WO2003075340A2

WO2003075340A2 - Method for obtaining metal to metal contact between a metal surface and a bonding pad.

Info

Publication number: WO2003075340A2
Application number: PCT/BE2003/000038
Authority: WO
Inventors: Pedro Banda; Meng Ho; Caroline Whelan
Original assignee: Interuniversitair Microelektronica Centrum
Priority date: 2002-03-01
Filing date: 2003-03-03
Publication date: 2003-09-12
Also published as: WO2003075340A3; AU2003209862A1

Abstract

A method for obtaining metal-to-metal contact between a bonding surface of a metallic bonding area and a second metal surface is disclosed. The method comprises the steps of : - coating said bonding surface of said metallic bonding area with a chemical composition that forms a self-assembled monolayer on said bonding surface of said metallic bonding area, and - bonding said second metal surface on said coated bonding surface through said selfassembled monolayer. The combination of the coating step and the bonding step result in a metal to metal contact between the bonding surface of he metallic bonding area and the second metal surface. The metallic bonding area can be a semiconductor bond pad, e.g. of a semiconductor device.

Description

METHOD FOROBTAININGMETAL TOMETAL CONTACTBETWEENA METAL SURFACEANDABONDINGPAD.

Field of the invention

The present invention relates to the field of protection of surfaces and more particularly the protection of a bonding surface of a semiconducting bonding pad. The present invention also relates to the field of wire bonding and the passivation of a copper surface in order to improve the wire bonding using a metal wire, especially Au and Cu wire as well as semiconductor devices including the wire bonding so produced.

Background of the invention and state of the art

The introduction of copper wafer metallization will drive the development of robust, highly reliable and low-cost interconnect solutions. Of great concern is the potential to bond onto the metal/dielectric BEOL features. The bondability requires addressing issues related to the use of novel materials and their related reliability. The metal bonding surface of a bond pad should be protected preventing the oxidation during all previous process steps.

The protection of copper surface becomes particularly relevant in the field of wire bonding. Wire Bonding on bare copper pad has always been a challenge to the semiconductor packaging industry. Copper has much different characteristics than gold or aluminium, making it more difficult to bond. Copper oxidises readily and the oxide continues to grow in thickness. This makes the surface condition a significant variable for ball bonding. Plasma cleaning prior to wire bonding does not ensure adequate wafer cleanliness for proper first bond and the process time window is usually within 15 minutes or oxidation will reoccur making it difficult wire bond.

Harman and Johnson ("Wire bonding to advanced Copper-Low-k Integrated circuits, the metal/dielectric stacks and material considerations", International Symposium on Microelectronics, Baltimore, USA, 9-11 October 2001) recites several coatings to enhance the bondability of the copper surface. A traditional approach using some metallic top surfaces such as Au or Ag requires some extra manufacturing steps.

Inorganic films are also presented, although the processing window is very limited if a breakable and clean bond is to be achieved.

WO 00/59029 describes a method and apparatus for performing wire bonding to a copper-based bond pad. The oxide on the copper surface is removed and a passivation layer is deposited on the copper. The passivation layer can be BTA, an azole-based coating. Prior to deposition of the passivation layer, the oxide on the surface has to be removed by e.g applying a solution of citric acid or hydrochloric acid. This requires an extra processing step. Moreover, the passivation layer is applied within 5 seconds of the oxide removal, resulting in a very narrow processing window.

US 2001/0028117A1 describes a method and a structure for preventing wetting or bleed of an adhesive onto the noble metal wire bond pads of a dielectric substrate when attaching an electrical component to the dielectric substrate. The method includes treating the bond pads of the dielectric substrate with a chemical compostion which will provide self-assembled monolayers. The protective coating is not applied on the electrical component, such that the bond pads are unprotected, which will result in a poor bond with the dielectric substrate.

US 6,323,131 relates to an device comprising copper interconnects wherein the copper surface is protected against corrosion by a passivation layer. The passivation layer comprises self-assembling monolayers (hereafter called SAM). The passivation layer can be formed on the copper surface and under the copper surface to provide a barrier against copper migration. The self-assembling monolayer can comprise long alkyl chain silanes, carboxylic acids and thiols. Preferred films of SAM are formed from molecules having the formula X[CH₂(CH₂)n-O-C(O)CH₂C(O)CH₃]₃ where X is S, Si or

N, n is between 2 and 6, although also longer chains are possible (n=18). The films are formed on a copper surface which is preferably clean prior to the deposition process. US 6,323,131 describes the use the SAM as passivation layer, barrier layer for ions or as adhesion promotor film in damascene structures in ULSI processing.

Aims of the invention

The present invention aims to provide a method for passivating a bonding surface of a metallic bonding area. Said bonding surface is preferably copper. Moreover, the present invention aims to provide a device comprising a metal-to-metal bond between a bonding area and a second metal surface.

The present invention aims to improve the wire bonding process by passivation of the copper bond pad and by passivation of a wire surface for the further improvement of adhesion of molding compound. Short description of the drawings

Fig. 1 : schematic diagram of the test structure for Au or Cu wire bonding onto a Cu/Low-k BEOL die. Fig. 2 a: wiring bonding process with wire on Copper bondpad in accordance with an embodiment of the present invention.

