Nickel Plating Overview:

One of the lowest cost and most flexible process alternatives for wafer bumping is to use electroless nickel and immersion gold (e-Ni/Au) as the under-bump-metallurgy (UBM).

This process is inherently low cost, and can be used for a variety of different applications, including:

1. Conventional flip chip (Ni/Au UBM + solder)
2. Polymer flip chip (Ni/Au UBM + conductive epoxies)
3. Anisotropic conductive adhesives (tall Ni/Au)
4. Chip scale and BGA packages (Ni/Au + large solder bumps)
5. Pad resurfacing of copper and aluminum for wire bonding (Ni/Au)
6. Pad resurfacing of copper pads for probe testing (Ni/Au)

The electroless nickel/gold under-bump-metallurgy (UBM) is formed by sequential immersion through a series of wet chemical baths. With careful control of the plating parameters, a layer of nickel, followed by a layer of gold, is deposited on the I/O pads (aluminum or copper). The nickel is typically deposited to a thickness within in a range of 1 to 30 um, depending on the application. The gold layer is deposited simply to protect the nickel from oxidation, and in the case where solder is subsequently applied; the gold diffuses into the solder during the reflow process.

High throughput, and consequently low cost, is accomplished by batch processing cassettes of wafers through an automated electroless plating line. The fact that the nickel plating process is highly selective, and will only plate on the exposed metal surfaces (aluminum or copper), translates into a major cost advantage for this UBM deposition technique. Compared to conventional techniques for depositing the UBM, the use of electroless nickel has the following advantages:

• There are no processing steps necessary to define the solderable area (such as vacuum metal deposition, photolithography, and mask etching).
• One system handles all wafer sizes without change over (3" to 12").
• The capital investment for plating technologies is relatively small.
• The operational costs (labor and overhead) are reduced.

Electroless plating on integrated circuits can, however, be challenging because of the fab-specific variations in materials and processes involved in creating the circuits. Aluminum (or copper) alloy composition, sub-structures under the pad metal, passivation material and quality, pad electrical potential, and energy sensitivity (radiation and grounding effects) all play a role in the plating rates, uniformity, and adhesion of the nickel.

Licenses for execution of electroless nickel processing of silicon wafers are available from several sources. Because the process details (inherent tricks of the trade) are not generally regarded as patentable, developers treat their processes as proprietary. Hence, particulars of electroless nickel plating are not readily available.

The common components for aluminum and copper based pad metallurgies include:

• Cleaning the pad of organics, silicon oxides, and/or nitrides (solvent and acid based solvation)
• Removing aluminum oxide or copper oxide (alkaline or acid based etch)
• activating the aluminum or copper (single/double zincate or palladium)
• electroless nickel plate (ammonia based phosphorous or boron chemistries)
• Gold strike plate (cyanide or sulfate based chemistries)

SEM micrographs of an I/O pad showing several steps in the electroless nickel/gold plating process:

The first three steps in the process are critical in determining the overall selectivity of the plating process, nickel morphology, and the adhesion of the nickel to the aluminum (or copper) pad. In general, a process that produces fine grained, uniform, thin layers of the catalyst (zinc or palladium) will produce the best nickel plated structures. The specific chemistries and absolute component ratios are critical in producing this desired structure. In addition to selecting the appropriate plating chemistries, one must also consider availability, place of origin, price, toxicology, bath life, waste treatment/disposal, and environmental issues related to the chemicals when implementing a process in a manufacturing setting.

Equipment:

Two major considerations must be fulfilled in order to make implementation of an electroless Ni/Au process cost competitive:

a) the equipment must be capable of high volume production, and
b) the process must be reproducible.

The automated plating line shown below was designed and built with these two criteria in mind. The system incorporates:

a) a processing side in which the wafers (in cassettes) are sequentially transported between the various chemical baths (6 different chemistries, with 4 duplicate nickel tanks for increased throughput, and 8 DI water rinse tanks), and

b) a chemical handling side in which the chemistries are prepared and maintained (see Figures below).

