United States Patent
`Yu
`
`[19]
`
`[11] Patent Number:
`
`.
`
`5,066,611
`
`[45] Date of Patent:
`
`Nov. 19, 1991
`
`[54]
`
`[75]
`
`[73]
`
`METHOD FOR IMPROVING STEP
`COVERAGE OF A METALLIZATION LAYER
`ON AN INTEGRATED CIRCUIT BY USE OF
`MOLYBDENUM AS AN ANTI-REFLECI‘IVE
`COATING
`
`Inventor: Chang Yu, Boise, Id.
`
`Assignee: Micron Technology, Inc., Boise, Id.
`
`[21]
`
`App]. No.: 575,942
`
`[22]
`
`Filed:
`
`Aug. 31, 1990
`
`[51]
`[52]
`
`[58]
`
`[56]
`
`Int. Cl.5 ............................................. H01L 21/26
`US. Cl. .................................... 437/173; 437/192;
`437/194
`Field of Search ........................ 437/173, 192, 194
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`4,758,533
`4,800,179
`
`....................... 437/173
`7/1988 Mageeetal.
`1/1989 Mukai .................................. 437/195
`
`FOREIGN PATENT DOCUMENTS
`
`54-150080 11/1979 Japan .
`57-198632 12/1982 Japan .
`01-303404 12/1989 Japan .
`
`OTHER PUBLICATIONS
`
`Y. Lai et al., “The Use of Ti as an Antireflective Coat-
`ing for Laser Planarization of Al .
`.
`. ” 6th International
`IEEE VLSI Multilevel Interconnect Conference, Cat.
`No. 89—TH0259—2, Jun. 12—13, 1989, p. 501.
`R. Liu et a1., “A Study of Pulsed Laser Planar'ization of
`Al . . . ”, 6th International IEEE VLSI Multilevel Inter-
`connect. Conference, Cat. No. 89TH0259—2,
`Jun.
`12-13, 1989 pp. 329—335.
`D. B. Tuckerman et 211., “Laser Planarization”, Solid
`State Technology vol. 29, No. 4, Apr. 1986, pp. 129—134.
`
`Primary Examiner—Brian E. Heam
`Assistant Examiner—Laura M. Holtzman
`
`Attorney, Agent, or Firm—Wells, St. John & Roberts
`
`[57]
`
`ABSTRACT
`
`A method for improving step coverage of metallization
`layers of an aluminum alloy on an integrated circuit
`involves use of a deposited layer of molybdenum as an
`anti-reflective coating to increase the efficient use of
`laser energy for planarization purposes where the un-
`derlying aluminum alloy covers a step, such as an open
`contact hole or via.
`
`6 Claims, 1 Drawing Sheet
`
`DEPOSIT
`BARR/ER
`METAL
`
`DEPOSIT
`INTERCONNECT
`METAL
`
`DEPOSIT
`Mo
`
`MSER
`PLANAR/ZA T/ON
`
`
`
`ETCH
`Mo
`(OPTIONAL)
`
`Nanya EX1011 - 1
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`Nanya EX1011 - 1
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`

`

`US. Patent
`
`Nov. 19, 1991
`
`_ 5,066,611
`
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`
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`
`DEPOSIT
`BARR/ER
`METAL
`
`DEPOSIT
`INTERCONNECT
`METAL
`
`
`
`DEPOSIT
`Mo
`
`LASER
`PLA NA R/ZA TlON
`
`ETCH
`Mo
`(OPTIONAL)
`
`Kg 3
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`Nanya EX1011 - 2
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`Nanya EX1011 - 2
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`

`1
`
`5,066,611
`
`METHOD FOR IMPROVING STEP COVERAGE
`OF A METALLIZATION LAYER ON AN
`INTEGRATED CIRCUIT BY USE OF
`MOLYBDENUM AS AN ANTI-REFLECTIVE
`COATING
`
`TECHNICAL FIELD
`
`This disclosure relates to metallization of integrated
`circuits, particularly to improvements in laser planariza-
`tion of deposited conductive metal film at steps and
`contact holes/vias about a covered surface on an inte-
`grated circuit die.
