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16

Chip Scale Review July • August • 2018

[ChipScaleReview.com]

through-metal wafer vias (TMV), as well

as fluid ports and bond pad openings for

wire bonding of the CSP made using a

metal multi-wafer stack. While many of

the same wafer-to-wafer bonding methods

developed for silicon wafers can, and

have been applied to metal wafers, some

practical limitation exists when mixing

wafers of different materials in the stack.

Differences in thermal coeff icients

of expansion (TCEs) between wafers,

even when relatively small, can result is

cracking or built-in stress after a higher

temperature wafer bonding step. Kovar

with a TCE of 5 to 6ppm/°C is often used

with silicon (TCE 2.6pp/°C) as a hermetic

package metal. However, anodically

bonding large diameter Kovar wafers

to borof loat glass (TCE 3ppm/°C) can

result in wafer bowing, and even cracking

across the wafer-to-wafer bond interface,

shearing off the majority of the glass

wafer, as shown in

Figure 3

. Just a thin

layer of glass is left, anodically bonded to

the patterned metal wafer after this TCE

related fracture. Like

Figure 1a

,

Figure

3

illustrates the impact of poor fracture

toughness (glass and silicon) versus high-

fracture toughness metals with regard to

cracking and fracture at the wafer level

during packaging. Care must be taken

when selecting the wafer bonding method

when using metal substrates. Softer

bonding materials like silicone or solder

may be preferred for many applications

where wafer TCEs don’t closely match

each other.

WLP using one or more metal wafers

offers improved heat sinking of the

CSP. For f luidic applications such as

pressure sensors, active chip cooling

and microf luidic sensors, a metal chip

adds the ability to weld the CSP directly

to metal tubing, as shown in

Figure 4

.

In these applications silicon chips often

use epoxy or silicone chip attachment

material that degrades over time when

used in liquid microfluidic applications.

On account of thermal shock cracking,

welding is not an option for silicon, so the

ability to weld a small diameter stainless

tube or tubes to a metal chip offers some

advantages in fluidic CSP applications.

CMOS wafer fab operators will not

allow transition metals like those used in

alloys such as stainless steel and Kovar,

to be processed in their equipment due to

potential contamination of silicon CMOS/

BiCMOS wafers. Transition met als

in silicon can cause high PN junction

leakage currents, emitter-collector pipes,

degraded minority carrier lifetimes and

gate oxide, and are known to diffuse

rapidly through silicon wafers [11]. Many

traditional MEMS foundries, which do

not process CMOS circuit wafers, will

allow the processing of sodium containing

borofloat glass and other substrates like

titanium wafers. This substrate flexibility

opens up the possibility of metal WLP

devices. Metal wafers of 100mm, 150mm

and 200mm are being processed in some

MEMS fabs.

3D-printed metal packages

WLP for ICs and MEMS opens up many

unique Internet of Things (IoT) applications.

To avoid some of the complications of

traditional WLP manufacturing like DRIE

cavity, via etching and obtaining wafer-

to-wafer bonded hermetic seals [12],

one alternative that is being explored is

using the 3D printing of metals to form

the complex 3D str uctures found in

many MEMS devices and packages. 3D

printing can eliminate the need for CSP

wafer bonding or welding in system-level

packages. This new fabrication method can

also directly merge the sensing elements

with the larger system-level package using

a single printing step.

Excellent metal electrical and thermal

conductors like copper and aluminum,

as well as corrosion resistant metals like

stainless steel alloys and titanium can be

3D printed. Various types of 3D metal

printing are available and include selective

laser melting (SLM) or direct metal laser

sintering (DMLS) that use metal powder as

a starting material. Electron-beam melting

(EBM) is also used for 3D printing in which

the raw material can be either metal powder,

filaments, rod, or wire. These printing

methods can be done under vacuum to

reduce porosity, oxidation and other defects.

Minimum printed feature sizes can range

from 50 to 200 microns depending on the

particle and beam size. The printing method

and particle size also controls the surface

roughness of the finished metal product.

Post-print processing is often required to

produce a polished finish.

3D metal printing has several advantages

over traditional machining and wafer

fabrication. It is often dramatically faster

than processing wafers and the combination

of machining and assembly. Material scrap

is lower because it is a direct print additive

process versus a subtractive machining

or etching process. By printing a single

piece instead of using assembly steps, the

manufacturing process can be simplified.

Metal and plastic 3D printing is ideal for

fast prototyping and low-volume, high-cost

products, or for the replacement of legacy

products.

Figure 5

shows a stainless steel,

3D-printed fiber optic pressure sensor that

combines a reflective metal diaphragm

along with the male threaded pipe fitting

and hex nut portion of the final package

into a single piece. 3D printing avoids

the welding step shown in

Figure 1c

for

pressure sensors. The ref lective finish

of the stainless diaphragm requires

polishing after 3D printing. This same

smooth diaphragm surface is ideal for

CVD dielectric deposition and patterning

Figure 3:

Shear fracture of the glass wafer at the

anodically-bonded metal to glass wafer interface.

Figure 4:

Stainless steel tube welded to a metal

microfluidic CSP.

Figure 5:

3D-printed stainless steel optical

pressure sensor prototype with a threaded metal

package fluid port.