Top View
Overall Dimensions 27’ x 9’ x 9’
6 Chemistries (+3 unused tanks)
7 DI water rinse tanks
Capacity: 50 wafers/hour (8")
Fully automated sequencing

The critical control variables on the wafer processing side of the plating system include:

a) process sequence
b) bath processing times
c) bath temperature
d) bath level

The critical variables on the chemical handling side of the plating system include:

a) chemical composition (6 chemistries)
b) pH levels
c) liquid flow rates

In addition to these process based variables, several product based variables must be monitored and control charted in order to maintain a robust plating process. These include:

a) nickel thickness
b) gold thickness
c) nickel adhesion (shear and tape peel)
d) surface roughness
e) morphology

The following figures are examples of control charts for the electroless nickel/gold plating process. The first figure shows the variation and control limits for pH of the nickel bath as monitored over a four month period. Similar charts are maintained for several other components of the nickel bath, as well as components of the other chemical baths: i.e., passivation clean, aluminum etch, etc. Tight control of these components is essential to maintain a consistent plating rate and ensure maximum adhesion of the nickel to the pad metallurgy. The second figure shows the variation in Ni/Au thickness for lots of a given device design processed over a four month period. The third figure shows the variation in nickel adhesion (shear) for lots of a given device design.

Control chart of pH for nickel plating bath.

Control chart of nickel thickness.

Control chart of nickel shear.

Nickle Plating Examples

The following figures show several alternative uses for electroless nickel and gold; in particular tall (20mm) nickel/gold for ACA and conductive adhesive applications.

SEM of 20um tall Ni/Au
for ACA and conductive
adhesive applications.

SEM crossection of tall
Ni/Au bump assembled
with a conductive adhesive.
(courtesy of Poly-Flex Circuits)

Wire bonding to Electroless Nickel/Gold Surfaces

The following pictures are examples of Electroless Nickel/Gold used to provide a stable surface for gold wire bonding.

SEM of 20um tall Ni/Au
for ACA and conductive
adhesive applications.

Electroless Nickel/Gold for Cavity Fill

Multi-layer Silicon wafer stack with cavity from first to second layer. Nickel provides improved step coverage and solderable surface.
(courtesy of Lucas NovaSensor)

Electroless Ni/Au Plating on Copper Conductors

The following pictures are examples of Electroless Nickel/Gold plating on Copper based semiconductors. This layer of nickel and gold provides a stable surface for probe testing, wirebonding, solder bumping, and protects the copper surface from corrosion.

Sheared Ni/Au Pad on Cu
The shear values for Electroless Nickel on Copper are comparable to those of Electroless Nickel on Aluminum. For the example on the left the week interface is the copper to substrate material, NOT the Nickel to copper interface. Severely damaged passivation is also observed.

Adhesion/Shear Force of Nickel/Gold on Copper
I/O Metallurgy	Shear Force per Unit Area
Aluminum	11-17 grams/sq.mil.
Copper	12-24 grams/sq.mil.

Advantages of Plating Nickel and Gold on Copper I/O pads:

• Creates a wirebondable surface
• Creates a barrier layer between interconnect material and I/O pad
• Protects the copper from corrosion and oxidation
• Increases probe yield due to increased electrical contact to probe
• Reduces the typical probe damage often seen with aluminum pads (lighter probe forces)
• Creates a solderable surface, opening up the IC to direct implementation of flip chip and/or CSP applications
• Creates a surface (tall nickel) compatible with anisotropic conductive adhesives (ACA)
• Creates a surface (thin nickel) compatible with conductive adhesives
• Much less expensive than capping the copper with Ti/Al