Fig. 2b : wire protection after bonding.

Fig. 3 : integration of SAM formation with the backend process of an IC in accordance with other embodiments of the present invention. Fig. 4 : Cyclic voltammograms in aqueous solutions of 0.1 M NaOH showing the influence of 1-decanethiol (■■■), benzenethiol ( ), benzenethanethiol ( ), and benzothiazole ( — ) films, labelled CIO, BT, BET, and BTA, respectively, on Cu oxidation and reduction (ooo). efficiency values (%).

Fig. 5 : wire bonding tests on copper bond pad coated with different type of coatings (ClOg, ClOl, BT, BTA) and non-coated copper bond pads. The wire is copper.

Fig. 6 : wire bonding tests on copper bond pad coated with different type of coatings (ClOg, ClOl, BT, BTA) and copper bond pads. The wire is gold.

Fig. 7 : ball shear strength vs ball bond diameter according to Q-1000 Standard.

Summary

In a first aspect of the present invention, a method for obtaining metal-to-metal contact between a bonding surface of a metallic bonding area and a second metal surface is disclosed. Said method comprises :

- coating said bonding surface of said metallic bonding area with a chemical composition that forms a self-assembled monolayer on said bonding surface of said metallic bonding area, and

- bonding said second metal surface on said coated bonding surface through said self- assembled monolayer.

Both coating step and the bonding step result in a metal to metal contact between the bonding surface of he metallic bonding area and the second metal surface. Said metallic bonding area can be a semiconductor bond pad.

Between the coating step and the bonding step, a drying step, a testing step or a dicing step can be performed without substantially oxidizing the metallic bonding surface. The self-assembling monolayer can withstand the dicing process, which occurs in an atmosphere including water vapour. Moreover, it withstands the curing processes, which is carried out at temperatures of about 150 degrees C. Furthermore, testing (also called probing) is improved, since no or reduced damage to the bonding surface takes place due to the coated bonding surface.

In an embodiment of the first aspect of the invention, a method as recited in any of the previous embodiments is disclosed wherein said self-assembled monolayer comprises at least one self-assembling molecule, said self-assembling molecule comprising a functional group that attaches to said bonding surface, a spacer group promoting the formation of the self-assembled monolayer and a terminal group essentially not attaching to said bonding surface.

In an aspect of the present invention said functional group attaching to said bonding surface is capable to reduce metal compounds being formed at said bonding surface of the metallic bonding area, thus resulting in a bare bonding surface coated with a self-assembled monolayer. The present invention provides a method to selectively protect bonding surfaces which is easy to integrate into semiconductor processing. An accurate control of the thickness of the monolayer is also possible by selecting the appropriate self-assembled monolayer.

Said self-assembling monolayer comprises at least one of the molecules selected from the group consisting of X-Ri-SH, X-Rι-S-S-R₂-Y and Rι-S-R₂, Ri and R₂ being independent from each other and forming a spacer of n carbon atoms, optionally interrupted by p heteroatoms, X and Y being independent from each other, and a terminal group essentially not attaching to said bonding surface. In an embodiment, X and Y are independent from each other and are a chemical group that is hydrophobia In a further embodiment X and Y are selected such that they promote the adhesion of protective material for device packaging. Said spacer may comprise at least a chemical group selected from the group consisting of alkyl, alkenyl, alkynyl, cyclic alkyl, aryl, alkyl bound to aryl, alkenyl bound to aryl and alkynyl bound to aryl.

In an embodiment of the first aspect of the present invention, a method as recited in any of the previous embodiments is disclosed wherein said metallic bonding area is a semiconductor bond pad. Said semiconductor bond pad is the layer being deposited on an interconnect metallization of a semiconductor device.

In an embodiment of the first aspect of the present invention, a method as recited in any of the previous embodiments is disclosed wherein said metallic bonding area comprises a metal.

In an embodiment of the first aspect of the present invention, a method as recited in any of the previous embodiments is disclosed wherein said metal is selected from the group consisting of copper, gold, silver, platinum, palladium, nickel, iridium, iron and aluminum.

In an embodiment of the first aspect of the present invention, a method as recited in any of the previous embodiments is disclosed wherein said bonding surface comprises copper. Preferably, said bonding surface consists essentially of copper. Said metal compound is copper oxide, formed by subjecting the substrate to an oxidizing atmosphere such as air, oxygen gases.

In an embodiment of the first aspect of the present invention, a method as recited in any of the previous embodiments is disclosed wherein said second metal surface is a part of a metal wire used in wire bonding. In a further embodiment, said metal wire is coated with a self-assembled monolayer. Said self-assembled monolayer coud be any monolayer described in the previous embodiments.

In an embodiment of the first aspect of the present invention, a method as recited in any of the previous embodiments is disclosed wherein said second metal surface is part of a solder ball.

