teight percent copper with a deposition rate of about 1,500A per minute.
~; 15 Onto said aluminum-copper interconnect metal-lization 14 is then deposited 50A of a Group IVA
metal similar to the previously deposited metal layer 12 discussed above. Deposition, composition and thickness of said layer 16 is identical to the previously deposited layer 12 (FIG. 4).
From FIG. 5, onto said metal layer 16 is then blanket deposited a suitable capping layer 18 to complete the interconnect metallurgy at this level.
The c~pping layer 18 is preferably comprised of -~ 25 aluminum-.05% copper deposited in the same manner as the conductor aluminum-.05% copper layer 14 as discussed above. The purpose of this layer is to: 1) prevent over-etch of metal layer 16; 2) lim-t the -' amount of light reflection during the subsequent '~ 30 photoresist steps, and 3) to act as a protective layer against corrosion during subsequent processing.
;' Therefore, any layer which would similarly satisfy the requirements of reducing the amount of light reflection and provide protective anodic capping j 35 during subsequent processing would be usable for this layer (e.g. pure aluminum).

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2~2~7 Referring now to FIG. 6, on top of metallization 18, a multilayered photoresist (20, 22 and 24) is then applied to pattern this blanket interconnect metallization. Any number of different photoresist techniques can be used. In particular, multilayered photoresists are well suited for this purpose, as well as, single-layered resists.
, ~ith a multilayered resist as shown in FIG. 6, a ,~, first resist 20 is applied to a thicknesc of approxi-~ 10 mately 1.8 micrometers. In the preferred embodiment `~ this resist is a diazo-quinone novolak photoresist.
;~ The resist 20 is baked in an oven in a nitrogen ! ambient at about 200C for 30 minutes. This resist 20 ~ serves as a sacrificial layer during subsequent metal ,j 15 reactive ion etching (RIE).
Onto said resist 20 is then deposited 200A of a silylating agent 22, such as, HMDS
~ (hexamethyldisilizane). The HMDS 22 serves as a J barrier to the cxygen reactive ion etching which is used to pattern the imaging laver resist ~4.
Onto said HMDS layer 22 is next deposited an .~3 imaging layer resist 24 to a thickness of about '~J, O ~ 9-1 ~ 2 micrometers. Similar to resist 20, imaging~' resist- 24 is a diazo-quinone novolak positive photoresist. The HMDS 22 and imaging resist layer 24 are then baked on a hot plate for 25 minutes at 85C.
The imaging layer resist 24 is then exposed for ~i the specific time required when used in conjunction with a specific exposure tool and associated mask.
The exposed image is developed using conventional developing for the required time depending on the exposure. The wafer is then rinsed and dried and the patterned top imaging layer is UV hardened by exposing it to ultraviolet light for a specific period of time, ~;î 35 typically, 5 to 10 minutes.
Following the patterning of the top imaging resist 24, the HMDS 22 and resist layer 20 are ready ;: .
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2~2~7 to be removed to expose the metal. The HMDS 22 and the resist 20 are removed by reactive ion etching.
This is accomplished by loading the wafer into a plasma tool and exposing the wafer to a plasma reactive to the HMDS layer 22 (e.g. CF4) and then to a different plasma (e.g. 2) reactive to resist 20. The polymer residues of the remains o~ the HMDS layer 22 and the resist 20 are then removed by dipping in a h solution of a conventional cleaning etch solution.
This reactive ion etching of the HMDS layer 22 and the resist 20, has put a lithographic mask into place for the subsequent reactive ion etching of the underlying blanket metal layers.
The metallurgy can now be reactively ion etched in a multi-step sequence. The first step is to break through any o~:ides which may exist on the top surface of the metallization. Next, most of the metal is ~; removed by reactive ion etching. An over etch is, ,~ then, performed to insure that all of the metal in the ,'~ 20 previous step has been etched away. Finally, a passivation step is performed to prevent any metal corrosion.