`BACKGROUND OF THE INVENTION
`
`In the metallization step utilized preparatory to etch-
`ing of conductors about the outer surface of an inte-
`grated circuit, step-coverage of conductive metal films
`(typically aluminum alloys) is poor over surface discon-
`tinuities, such as recesses and contact holes/vias. Pla-
`narization of the conductive surface is particularly de-
`sirable when vias are stacked vertically in multilevel
`metallization.
`Step coverage of film deposited conventionally by
`evaporation or sputtering becomes progressively worse
`as the dimensions of components on the integrated cir-
`cuit shrink. The poor step coverage is a result of the
`shadow effect in the deposited film at the sidewalls of
`steps or holes.
`The poor step coverage problem can be solved by
`selective chemical vapor deposition of tungsten, by
`metal deposition using high temperature and/or bias
`sputtering, or by supplemental metallic deposition,
`using multiple alternation sequences involving a combi-
`nation of evaporation and resputtering. However, the
`surfaces resulting from these procedures are not planar.
`The use of a pulsed laser to melt and planarize Al thin
`films to fill high aspect ratio contact vias is a very at-
`tractive approach to Ultra Large Scale Integrated
`(ULSI) circuit metallization. Laser planarization is a
`low thermal budget, simple, and effective technique for
`planarizing metal layers and filling interlevel contacts at
`the cost of only one additional step to the standard
`process flow. Excimer Laser planarization relies on a
`very short laser pulse to rapidly melt an absorbing metal
`layer. During the molten period, mass transport of the
`conductive metal occurs, which results in flow of the
`metal into contact holes/vias and drives the surface flat
`due to the high surface tension and low viscosity of
`molten metals.
`Recently,
`the technique of laser planarization has
`shown promise in improving the step coverage of alu-
`minum alloy films in micron/submicron geometry
`contacts and contact vias. However, due to the high
`reflectivity of aluminum (approximately 93% for wave
`lengths in the region down to 200 mm) and its relatively
`low evaporation temperature (2467n C.), aluminum
`alloys suffer from the following disadvantages: (1) inef-
`ficient use of laser energy, (2) low optical ablation limit,
`and (3) low process window between the ablation limit
`and the via-fill limit.
`Planarization systems utilizing excimer laser irradia-
`tion show particular promise for filling submicron-
`diameter contact holes/vias and planarizing the result-
`ing surface, Lessening of the surface reflectivity nor-
`mally encountered in heating of aluminum alloys by
`laser energy has already been reported as widening the
`“process window” between the “ablation limit”, or
`
`5
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`10
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`15
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`20
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`25
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`30
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`65
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`2
`temperature at which the conductive metal boils or
`evaporates, and the “via-fill limit,” or temperature at
`which sufficient flow of the conductive metal occurs to
`fill the circuit recesses.
`A general discussion of laser planarization can be
`found in a paper titled “Interconnects on Integrated
`Circuits Improved by Excimer Laser Planarization for
`Multilevel Metallization” by Mukai, et al., pp. 101—107,
`i.e., VLSI Multilevel
`Interconnection Conference,
`Santa Clara, CA (1988), which is hereby incorporated
`into this disclosure by reference. It describes the use of
`a thin copper overcoating to enhance aluminum planari-
`zation processing by increasing the initial optical absor-
`bance of the laser beam in the conductive metal film.
`. The paper fails to address the low oxidation resistance
`and the recognized difficulty of etching copper coat-
`ings.
`Use of titanium as an anti-reflective coating for laser
`planarization processes has also been proposed. How-
`ever,
`reported improvements in planarization were
`achieved at the expense of several drawbacks, including
`high resistivity and stress. The higher resistivity of the
`Ti—Al alloys that result from this process diminishes
`the advantage of Al metallization over an alternative
`metallization scheme using chemical vapor deposited
`tungsten as the primary conductive medium. Moreover,
`the higher stresses in the Ti—Al alloys imposes reliabil-
`ity concerns such as adhesion, cracks and stress voiding.