In an embodiment of the first aspect of the present invention, a method as recited in any of the previous embodiments is disclosed wherein said metal surface comprises a metal. Said metal is selected from the group consisting of copper, gold, silver, platinum, palladium, nickel, iridium, iron and aluminum. In a further embodiment of the first aspect of this invention, said metallic bond pad is a semiconductor bond pad, said semiconductor bond pad being part of a substrate, said method further comprising the steps of dicing said substrate into individual chips.

In a further embodiment of the first aspect of this invention, a method as recited in any of the previous embodiments is disclosed further comprising the step of coating said second metal surface with a chemical composition that forms a self-assembled monolayer on said second metal surface prior to said bonding step

In a further embodiment of the first aspect of this invention, a method as recited in any of the previous embodiments is disclosed further comprising the step of coating said metal-to-metal bond with a chemical composition that forms a self-assembled monolayer.

In a second aspect of this invention, a device comprising a metal-to-metal contact between a bonding surface of a metallic bonding area and a second metal surface is disclosed. Said metal-to metal contact is obtained by providing a coated bonding surface of a metallic bonding area, said coated bonding area comprising a self- assembling monolayer being deposited on said bonding surface and then bonding said second metal surface to at least a part of said coated bonding surface through said self- assembled monolayer. In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said bonding surface comprises at least a first and a second region, said first region being bound to said second metal surface and said second region being coated with a self-assembled monolayer.

In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said self-assembled monolayer comprises at least one self-assembling molecule, said self-assembling molecule comprising a functional group that attaches to said bonding surface, a spacer group promoting the formation of the self-assembled monolayer and a terminal group essentially not attaching to said bonding surface. In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said functional group that attaches to said bonding surface is capable of reducing metal compounds being formed at said bonding surface of the metallic bonding area.

In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said self-assembling monolayer comprises at least one molecule selected from the group consisting of X-Rι -SH, X-Ri- S-S-R₂-Y and Rι-S-R₂, Ri and R₂ being independent from each other and forming a spacer of n carbon atoms, optionally interrupted by p heteroatoms, X and Y being independent from each other, an end group essentially not attaching to said bonding surface.

In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said metallic bonding area is a semiconductor bond pad. In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said metallic bonding area comprises a metal. In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said metal is selected from the group consisting of copper, gold, silver, platinum, palladium, nickel, iridium, iron and aluminum.

In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said bonding surface comprises copper. In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said bonding surface consists essentially of copper.

In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said metal compound is copper oxide.

In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said second metal surface is a part of a metal wire used in wire bonding.

In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said second metal surface is part of a solder ball.

In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said metal surface comprises a metal. In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said metal is selected from the group consisting of copper, gold, silver, platinum, palladium, nickel, iridium, iron and aluminum.

In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein X and Y are independent from each other and each are a chemical group that is hydrophobia

In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said second metal surface is coated with a self-assembling monolayer prior to said bonding step.

In an embodiment of the second aspect of this invention, a device as recited in any of the previous embodiments is disclosed wherein said metal-to-metal contact is coated with a self-assembling monolayer.

Detailed description of the invention

The device and method as disclosed in this invention can be used in the field of semiconductor processing, especially IC manufacturing, IC packaging, wire manufacturing or IC package material manufacturing. The present invention discloses a method for obtaining a metal to metal contact between the bonding surface of a metallic bonding area and a second metal surface. Said method comprises the steps of :

- coating said bonding surface of said semiconductor bond pad with a chemical composition that passivates said bonding surface of said semiconductor bond pad, and

- bonding said second metal surface to said coated bonding surface through said coating that passivates said bonding surface.

Said coating step can have the additional effect of reducing chemical compounds formed on said bonding surface. This is of particular importance when metal oxides are formed at the bonding surface of the metallic bonding area. In this case the oxide reduces to metal. Moreover, said chemical composition is selected such that it provides a self-assembled monolayer on said bonding surface. The present method results in an improved bonding between the second metal surface and the bonding surface of the metallic bonding area since the bonding is through the self-assembled monolayer. This means that there is essentially no layer between the both bonding surfaces. Improved bonding means a metal to metal contact between both surfaces, i.e. not disturbed by oxide. This can be characterized by the following parameters :

- bonding of the second surface to the first surface. Metal to metal contact means that a bonding is observed. ^" ball shear test should be at least 2.0 gf/mil², preferably at least 4.0 gf/mil²

- resistance between the electrical pads and the second surface may increase at the highest 10 %. The invention is of particular importance in the field of bonding a semiconductor substrate to a second metal surface such as, but not limited hereto, a metal wire or a solder ball in semiconductor packaging technology. The metallic bonding surface can be a semiconductor bonding pad, e.g. of a semiconductor device or one in processing. The semiconductor bond pad is typically formed after the final layer of interconnect metallization on a semiconductor substrate (see Fig. 1). The bond pad (14) can be integrated in standard IC backend processing which includes interconnections (11), intermetal dielectrics (12) such as silicon oxide, Low-k materials, and passivation films (13) such as silicon nitride and silicon carbide. Thus, the coating step can be performed at substrate (such as, but not limited hereto, a wafer) level, before dicing the individual chips, providing protection up to the wire bonding process (15 : wire bonded to bond pad).