~ The reactive ion etch is typically performed in a ; single ~wafer tool under a low pressure. Typical plasma composition, pressure, power and time combinations, for performing the above etches in a step-by-step process can be seen from the following ' Table I. These compositions, pressures, powers and : times should be recognized by those skilled in the art as being flesigned for a specific tool under specific conditions. Any comparable times, compositions, pressure, etc., could be similarlv fabricated to insure the etch of the blanket metallization.
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i Gas Flow (cc/min) Step 1 Step 2 Step 3 Exit BCl 20 12 12 ---, 2 11 11 8 ___ ~, CHC13 5 16 16 ---'I N2 50 50 50 ---CF4 --- ___ ___ 180 ~ 2 20 -, 10 Pressure 375 375 375 0.5 Torr (milli-Torr) Power (Watts) 485 350 350 130 ~ Typical Times 15 sec2-3 min 40 sec 20 sec ~, .
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With the completion of the reactive ion etch, the wafer can then be rinsed and dried.
Referring now to FIG. 7, it can be seen that the reactive ion etch of the metal removes any of the remaining imaging layer resist 24 and most of the the HMDS layer 22 leaving on the surface of the m~tal the resist 20. This resist 20 can be removed by placing ~ the wafer in an oxygen plasma for approximately 45 ! minutes. The wafer is then placed in a developer at ~l room temperature for a short period of time to remove ; any oxides that may have formed in the previous step.
The wafer is again rinsed and dried.
~ With removal of this final layer of the resist ;l 20, the metallization stack can now be annealed by placing the wafer in an oven at 400C in forming gas for 1 hour in order to form TiA13 intermetallic layers 13 and 15 on the top and the bottom of the aluminum-copper layer 14 and to allow grain growth to occur in ;~ . .
the aluminum-copper layer 14.
From FIG. 8, it can be seen that once the metallization stack has been annealed (to the ... .
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The superior performance of the interconnect metallurgy of the subject invention over that which is ,' 10 known in the prior art can be seen in the following figures. FIG. 9 is a lifetime (hours) versus weight ~ percent copper graph for the electromigration .~ characteristics of both the above-described sputtered four-layered structure and an alternative sputtered three-layer structure (Al/Cu/refractory metal/Al-Cu), , as compared to an evaporated three-layer structure , patterned by lift-off and an evaporated four-layer I, structure, patterned by RIE. From FIG. 9 it can be ;i seen that for all weight percent copper compositions, ~, 20 the sputtered interconnect metallurgies are vastly superior to the evaporated metallurgies.
Fig. 10 is a graph of the resistivity versus weight ~ percent copper for various alternative `~ embodiment metallurgies of the subject invention. The ; 25 metallurgies have been subjected to a 400C forming gas anneal wherein the plots have been taken both before and after said anneal. From the plots it can clearly be seen that the resistivity of the 0.5 weight i percent copper structures are lower than that of the ,~ 30 higher weight percent copper films. Additionally, it can also be seen that the annealed films of the four-layer structure have a lower resistivity than the , annealed films of the three-layer structure.
-I The following Table II is a further comparison of the electromigration characteristics of sputtered ` Al-0.5%Cu metallurgy after annealing with .~
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;~ ALLOY RESISTIVITY
Q - cm) T(50%) S lhr, 400C250C,2.5E+06A/cm2 Forming Gas(Hours) Al-0.5~Cu ~Ref.~l) 3.5 9000 Al-0.5%Cu (Ref.#2) 3.4 12000 Evap (Ref. #3) 3.7 400-500 Evap (Ref. #4) 3.8 400-500 Cr/Al-4%Cu 3.0 400 Al 2.8 15 Al-0.5%Cu 2.9 50 , Al-1.2~Si-0.15%Ti 3.1 23 Al-1.2%Si (Ref.#5) 2.9 156*
. Al- 1 %Ti 6.6 2 Al-Si/Ti (Ref.#6)3.1 300*

