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Definitions

• the invention relates to a solder or tin contact bump structure according to the preamble of claim 1 for unencapsulated microcircuits.
• the invention also relates to a process for producing the solder bump structure.
• One interesting bonding method is the so called flip-chip method which comprises bonding a microcircuit directly to its mounting substrate in exposed, unencapsulated form.
• the method is based on depositing a solder contact bump on the aluminium contact areas of the microcircuit and then melting the bump to bond the circuit to its substrate.
• the benefits of the method include the small size of the bonding footprint and the lightness of the structure. The method has, however, been hampered by the inadequate reliability of the bonding structure.
• Japanese Patent Specification No. 07321114 discloses a contact bump structure where a titanium-tungsten layer is first formed on the contact substrate followed by a gold layer formed in two steps, whereafter a nickel layer is formed. The actual tin contact bump is deposited on this multilayer structure. The deposition of the gold layer in two steps complicates the process, and the titanium-tungsten thin film is, in the manner described, left with weaker diffusion-barrier properties, thus impairing the reliability of the structure.
• the present invention aims at eliminating the drawbacks of the above-described techniques and at obtaining an entirely novel type of solder contact bump structure for unencapsulated microcircuits.
• the invention is achieved by forming the gold layer of the contact bump structure in one step only, and by processing the titanium-tungsten thin film in a nitrogen- argon environment and subjecting it to oxygen treatment. More specifically, the solder or tin contact bump structure is characterized by what is stated in the characterizing part of claim 1.
• the invention provides considerable benefits.
• the compatible metal layers have improved the reliability of the structure.
• the TiW thin film has good adhesion to aluminium and offers good diffusion-barrier properties.
• an Au thin-film layer has been used as the adhesion-fo ⁇ ning film in combination with a gold bump.
• electrolytically deposited nickel provides good adhesion to a gold layer. Having both an exceptionally low stable and metastable solubility in most tin-based alloys when these are in their molten state, nickel is most suitable for use as an adhesion layer for tin-based solders. Primarily for this reason, nickel has a clearly lower rate of dissolution in a eutectic Sn37Pb solder than, e.g., copper which is conventionally used as an adhesion layer in solder bump structures. Particularly for this reason nickel forms clearly thinner intermetallic compound layers than copper. What is more, the oxidation and nitrogen treatment of the titanium-tungsten essentially improve the diffusion barrier properties of the titanium-tungsten layer.
• Fig. 1 is a longitudinally sectioned side view of the UMB structure in a solder bump made using conventional techniques.
• Fig. 2 is a graph of the dissolution rate of different metals into eutectic Sn37Pb solder as a function of temperature (ref. Klean Wassink, Soldering in Electronics, 2nd
• Fig. 3 is a longitudinally sectioned side view of a contact bump structure according to the invention.
• Fig. 4 is a detail of the structure shown in Fig. 3.
• Figure 5 is a more detailed view of the structure shown in Fig. 3.
• Figures 6a to 6c are longitudinally sectioned side views of alternative contact bump geometries according to the invention.
• FIGS 7a to 7i are process steps of the process according to the invention.
• both the TAB and flip-chip techniques conventionally used for bonding components are based on depositing contact bumps 6 typically having a height of 10 to 100 ⁇ m on the electric bonding pad areas 3 of a microcircuit 15, after which the microcircuit 15 itself is bonded on a desired bonding substrate (e.g., a TAB film, FR4, or a so called flexible substrate).
• a desired bonding substrate e.g., a TAB film, FR4, or a so called flexible substrate.
• a required number of different thin metal or metal alloy layers must be processed between the bump 6 and the electric contact pad area 3.
• the structure formed by these metallization layers is generally termed UBM (Under Bump Metallurgy) or BLM (Ball Limiting Metallurgy) and serves to improve adhesion and retard or even prevent diffusion therethrough.
• UBM Under Bump Metallurgy
• BLM Bit Limiting Metallurgy
• the metal layers may be two or more in number, with a typical thickness of 0.1 to 5 ⁇ m.
• UBM structure is formed from TiW and Au thin-film layers.
• the most widely known UBM structure is the Cr-Cu-Au used by IBM Corp.
• each of the layers serves for the above-mentioned purposes to improve the reliability of the structure.
• the metals used and their layer thicknesses must be selected such that they will also meet the basic metallurgic requirements. Furthermore, they need to retain their properties under normal operating and processing conditions and temperatures, meaning that the structure must not exhibit any undesirable reactions or changes in its properties.
• the purpose of the first intermediate metal layer 11 is to provide good adherence to the silicon- alloyed contact pad area 3 of aluirjinium. When required, it also functions as a diffusion barrier layer serving to prevent intermetallic reactions between the aluminium 3 and other metals in the structure. Furthermore, the layer acts to protect the contact pad areas 3 of the microcircuit 15 against possible corrosion.