The semiconductor bond pad can also be integrated in a redistribution layer processed on top of a standard IC device. This redistribution layers include polymer based intermetal dielectrics such as BCB. Preferably, said bond pad is part of an IC formed on a substrate. More preferably, said substrate can be an IC with Cu backend metallisation integrated in IC processing or IC with Cu backend metallisation in redistribution layers postprocessed on top of the device.

The substrate can be, but is not limited hereto, a substrate made of a semiconductor material such as silicon or germanium, glass, quartz, polymeric material. Said substrate may include ceramics with a patterned Cu surface and organic laminates with a patterned Cu surface.

According to present the invention, the semiconductor bond pad can be coated, such that it can withstand further processes such as drying, testing and dicing. The bonding surface of the semiconductor bonding pad comprises at least one metal. The metal can be selected from the group copper, gold, silver, platinum, palladium, nickel, iridium, iron, aluminium and alloys thereof.

Said coating step results in the deposition of a layer that provides passivation of the bonding pad. Passivation means protection against oxidation, dust, particle contamination, for example. Preferably, the coating results in an essential coverage of the bonding surface in the region where the second metal surface is bound to the bonding pad such that a metal to metal contact is obtained.

The second metal surface comprises a metal selected from the group copper, gold, silver, platinum, palladium, nickel, iridium, iron, aluminium and alloys thereof. The second metal surface can be part of a wire or a solder ball or other semiconductor bonding structure.

Said chemical composition is selected such that it provides a self-assembling monolayer (29) on the bonding surface (24) of the semiconductor bond pad (22) (Fig. 2a (ii)). The bond pad (22) can be formed on any suitable substrate (21) - see Fig. 2a (i). The self-assembling monolayer is a temporary layer that will be punched through by the bonding process (see Figs. 2a (iii) and 2b). Consequently, there is a direct contact between the bonding surface (22) of the bond pad (24) and the second metal surface (26). The self-assembling monolayer prevents the reaction between the bonding surface and molecules being present in the external medium such as oxygen, water, water vapour, dust. Moreover, if a thin layer of metal oxide is spontaneously formed on the bonding surface, the chemical composition providing the SAM has a reducing effect on the metal oxide, thus avoiding an extra cleaning step. Partial removal of the SAM layer (24) leaving portions (29) may form part of the bonding process in accordance with an embodiment of the present invention (Fig. 2a(iii)). The area of underlying metal surface (22) which is exposed is preferably smaller than the final bonding area of as shown in Fig. 2b.

For the purpose of this invention, self-assembled monolayers (SAM) should be understood as a relatively ordered assembly of molecules that spontaneously adsorb

(also called chemisorb) from either the vapour or liquid phases on a surface. The self- assembly is driven by preferential bond formation of an appropriately functionalised headgroup onto specific substrate surface sites. A self-assembling molecule may comprise at least a functional group attaching to the bonding surface, a spacer group and a terminal group. The functional group attaching to the bonding surface can vary, depending on the material characteristics of the bonding surface material, e.g. thiols on metals - see table 1. Lateral interactions between the spacer segments, such as but not limited hereto, hydrocarbon segments of the molecules, comprising alkane chains and/or aromatic groups. A variable terminal group (exposed SAM-gas/liquid interface) such as a methyl, phenyl, amine, carboxylic acid, or alcohol unit, is also of importance. The molecules are preferably oriented perpendicular with respect to the substrate surface plane with all trans extended hydrocarbon chains oriented close to the surface normal.

The thickness of the self-assembling monolayer depends on the chemical nature of the self-assembling monolayer.

Table 1

When the bonding surface of the bond pad is copper, said self-assembling monolayer comprises at least one molecule with the chemical formula :

X-Ri-SH or X-Rι-S-S-R₂-Y or R S-R₂ wherein Ri and R₂ are spacers of n carbon atoms, optionally interrupted by p heteroatoms. Ri and R₂ are independent of each other can have the same chemical formula or can be different. It should be understood that the sulphur atom can be replaced by any atom listed in table 1, such that the SAM becomes suitable for coating other metals.

X and Y can have the same chemical formula or can be different. X and Y are chemical groups selected such that there is essentially no chemical reaction between X and the bonding surface. This means that there is no competition between the sulphur atom and X related to chemisorption on the metal Bonding surface. Preferably, X or Y is a non-oxidizing chemical group. X or Y can be a hydrophobic chemical group. X or Y can be selected form the group consisting of a methyl (CH₃), COOH, NH₂, OH, halogenide (F, Cl, Br, I), -SH. Preferably, X is -CH₃.

Ri or R₂ are a spacer of n carbon atoms, optionally interrupted by p heteroatoms wherein n or (n+p) is an integer between 1 and 30, between 1 and 15, between 1 and 12, between 4 and 15, between 4 and 12. Preferably, n or (n+p) is an integer between 6 and 12. In an embodiment, n or (n+p) equals 10.