3 * 150C, lE+06 A/cm , unpassivated 1. Sputtered 4250A Al-0.5%Cu/1500A TiA13/4250A
~` 20 Al-0.5~Cu and annealed in forming gas at 400C.
2. SputteredO700A TiA13/8500A Al-0.5%Cu/700A
TiAl /250A Al-0.5%Cu and annealed in forming gas at 4~0C.
3. Evaporated 4250A Al-0.5%Cu/1500A TiA13/4250A
;~ 25 Al-C.5~Cu and annealed in forming gas at 400C.
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~ 4. Evaporate~ 700A TiA13/8500A Al-0.5%Cu/700A
- TiAl~/250A Al-0.5%Cu and annealed in forming gas at 400C.
5. F. Fisher, Siemens Forsch-U. Entwickl-Dec. 13, 21 (1984).
~ 6. D.S. Gardner, T.L. Michalka, P.A. Flinn, T.W.
i' Barbee Jr., K.C. Saraswat & J.D. Meindl, Proc. 2nd IEEE VMIC, pp. 102-113 ~1985).

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20~2~7 From the table it can be seen th~t the sputtered 0.5~ copper metallurgy provides the longest electro-migration capabilit~, with the lowest resistivity.
In general, while the corrosion resistance of bulk aluminum is greatly decreased by the addition of copper, it is known and recognized in the 2rt (see, for example, J. Zahavi, M. Rotel, H.C.W. Huang, P.A.
~ Totta, "Corrosion behavior of Al-Cu Alloy Thin Films `~ in Microelectronics." Proceeding~ of International Congress of Metallic Corrosion. Toronto, June, 1984.) that the corrosion recistance of reactive ion etched low copper containing films of aluminum (e.g., less ~¦ than 1% copper) are at least as good as bulk aluminum.
This is in contradiction to that skill in the art which recognizes that the corrosion resistance of higher percentage (above l~) copper-aluminum films significantly degrades below that of pure aluminum.
While detaile~ understanding of the mechanism of the ' performance of the subject interconnect metallurgy isnot known, several principles have been extended by the inventors to e~:plain the superior electromigration - and resistivity results as seen above.
The solubility of copper and aluminum is known to decrease from 5.65 wt.% at 548C to 0.25 wt.% at room temperature. Therefore, the 0.5~ copper film composition of the subject invention has enough copper without theta phase formation to improve both the il mechanical properties and reliability, (e.g., electro-migration properties) of the alloy over pure aluminum.
Moreover, it is recognized that there is enhanced copper uniformity in the subject films due to the fact that said films were sputtered versus the ~ non-uniformity in copper distribution as can be seen `, in the evaporated aluminum-copper films. In addition,t 35 it is also recognized that evaporation results in the~'l uneven distribution of theta particles in the .~
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While the invention has been particularly shown and described with reference to the preferred embodi-ments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

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Claims

1. A metallization structure for the interconnection of semiconductor devices on a substrate compris-ing:

a) a layer of sputter deposited aluminum-copper of greater than 0.0% and less than 2.0% copper by weight; and b) an intermetallic layer formed at the surface of said aluminum-copper layer, said intermetallic formed of a sputter-deposited Group IVA metal and aluminum of said aluminum-copper layer.

2. A metallization structure according to Claim 1 wherein said structure comprises:

a) a first intermetallic layer;

b) a layer of sputter deposited aluminum-copper containing greater than 0.0% and less than 2.0 by weight;

c) a second intermetallic layer similar to (a) above deposited on layer (b) above; and d) a metal overlayer deposited on layer (c).

3. A metallization structure according to Claim 2 wherein said metal overlayer is selected from the group comprising aluminum, aluminum-compounds or titanium-compounds.

4. A metallization structure according to Claim 2 wherein said metal overlayer is aluminum-copper alloy containing greater than 0.0% to less than 2.0% copper by weight.

5. A metallization structure according to Claim 2 wherein said intermetallic layer is approximately 700.ANG. thick.

6. A metallization structure according to Claim 2 wherein said aluminum-copper layer is approxi-mately 8,500.ANG. thick.

7. A metallization structure according to Claim 2 wherein said metal overlayer is 100.ANG. - 500.ANG.
thick.

8. A metallization structure according to Claim 2 wherein one metal of said intermetallic layer is selected from the group comprising the Group IVA
metals of the Periodic Table.

9. A metallization structure according to Claim 1 wherein said structure comprises:

a) a first layer of sputter-deposited aluminum-copper containing greater than 0.0% and less than 2.0% copper by weight;

b) an intermetallic layer deposited on layer (a) above;

c) a second layer of the metallurgy of (a) above deposited on (b).

10. A metllization structure according to Claim 9 wherein one metal of said intermetallic layer is selected from the group comprising the Group IVA
metals of the Periodic Table.

11. A metallization structure according to Claim 9 wherein the intermetallic layer is approximately 1500.ANG. thick.

12. A metallization structure according to Claim 11 wherein the aluminum-copper layers are approximately 4,250.ANG. thick.

13. A metallization structure according to Claim 1 wherein said structure comprises:

a) a first layer of sputter deposited aluminum-copper containing 0.5% copper by weight approximately 4,250.ANG. thick, b) a layer of TiAl3 deposited on layer (a) above and approximately 1500.ANG. thick, and c) a second layer of the metallurgy of (a) above deposited on (b).

14. A metallization structure according to Claim 1 wherein said structure comprises:

a) a first layer of TiAl3 approximately 700.ANG.
thick, b) a layer of sputter deposited aluminum copper containing 0.5% copper by weight deposited onto said TiAl3 and being approximately 8,500.ANG. thick, c) a second layer of the metallurgy of (a) above deposited onto said layer onto (b) above, and d) a layer of sputter deposited aluminum copper containing 0.5% copper by weight deposited on layer (c) above and being approximately 250.ANG.
thick.

15. A method of forming a structure for connecting semiconductor devices on a substrate comprising the steps of:

a) sputter depositing a first layer of at least one of the Group IVA metals onto said substrate, b) sputter depositing onto said layer of the Group IVA metals a layer of aluminum copper of greater than 0.0% and less than 2.0% copper by weight, c) sputter depositing a second layer of the same Group IVA metal as in step a, and d) annealing the structure to form an intermetallic of the Group IVA metals and aluminum.

16. A method according to Claim 15 wherein said annealed interconnect structure is patterned to connect the devices.