`It has been therefore concluded that titanium is not a
`desirable anti-reflective coating for aluminum and alu-
`minum alloys in metallization procedures.
`Despite the shortcomings in presently-reported sys-
`tems for laser planarization, the value of an anti-reflec-
`tive coating in widening the process window has been
`concluded to be important and to have demonstrated
`usefulness in increasing the thickness of a layer of con-
`ductive metal across a step or via.
`A search for alternative anti-reflective coatings has
`led to identification of molybdenum as a useful coating.
`Molybdenum film is proposed as an anti-reflective coat-
`ing on aluminum alloys or other low boiling point met-
`als used for metallization purposes. The addition of a
`molybdenum film prior to laser planarization results in
`more efficient use of laser energy, less ablation of the
`aluminum layer at a given optical fluence, and widening
`of the process window.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The preferred embodiment of the invention is illus-
`trated in the accompanying drawings, in which:
`FIG. 1 is a diagrammatic cross-sectional view show-
`ing the initial metallization films;
`FIG. 2 is a diagrammatic view of the films following
`laser planarization; and
`FIG. 3 is a flow diagram of the steps carried out in
`this process.
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`The following disclosure of the invention is submit-
`ted in furtherance with the constitutional purpose of the
`Patent Laws “to promote the progress of science and
`useful arts” (Article 1, Section 8).
`The use of excimer lasers to melt and planarize an
`aluminum alloy film used in the metallization of inte-
`grated circuits has been recognized as holding out possi-
`bilities of both improved step coverage and economical
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`5,066,611
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`4
`3
`manufacturing of Very Large Scale integrated (VLSI)
`underlying low temperature melting aluminum alloy 13.
`circuits. However, several difficulties are inherent in
`As shown in FIG. 2, this causes the aluminum alloy 13
`this technique: (1) aluminum and its alloys are highly
`to flow into the open hole, resulting in a relatively pla-
`nar outer surface in the resulting laminate.
`reflective to light, down to approximately 200 nm, lead-
`As the structure shown in FIG. 1 is subjected to laser
`ing to inefficient use of the laser power directed to an 5
`irradiation, the layer of conductive metal 13 is melted.
`aluminum film; (2) the low absorbance of aluminum and
`This is facilitated by the anti-reflective nature of the
`its alloys enhances localized ablation where surface
`covering layer of molybdenum. The anti-reflective mo-
`irregularities and precipitates, such as silicon and cop-
`lybdenum, with respect
`to the laser energy wave
`per, absorb more light than the areas surrounding them;
`and (3) the process window of :6—8% for laser planari- 10 lengths, widens the planarization process window by
`zation of aluminum and its alloys is relatively narrow.
`the resulting increase in efficient use of laser energy to
`While reports have been published with respect to
`melt the conductive metal and a lowering of the via-fill
`using copper and titanium as anti~reflective coatings
`limit. This can be attributed to a combination of the
`(ARC) to solve these problems, the choice of these
`reflective properties of molybdenum, aswellasits resis-
`metals has apparently been dictated solely by their re- 15 tance to oxidation, thermal conductivity and specific
`flective qualities. The present improvements to these
`heat, without a proportional lowering of the ablation
`systems take into account not only the reflectivity of the
`limit, which is attributable to the evaporation tempera-
`coating metal, but also its electrical and thermal proper-
`ture of molybdenum.
`ties and resistance to oxidation. Resistance to oxidation
`At low laser energies, there will be little intermixing
`is of particular importance because the processed wafer 20 of the aluminum and molybdenum layers apart from the
`typically is exposed to air when moving from the pla-
`area of the filled via itself, where there will be signifi-
`narization equipment to the next processing station.
`cant homogenous mixing of aluminum and molybde-
`The method of this invention is outlined in FIG. 3.