Said spacer promotes the formation of a self-assembling monolayer. Said spacer may be understood as including an alkyl, alkenyl, alkynyl, cyclic alkyl, aryl, alkyl bound to aryl, alkenyl bound to aryl, alkynyl bound to aryl. Said spacer could also represent a hydrocarbon interrupted by a -CO-(ketone), -CONH, -CONHCO-, -CSNH-, -CS-, and the like. Said hydrocarbon chain can be branched. Said heteroatom could be selected from the group consisting of-NH-, -O-, -S-, -CS-, -N-.

Options for Ri or R₂:

1. Ri or R₂ could be an alkyl, alkenyl, alkynyl, cyclic alkyl, aryl, alkyl bound to aryl, alkenyl bound to aryl, alkynyl bound to aryl. Ri or R₂ could be modified by :

- Number of carbon atoms n is an integer between 1 and 30, between 1 and 15, between 1 and 12, between 4 and 15, between 4 and 12. Preferably, n or (n+p) is an integer between 6 and 12. In an embodiment, n or (n+p) equals 10.

- Interruption by p heteroatoms : heteroatom being selected from the group consisting of -NH-, -O-, -S-, -CS and -N-. For each heteroatom (n+p) is an integer between 1 and 30, between 1 and 15, between 1 and 12, between 4 and 15, between 4 and 12. Preferably, n or (n+p) is an integer between 6 and 12. In an embodiment, n or (n+p) equals 10. All combinations of RI and the number of carbon atoms and the interuption by heteroatoms are possible.

2. Ri or R could be an alkyl, alkenyl, alkynyl, cyclic alkyl, aryl, alkyl bound to aryl, alkenyl bound to aryl, alkynyl bound to aryl interrupted by and -CO-(ketone), -CONH, -CONHCO-, -CSNH-, -CS-.

- Number of carbon atoms n is an integer between 1 and 30, between 1 and 15, between 1 and 12, between 4 and 15, between 4 and 12. Preferably, n or (n+p) is an integer between 6 and 12. In an embodiment, n or (n+p) equals 10

- Interruption by p heteroatoms : heteroatom being seleceted from the group consisting of -NH-, -O-, -S- and -CS. For each heteroatom (n+p) is an integer between 1 and 30, between 1 and 15, between 1 and 12, between 4 and 15, between 4 and 12. Preferably, n or (n+p) is an integer between 6 and 12. In an embodiment, n or (n+p) equals 10.

All combinations of Ri, R₂, the number of carbon atoms and the interruption by heteroatoms are possible.

In an embodiment, the molecule forming the SAM is X-Ri-SH, R_t being an alkyl chain -(CH₂)_n-. n is between 1 and 30 , between 4 and 12 and preferably 10.

X

I

(CH₂) n

I

SH X could be selected from the group consisting of H, CH₃, (CH₂)_n, halogenide (F, Cl, Br, I), SH, OH, COOH, NH₂

In another embodiment, the molecule forming the SAM is X-Ri-SH, Ri comprising an alkyl group bound to an aryl group. The alkyl part of the group binds the sulphur atom to the aryl group.

SH

n is between 1 and 22, between 2 and 22, between 4 and 12. Zi, Z , Z , Z₄, Z₅ and Z₆ could be independently selected from the group consisting of H, CH₃, (CH₂)_n. halogenide (F, Cl, Br, I), SH, OH, COOH and NH₂.

In another embodiment, the molecule forming the SAM is X-Ri-SH, Ri comprising an aryl group.

Zi, Z₂, Z₃,Z , Z5 and Z₆ could be independently selected from the group consisting of H, CH₃, (CH₂)_n, halogenide (F, Cl, Br, I), or SH.

A self-assembling monolayer can be formed from at least one of the molecules described above. The self-assembling monolayer can also be a layer of mixed self- assembling monolayers. Moreover, stacks of self-assembled monolayers can be formed. The self-assembling monolayer chemisorbing at the bonding surface can serve as substrate for depositing a second self-asssembling monolayers. The chemical nature of those layers is determined by the chemical interactions between both self-assembling monolayers.

In an embodiment of the present invention, the second metal surface can be coated with a chemical composition that passivates said second metal surface and optionally reduces chemical compounds formed on said second metal surface. Said chemical composition can be selected such that a layer of self-assembled molecules (also called self-assembled monolayer) is formed. The self-assembled monolayer can be selected from the molecules described in this application.

In case of wire bonding, the wire (see Fig. 2b) and the wire spool can also be coated with the self-assembling monolayer (27). This would increase the storage life of Cu wire spool. The description below is directed mostly to copper based process technologies for semiconductor integrated circuit bond pads. It is understood that a person skilled in the art can imagine several other applications wherein the material is susceptible to aggressive or unwanted oxide formation. It is also understood that the teaching can be applied to all existing packaging technologies, such as wire bonding, flip chip bonding or ball grid array bonding or pin grid array bonding.