Plating Over Defects

Shear Evaluation

FC1 6.5 mil octagonal bond pads & 3.6 mil sq. test pads 20 mm nickel thickness	FC2 4 mil round bond pads 20 mm nickel thickness
Shear force = >500 g & 195 g Standard deviation = 23 g  Load/area = >13.2 g/sq.mil & 15 g/sq.mil  Failure mode = nickel/aluminum	Shear force = 201 g Standard deviation = 18 g Load/area = 16 g/sq.mil Failure mode = nickel/aluminum

As Received Al I/O pad	Ni/Au Plated I/O Pad	Sheared Ni/Au I/O Pad

The shear force for Nickel/Gold plated pads on aluminum is typically between 12 and 16 grams of shear force per square mil of aluminum surface. Several failure mechanisms are commonly observed, including: aluminum cohesive failure, aluminum to silicon adhesive failure, and silicon fracture. Also observed along with these gross failure modes is: passivation cracking, passivation delamination, and metal trace liftoff.

Shear at Nickel/Aluminum interfacial

Shear at Nickel/Aluminum interfacial
with silicon cratering

Tall Nickel(20umNi/Au) Shear
after multiple reflows
Devices reflowed on belt furnace using Eutectic Solder reflow profile (220 degC maximum temperature).

Devices reflowed on belt furnace using High Lead Solder reflow profile (330 degC maximum temperature).

Tall Nickel (20umNi/Au) Shear
after high temperature storage
Devices stored in oven at 200 deg C °

Nickel/Gold Surface Roughness

	Aluminum I/O Pad Surface (Ra =1035±105)	Ni/Au Plated I/O Pad Surface (Ra =1157±126)
Device #1 (Rough Al)
	Aluminum I/O Pad Surface (Ra =184±65)	Ni/Au Plated I/O Pad Surface (Ra =203±46)
Device #2 (Smooth Al)

Ni/Au Analysis

The following figures are crossections of Ni/Au plated structures. The nickel morphology is observed to be very fine grained in nature and the gold layer crystalline.

	SEM crossection of plated Ni/Au.
TEM crossection of plated Ni/Au

The consistency of the plated out UBM is a critical factor in the robustness of any subsequent I/O bonding process. Auger analysis is an effective technique for such studies. Analysis of the plated films reveals the surface of the gold layer has absorbed commonly observed elements from the atmosphere, carbon and oxygen (see Figure 1 below). Removal of this residue by argon back-sputtering results in a layer which is nearly isotropic in gold composition (see Figure 2). Further sputtering through the gold and into the nickel reveals the constituents of the nickel layer (see Figure 3).

Figures 4 and 5 are depth profiles of the electroless nickel and immersion gold layers as measured by Auger spectroscopy.

Figure 1. Auger Spectrum of Ni/Au plated bond pad surface.

Figure 2. Auger Spectrum on Ni/Au plated bond pad after sputtering.

Figure 3. Auger Spectrum of Ni/Au plated bond pad after sputtering through gold layer.

Figure 4. Auger survey profile of Ni/Au plated bond pad.

Figure 5. Auger survey profile of Ni/Au plated bond pad .

Selection of Nickel Chemistry and Process

IC Interconnect evaluated four different suppliers of electroless nickel technology. The evaluation was based on each supplier processing wafers of four different designs. Yield, absolute thickness, thickness uniformity, adhesion, solderability, shear failure mode after solder deposition, and shear failure mode after multiple reflows, were all used as criteria for process selection.

The following figures are representative examples of the variability observed among nickel plating technologies. In some cases, pads had dramatically different morphologies across the wafer, and the nickel to aluminum adhesion in some cases was very poor. Clearly not all electroless nickel processes perform equally, and licensing costs also varied extensively.

Optical micrographs of aluminum bond pads plated with nickel and gold from two vendors:

55x	350x
Fine grained Ni/Au with uniform plating (Vendor A)
55x	350x
Course grained Ni/Au with uneven plating (Vendor B)

Nickel shear results of test die from electroless nickel Vendor A and Vendor B.

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