• the purpose of the second intermediate metal layer 10 is to provide good adhesion between the adherence layer 9 of the bump 6 and the metallized contact pad area 3.
• the adherence layer 9 of the contact bump 6 forms the actual base layer for the solder bump 6 itself.
• the adherence layer 9 must be metallurgically compatible with the solder used.
• Fig. 2 shows the dissolution rate of metals commonly used in electronics into eutectic Sn37Pb solder as a function of temperature.
• the structure according to the invention is selected such that all metallizations are metallurgically more compatible with each other than those used in the prior art, and what is more, the layer structure obtained is stable and easy to manufacture.
• the substrate of the microcircuit 15 is formed by alu ⁇ iinium contact pad areas 3 on the surface of a substrate wafer 1 and a passivation layer 2 which partly covers the contact pad areas and which entirely covers the wafer 1.
• the tJam-film layer 7 is formed on the surface of the contact pad area 3 of the microcircuit 15 from a thin film 7 of TiW (10/90) alloy sputtered in the presence of oxygen and nitrogen, whereby the alloying ratio is typically 10/90 (Ti/W).
• the alloying ratio may typically be varied in the range 1/99 - 50/50.
• the thickness of the thin film 7 is typically approximately 100 to 350 nm, most appropriately about 140 nm.
• the TiW ⁇ in-film layer 7 acts as an adhesion layer thus providing good adherence to the contact pad area 3 of the microcircuit 15.
• the layer also acts as a diffusion barrier layer preventing undesirable reactions between the aluminium metallization and the gold layer of the contact bump.
• Another function of the Au thin-film layer 4 is to protect the TiW thin-film layer 7 against oxidation prior to the electrolytic deposition of the contact bumps.
• the nickel layer 5 serves as the base metallization layer of the actual contact bump 6.
• the height of the contact blimp 6 is typically 10 to 100 ⁇ m.
• the height is typically in the range from 10 to 60 ⁇ m, and correspondingly, for a solder bump it is typically in the range from 20 to 100 ⁇ m
• the contact bump 6 may be either straight-walled as in Fig. 6b, mushroom-shaped as in
• the height ratios of the composite metal structures are, e.g., 1: 1:20 (TiW: Au:Ni).
• the contact bump 6 can be made from different compositions of tin-lead alloys, tin or other electrolytically depositable tin-based solder alloys.
• the wafers are RF etched at a slightly elevated temperature.
• the structure then consists of a substrate 1, an insulation layer 20 on the substrate, and a passivation layer 2.
• the TiW layer 7 is DC sputtered in two parts.
• the total thickness is typically 1400 ⁇ 10 ⁇ A.
• the first TiWN (700 A) is sputtered in a N 2 (1 part)/Ar (three parts) atmosphere and the second TiW layer in a pure Ar atmosphere.
• the total thickness of the layer is controlled periodically.
• the TiWN/T ⁇ W layer 7 is oxidized in an oxygen chamber in order to improve its diffusion barrier properties. During this treatment, the oxygen is dissolved into the TiWN/TiW structure. Before etching the next layer, the wafers are RF etched.
• the second intermediate metal layer U (the Au thin film) is RF sputtered into a thickness of 1200 ⁇ 10 ⁇ A in an Ar atmosphere.
• the thickness of the layer is controlled periodically.
• the photoresist of Fig. 7d is made by conventional methods using a thick photoresist layer 21.
• the thickness is typically 23 mm
• the pre-light exposure baking is carried out on the line at a temperature of about 100 °C.
• the mask is aligned and the exposure is carried out by means of commercially available apparatuses.
• the wafers are exposed to UV light.
• the exposed areas are dissolved by means of a developing phase carried out either by the spray or the immersion method.
• the thickness of the photoresist is measured and registered.
• the after-baking of the wafers takes place in the oven.
• the exposed areas are purified by oxygen plasma just before the electrolytic deposition of the bumps.
• an Ni layer 5 is electrolytically deposited into the openings in the photoresist
• the nickel bath is periodically monitored and the deposition parameters are registered.
• a tin solder bump 6 is electrolytically deposited onto the thin nickel bump 5. Even this bath is monitored and the parameters are registered.
• the photoresist is removed using wet chemistry, and rinsed with ion-exchanged (DI) water.
• DI ion-exchanged
• the removal of the photoresist is checked on one wafer.
• the height of the bumps is measured by means of a Profilometer at five points on a wafer, and at least two wafers in one production lot are subjected to such measurement.
• the Au layer is removed by wet etching from the areas outside the contact bumps. The etching times are registered and the wafers are rinsed and examined.
• the wafers are reflow heat treated in glycerol at a temperature of 210 °C in order to obtain the final shape and structure of the tin bumps 6. After this treatment the wafers are rinsed and the end result is examined.

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• 1996
  • 1996-05-31 FI FI962277A patent/FI962277A0/fi not_active Application Discontinuation
• 1997
  • 1997-05-30 AU AU29642/97A patent/AU2964297A/en not_active Abandoned
  • 1997-05-30 EP EP97924046A patent/EP0958606A1/en not_active Withdrawn
  • 1997-05-30 JP JP09541692A patent/JP2000511001A/ja not_active Ceased
  • 1997-05-30 WO PCT/FI1997/000331 patent/WO1997045871A1/en not_active Application Discontinuation

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