In an embodiment of the present invention, the bonding surface of the semiconductor bond pad consists essentially of copper. The implementation of copper in semiconductor processing poses a challenge for the implementation due to the formation of an unstable oxide. Since the bond pads are used for input-output connections for the semiconductor chip to the package, the native oxide formed on the copper bond pad prevents an appropriate bond. The bond pad structure is typically formed after the last metallization layer in the interconnect structure. After the last metallization layer, a polishing step is performed, followed by a cleaning step for removing residues of the polishing step. After this step, native oxide can be formed on the copper bond pad. In a next step, the substrate is tested, (also called probed) in order to test the electrical connections on the substrate. Probing on an oxide covered bond pad can result in scratching the bonding surface. In the present invention, the formation of native oxide on the copper bond pad can be avoided, since the copper bond pad is coated with a chemical composition that passivates the copper surface. The chemical composition provides the formation of self- assembling monolayer on the copper surface. The self-assembling monolayer can be any SAM described above. Particularly, the SAM is alkanethiol based. If copper oxide is already formed on the copper surface, the SAM will preferably reduce the copper oxide and the sulphur group will chemisorb on the surface such that further oxidation is avoided. As such, the SAM has a cleaning effect.

The use of SAM to protect copper bondpads has the advantage of reducing the process time. The coated Cu bondpad permits the bonding process to be carried out without prior plasma cleaning process. This is advantageous as it provides larger flexibility in wire bonding packaging assembly process. Currently, typical production process time window for aluminium bondpad after plasma cleaning is 24 hours. This is not possible for Cu bondpad because the process time window after plasma cleaning is very short, typically 15 minutes (due to oxidation of the copper surface). Moreover, the SAM will form a passivation layer such that e.g. dust, water vapour do preferably not contact the copper surface.

Fig. 3 discloses a method for a metal to metal contact between a copper bond pad and a metal wire which is a further embodiment of the present invention. Said method can comprise the steps of (see Fig. 3): - define the Cu bonding surface of the bond pad (step 1)

Cu bonding surface exposed after the contact pads have been defined, as in a standard IC process. Cu passivation in the back-end of line includes the deposition of a passivation dielectric (32) together with a resist or sacrificial hard mask (31) for bond pads opening (step 1). After that, the bond pads are opened by dry etch (step 2). This might cause the formation of residues. Remaining resist and etch residues are subsequently removed by dry strip (step 3). However, after dry strip, some residues remain. Moreover, the use of certain gases might enhance Cu oxidation. Remaining residues are finally removed either by wet strip (step 4A) or dry, by plasma treatment (step 4B).

- SAM deposition process:

The SAM can have any chemical composition as described in this application. Stripping of the copper oxide that might be present on the copper surfaces, is done immersing the sample in a 7 M HNO3 solution. Rinse with water and place the sample in a highly concentrated thiol solution in ethanol (range). Everything is best done under an N2 atmosphere. In separate embodiments of the present invention, the SAM (36) can be deposited from solution or from the vapour phase (step 4A or step 4B, respectively).

- The step of stripping copper oxide may be omitted, since the SAM will reduce the copper oxide and consequently clean the copper surface.

- Wafer Dicing process.

Singulation of die from wafer is carried out by wafer dicing process. This involves sawing the wafer with a saw blade. The copper bond pad is protected by the layer of self-assembling monolayers. As sawing process would generate high temperature, deionised (DI) water is sprayed onto the cutting parts for cooling purposes. If the copper bonding surface would not have been protected, the submersion of entire wafer under DI water would readily oxidise the Cu bondpad during the dicing process. This is substantially avoided in the current method.

- Die attach process. To attach the die onto the substrate, the die is picked and placed onto the substrate with adhesive material in between. The adhesive material is then cured in oven to achieve the mechanical properties. Depending on the type of adhesive materials used, the curing temperature range from 100C to 180C and curing time range from 1 minute to 30 minutes. The exposure of the die under this temperature would readily oxidises the Cu bondpad during the adhesive curing process. This is also prevented by coating the copper bonding surface with self-assembling monolayers

- Wire bonding process

Electrical connection from the chip to the substrate (leadframe, ceramic or organic interposed) could be achieved by several bonding techniques, such as wire bonding. For thermosonic ball-wedge bonding, the bonding temperature could range from 80°C to 220°C depending on the type of substrate used. In typical packaging assembly process, prior to wire bonding, the sub-assembled module is plasma cleaned with selected gases to remove the oxide layer from the top of the bond pad. For aluminium bondpad, the time process window is 24 hours whereas for Cu bondpad, the time process window could be only 15 minutes. This implies that wire bonding has to be completed within 15 minutes after plasma cleaning in the case of Cu bondpad. The method in this invention foresees the deposition of a SAM on the surface of the Cu pad prior to wirebonding, so as to extend the process time window. During the bonding process, the SAM deposited in the interface coated Cu bondpad will be eliminated by the bonding process.

The method can further comprise the steps : Deposition of the SAM can be performed on a device which has been already wire bonded. All metal surfaces will be then protected and adhesion promotion of a protective coating can be then enhanced (see Fig. 4).