`num. In most instances the outer molybdenum layer can
`Following the use of standard processes to produce an
`remain on the underlying aluminum layer. The low
`integrated circuit, the metallization steps first involve 25 electrical resistivity of Molybdenum insures that there is
`depositing a barrier metal by sputtering, in the conven-
`no significant increase in the resistivity of the Mo/Al
`tional manner. The barrier metal can be any suitable
`alloy composite that might be formed.
`metal or alloy, such as Ti:W or an aluminum alloy.
`At high laser energies, it is expected there will be
`Then the interconnect layer, which can be an aluminum
`significant mixing of molybdenum and aluminum
`alloy or other low boiling point metal (such as copper 30 throughout the irradiated area. The etching of Mo/Al
`or copper alloys), is deposited in a covering film by
`alloy, which is less conventional, is an optional step in
`sputtering or other conventional deposition processes
`the method, to be utilized only as necessary for quality
`suitable for the electrically conductive metal. Finally, a
`control reasons. For this reason, the use of low energy
`layer of molybdenum is deposited over the interconnect
`laser irradiation is preferred, to minimize mixing of the
`or conductive metal, using sputtering techniques, chem- 35 metals and to leave a layer of molybdenum that can be
`ical vapor deposition, or other suitable known proce-
`readily removed by conventional wet etching at the end
`dures.
`of the process.
`The resulting physical structure at a through via is
`A summary of physical properties of molybdenum
`shown in FIG. 1, where the underlying substrate is
`and a comparison of those properties with the corre-
`designated by reference numeral 10. A covering insulat- 40 sponding properties of copper, titanium and aluminum,
`ing layer 11 overlies the semiconductor components
`is listed in the following table, where Tm is the melting
`(not shown). The exterior surface of the layer 11 and the
`temperature, Tb is the boiling point or evaporation tem-
`exposed surfaces on substrate 10 are apertured to form
`perature, p is the electrical resistivity, K is the thermal
`a hole, which might be a contact hole or a via. The
`conductivity, c is the specific heat, and R is the optical
`surfaces are covered by a barrier metal 12, such as an 45 reflectivity. All data is taken from The Handbook of
`aluminum alloy or Tizw, which serves both as a wetting
`Physics and Chemistry.
`
`
`R
`c
`K
`p
`Density
`Tb
`Tm
`(‘70)
`(cal/gK)
`(W/ch)
`(uohmrm)
`(g/cm)
`(C)
`(C)
`Element
`92.5
`0.215
`2.37
`2.65
`2.70
`2467
`660
`Al
`36.4
`0.092
`4.01
`1.68
`8.96
`2567
`1083
`Cu
`Ti
`1660
`3287
`4.54
`42.0
`0.22
`0.125
`38.9
`Mo
`2617
`4612
`10.22
`5.2
`1.38
`0.060
`57.6
`
`
`55
`
`layer and a diffusion barrier.
`A covering layer of aluminum alloy 13 extends across
`the hole. The thickness of the aluminum alloy along the
`walls and bottom surface of the hole is relatively thin
`due to the shadow effect that occurs when coating a
`hole. The deposited layer of molybdenum is illustrated 60
`at 14.
`The final step leading to covering of the hole is laser
`planarization. This is accomplished by directing optical
`pulse irradiation from an excimer laser onto the area of
`the via or contact hole. The pulsed laser beam is 65
`scanned across the whole wafer, causing absorption of
`the laser beam within the molybdenum coating and
`thereby heating it. The heat is then conducted into the
`
`The advantages of this process over the previously
`described processes using anti-reflective coatings of
`copper or titanium are as follows:
`(1) Compared with copper, molybdenum has a much
`better etchability, since copper is basically not
`etchable.
`(2) Compared with titanium, molybdenum has a
`much lower electrical resistivity.
`(3) The positioning of the molybdenum thin film on
`the surface of an underlying aluminum alloy raises
`the ablation limit of the laser planarization process
`due to the higher evaporation temperature of mo-
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`5,066,611
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`titanium
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`5
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`5
`lybdenum in comparison to aluminum,
`and copper.
`to aluminum, copper and titanium
`(4) In contrast
`films, which are readily oxidized at room tempera-
`ture, molybdenum has a high oxidation resistance.