Moreover, the SAM coating can be deposited on Cu leadframe to replace silver plated surface finished bond finger. Currently, the second bond finger on the copper leadframe is plated with silver to prevent Cu oxidation so that wire bonding is possible with gold wire. The monolayer coating could be applied to the bond finger of the Cu leadframe to replace the silver coating to enable wedge bonding with both Au and Cu wires. In another embodiment, the SAM can replace Ni/Au surface finish bond finger. Currently, the second bond finger on the BT (Bismaleimide Triazine) substrate for BGA package is plated with Ni/Au to prevent Cu trace oxidization so that wire bonding is possible with gold wire. The monolayer coating could be applied to the bond finger of the Cu bondpad of BT substrate to replace the Ni/Au coating to enable wedge bonding with both Au and Cu wires.

Examples

Experiment 1 The coating of a copper bond pad has been performed on Cu/SiO2 BEOL substrates and on blanket wafers. Different types of coating were used (see table 2).

Table 2 : types of coating and blocking efficiency against oxidation/reduction of the copper surface based on voltammographic measurements.

Cyclic voltammograms are measured in aqueous solutions of 0.1 M NaOH showing the influence of 1-decanethiol (■■■), benzenethiol ( ), benzenethanethiol ( ), and benzothiazole ( — ) films, labelled CIO, BT, BET, and BTA, respectively, on Cu oxidation and reduction (ooo). Films were formed by immersion (> 12 hr.) in a 10^"3 M CIO, BT, BET, and BTA solutions in methanol. Scan rate 50 mVs^"1.

(see Fig. 4). It is observed that the coating BT, BET and CIO result in a decrease of the oxidation of the copper surface compared to bare copper (with the formation of the native oxide).

Experiment 2:

Then, a copper wire is bound to the copper bond pad to test the effectiveness of coating layers. Ball shear tests were performed. Ball shear tests are only performed on test samples that result in a completed bond between the bonding pad and the wire. Non-coated copper bond pad showed a large number of non-attached bonds (71 % non- attached bonds), in comparison to 100 % effective bonds realized on coated bond pads.

Comparison of ball shear force test results for Cu wire onto Cu blanket die with and without passivation is shown in Fig.5.

With the exception of C10_g formed by adsorption of 1-decanethiol from the gas phase, passivation layers ClOi, BT, and BTA were formed by immersion (> 12 hr.) in a 10^"3 M solutions in methanol of 1-decanethiol, benzenethiol, and benzothiazole, respectively.

It is observed that a good bonding is obtained for the coating that are self- assembling monolayers (see Fig. 5). Experiment 3:

In a next experiment, a gold wire is bound to the copper bond pad to test the effectiveness of coating layers.

Ball shear tests were performed form gold wire bonding. Ball shear tests are only performed on test samples that result in a completed bond between the bonding pad and the wire. Non-coated copper bond pad showed a large number of non-attached bonds (71 % non-attached bonds), in comparison to 100 % effective bonds realized on coated bond pads.

Fig. 6 illustrates a good bond between copper coated with a self-assembling monolayer and the gold wire.

Experiment 4

The coating SAM CIO had been selected to protect the copper bond pad in the next experiment which involved more elaborated wire bond optimization. The optimized ball shear strength for both gold and copper wire ball bonds are

5.8gf/mil² and 7.3 gf/mil² respectively. The use of ball shear strength with unit "gf/mil² is to normalize the ball shear strength so that the result is not dependent on the ball diameter. The specification of a minimum ball shear strength is 4.0 gf/mil² according to Q-1000 Standard (see Fig. 7). Therefore, both the gold and copper ball shear strength had reached the minimum ball shear strength requirement.

Experiment 5.

Two types of reliabilty tests have been completed: the High Temperature Storage test on non-encapsulated wire bond and the Thermal Cycling test on encapsulated test vehicle. The minimum requirement for both tests is 1000 hours and 1000 cycles respectively.

Ball bonds formed by both gold and copper wire bond on the SAM CIO organic coated copper bondpad are thermally stable in ball shear strength up to a period of 1500 hours at 150°C. The encapsulated daisy chain test vehicle with both gold and copper wires bonding have passed 1400 cycles of thermal cycling test (-65°C to 150°C).

Claims

1. A method for obtaining metal-to-metal contact between a bonding surface of a metallic bonding area and a second metal surface, comprising the steps of : - coating said bonding surface of said metallic bonding area with a chemical composition that forms a self-assembled monolayer on said bonding surface of said metallic bonding area, and

- bonding said second metal surface to said coated bonding surface through said self- assembled monolayer.

2. A method as recited in claim 1 wherein said self-assembled monolayers comprises at least one self-assembling molecule, said self-assembling molecule comprising a functional group that attaches to said bonding surface, a spacer group promoting the formation of the self-assembled monolayer and a terminal group essentially not attaching to said bonding surface.

3. A method as recited in claim 2 wherein said functional group that attaches to said bonding surface is capable of reducing a metal compound formed at said bonding surface of the metallic bonding area.

4. A method as recited in any of claims 1, 2 or 3 wherein said self-assembling monolayer comprises at least one molecules selected from the group consisting of X-Ri- SH, X-Rι-S-S-R₂-Y and Rι-S-R₂, Ri and R₂ being independent from each other and forming a spacer of n carbon atoms, optionally interrupted by p heteroatoms, X and Y being independent from each other, and an end group essentially not attaching to said bonding surface.

5. A method as recited in any of claims 1 to 4 wherein said metallic bonding area is a semiconductor bond pad.

6. A method as recited in any of claims 1 to 4 wherein said metallic bonding area comprises at least one metal.

7. A method as recited in claim 6 wherein said metal is selected from the group consisting of copper, gold, silver, platinum, palladium, nickel, iridium, iron and aluminum.

8. A method as recited in claim 7 wherein said bonding surface comprises copper.

9. A method as recited in claim 8 wherein said bonding surface consists essentially of copper.

10. A method as recited in claim 3 wherein said metal compound is copper oxide.

11. A method as recited in any previous claim, wherein said second metal surface is a part of a metal wire used in wire bonding.

12. A method as recited in any of the claims 1 to 10, wherein said second metal surface is part of a solder ball.

13. A method as recited in any of the claims 1 to 11 wherein said second metal surface comprises at least one metal.

14. A method as recited in claim 13 wherein said metal is selected from the group consisting of copper, gold, silver, platinum, palladium, nickel, iridium, iron and aluminum.

15. A method as recited in any of claims 4 to 14, wherein X and Y are independent from each other and are a chemical group that is hydrophobia

16. A method as recited in any of the claims 2 to 15 wherein the spacer comprises at least a chemical group selected from the group consisting of alkyl, alkenyl, alkynyl, cyclic alkyl, aryl, alkyl bound to aryl, alkenyl bound to aryl and alkynyl bound to aryl.

17. A method as recited in claim 1 to 16 further comprising the step of coating said second metal surface with a chemical composition that forms a self-assembled monolayer on said second metal surface prior to said bonding step

18. A method as recited in any of claims 1 to 17 further comprising the step of coating said metal-to-metal bond with a chemical composition that forms a self-assembled monolayer

19. A method as recited in any of claims 1 to 18 wherein said metallic bond pad is a semiconductor bond pad, said semiconductor bond pad being part of a substrate, said method further comprising the steps of dicing said substrate into individual chips.

20. A device comprising a metal-to-metal contact between a bonding surface of a metallic bonding area and a second metal surface, wherein said metal-to metal contact is obtained by providing a coated bonding surface of a metallic bonding area, said coated bonding area comprising a self-assembling monolayer being deposited on said bonding surface and at least a part of said second metal surface bonds to at least a part of said coated bonding surface through said self-assembled monolayer.

21. A device as recited in claim 20, wherein said bonding surface comprises at least a first and a second region, said first region being bound to said second metal surface and said second region being coated with a self-assembled monolayer.

22. A device as recited in claim 19 or 20 wherein said self-assembled monolayer comprises at least one self-assembling molecule, said self-assembling molecule comprising a functional group that attaches to said bonding surface, a spacer group promoting the formation of the self-assembled monolayer, and a terminal group essentially not attaching to said bonding surface.

23. A device as recited in claim 22 wherein said functional group that attaches to said bonding surface is capable of reducing metal compounds being formed at said bonding surface of the metallic bonding area.

2^ A device as recited in any of claims 20 to 23 wherein said self-assembling monolayer comprises at least one molecule selected from the group consisting of X-Ri- SH, X-R]-S-S-R₂-Y and Rj-S-R₂, i and R₂ being independent from each other and a spacers of n carbon atoms, optionally interrupted by p heteroatoms, X and Y being independent from each other and an end group essentially not attaching to" said bonding surface.

257 A device as recited in any of claims 20 to 2f wherein said metallic bonding area is a semiconductor bond pad.

2 . A device as recited in any of claims 20 to 25^" wherein said metallic bonding area comprises a metal.

2?. A device as recited in claim I wherein said metal is selected from the group consisting of copper, gold, silver, platinum, palladium, nickel, iridium, iron and aluminum.

2i?. A device as recited in any of claims 20 to 2? wherein said bonding surface comprises copper.

29. A device as recited in any of claims 20 to 28 wherein said bonding surface consists essentially of copper.

30. A device as recited in claim 23 wherein said metal compound is copper oxide.

31 . A device as recited in any of claims 20 to 30 wherein said second metal surface is a part of a metal wire used in wire bonding.

32. A device as recited in any of claims 20 to 30 wherein said second metal surface is part of a solder ball.

33. A device as recited in any of claims 20 to 32. wherein said second metal surface comprises a metal.

31-. A device as recited in claim 33 wherein said metal is selected from the group consisting of copper, gold, silver, platinum, palladium, nickel, iridium, iron and aluminum.

357 A device as recited in any of claims 20 to 3Ψ- wherein said second metal surface is coated with a self-assembling monolayer prior to said bonding step.

367 A device as recited in any of claims 20 to 3 ^" wherein said metal-to metal contact is coated with a self-assembling monolayer.

3^. A device as recited in any of the claims 24 to 36 wherein X and Y are independent from each other a chemical group that is hydrophobia