`(5) Since the aluminum film used for metallization
`purposes is highly reflective and has a melting
`point of 2050‘ C., a molybdenum coating over such
`a film will remain at least partially intact to prevent
`the underlying aluminum alloy from oxidizing, 10
`resulting in more efficient coupling between the
`laser energy and the aluminum alloy.
`As a result of the listed advantages, the process win-
`dow of the laser planarization system is improved by
`use of molybdenum with no degradation to the inter- 15
`connect film quality and no added difficulty in the sub-
`sequent processing steps, such as etching, as compared
`to use of copper or titanium.
`The high thermal conductivity of molybdenum (in
`comparison with Titanium) and its low specific heat 20
`insure a higher temperature rise at a given optical flu-
`ence and a more efficient heat transfer to the underlying
`aluminum alloy. In addition, the low electrical resistiv—
`ity (5.2 ohm-cm) of molybdenum is ideal for VLSI
`metallization techniques because it insures low resistiv- 25
`ity for any resulting molybdenum-aluminum alloy com-
`posites. The result is an increase in efficient use of laser
`energy to melt the conductive metal and a lowering of
`the via-fill limit, attributable to a combination of the
`reflective properties of molybdenum, as well as its resis- 30
`tance to oxidation, thermal conductivity and specific
`heat, without a proportional lowering of the ablation
`limit, which is attributable to the substantially higher
`boiling point or evaporation temperature of molybde-
`num in comparison to aluminum, titanium and copper. 35
`To summarize, molybdenum has many advantages
`over titanium and copper as an anti-reflective coating.
`Its low optical reflectivity, low electrical resistivity,
`low heat capacity, and high melting and evaporation
`temperatures, high thermal conductivity and resistance 40
`of oxidation make it ideal as a choice for this purpose.
`In compliance with the statute, the invention has been
`described in language more or less specific as to struc-
`tural features. It is to be understood, however, that the
`invention is not limited to the specific features shown, 45
`since the means and construction herein disclosed com-
`prise a preferred form of putting the invention into
`
`6
`effect. The invention is, therefore, claimed in any of its
`forms or modifications within the proper scope of the
`appended claims appropriately interpreted in accor-
`dance with the doctrine of equivalents.
`I claim:
`
`1. A method for improving step coverage of a metalli-
`zation layer comprising an aluminum alloy or other low
`boiling point or high optical reflectivity metal on an
`integrated circuit during production of micron/submi-
`cron geometry contacts,
`surface recesses, and/or
`contact vias, comprising the following sequential steps:
`depositing a layer of electrically conductive metal
`over an integrated circuit assembly;
`depositing a layer of molybdenum over the conduc-
`tive metal; and subjecting the resulting laminate to
`laser planarization at a fluence level sufficient to
`melt the underlying conductive metal while using
`the layer of molybdenum as an anti-reflective coat-
`ing,
`thereby widening the planarization process
`window by the resulting increase in efficient use of
`laser energy by absorption of such energy into the
`underlying conductive metal to melt the conduc-
`tive metal and a lowering of the via-fill limit, attrib-
`utable to a combination of the reflective properties
`of molybdenum, as well as its resistance to oxida-
`tion, thermal conductivity and specific heat, with-
`out a proportional lowering of the ablation limit,
`which is attributable to the evaporation tempera-
`ture of molybdenum.
`2. The method of claim 1 wherein the molybdenum is
`deposited by sputtering.
`3. The method of claim 1 wherein the molybdenum is
`deposited by chemical vapor deposition.
`4. The method of claim 1, further including the fol-
`lowing additional sequential step:
`removing the remaining molybdenum after the laser
`planarization step.
`5. The method of claim 1 wherein the conductive
`metal is an aluminum alloy.
`6. The method of claim 1, further including the fol-
`lowing additional step:
`depositing a layer of barrier metal upon the exposed
`outer surface of the integrated circuit prior to the
`depositing of the layer of electrically conductive
`metal.
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