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Chip Scale Review July • August • 2018

[ChipScaleReview.com]

Metal-based wafer-level and 3D-printed packaging

By Doug Sparks

[Hanking Electronics]

ew technologies and materials

are being incorporated into

IC and sensor packaging

each year. Wafer-level packaging (WLP)

traditionally has utilized silicon, or perhaps

glass substrates to form chip-scale packages

(CSP). More extreme corrosive or high-

temperature sensor applications as well as

needs for better electromagnetic interference

(EMI) shielding or perhaps cooling for high-

power chips have pushed some packaging

materials towards incorporating metals

either in WLP and/or into the larger system-

level package. This paper will discuss metal-

based WLP of ICs and sensors as well as

new developments in 3D-printed metal

sensor, circuit and microfluidic packaging.

Adding circuits to metal substrates

began in the 1980s with screen-printed

hybrid technology using porcelainized

steel [1]. This glass or glaze coated steel

substrate material was seen as a lower cost

option over alumina substrates and as a

way of making large area, more complex

substrate shapes and improving heat

sinking for industrial applications. Thick-

film piezoresistive pressure sensors were

even demonstrated using coated steel

substrates [2].

The f irst t r uly thin-f ilm, IC-li ke

device made on metal was an industrial

pressu re sensor. The d r ivi ng force

behind developing a sensor on steel was

overcoming the poor fracture toughness

of silicon.

Figure 1a

shows what happens

to a differential silicon pressure sensor

under overpressure conditions, the silicon

diaphragm breaks like glass due to its

low fracture toughness. Using a metal

diaphragm instead of fragile silicon

allows the sensor to be used in high-

pressure applications without the worry

of diaphragm breakage and release of the

gas or liquid.

I n t he 1990 s a “wa fe r ” f ab wa s

const r ucted by Nagano Keiki [3,4],

dedicated to only thin-f ilm IC-type

proce ssi ng on st ai n le ss st eel. The

pressure sensors shown in

Figure 1b

are

made using plasma-enhanced chemical

vapor de posit ion ( PECVD) ox ide,

doped polysilicon and silicon nitride, as

well as metal layers, all patterned with

conventional photolithography techniques

on stainless steel. These small steel sensor

elements are first machined from rod

stock, polished on one side and then fab-

processed in trays. However, connected

panels of the elements, more like a wafer

or circuit board, with multiple sensors

on a substrate, can also be used to run

the metal parts through the dedicated fab

[5]. After fabrication, the round sensing

elements are welded to the threaded, hex

nut pressure sensor package as shown in

Figure 1c

. The corrosive or hazardous

liquid is only exposed to the bottom side

of the stainless steel diaphragm, while

the thin-film Wheatstone bridge circuit

is safely located on the opposite, top

steel surface. These steel sensors are

still being fabricated in high volume for

automotive and industrial applications.

Other pressure sensor suppliers make

competing products by attaching thinned

single-crystal silicon strain gauges to

steel diaphragms using ref lowed glass

and epoxy [6].

Another metal MEMS technology

developed in the 1990s and 2000s was

LIGA, a German acronym for Lithographie,

Galvanoformung, Abformung. LIGA

involves the electrochemical plating of metal

structures in a photoresist mold [7]. This 2D

additive technology was used to grow nickel

resonators for gyroscopes on CMOS wafers

[8]. The CMOS integrated metal sensor was

incorporated into a silicon to silicon wafer

bonded stack to encapsulate the metallic

sensor in a silicon CSP.

Metal WLP

Grounded EMI shielding in the form

of metal covers are often placed over

silicon-based CSPs at the system board

level. EMI shielding is one driving force

for RF-MEMS, metal CSP applications.

Me t a l s l i ke Kova r, s t a i n le s s s t e el

and titanium have been employed in

microelectromechanical (MEMS) sensors

and actuators. For over ten years, Professor

McDonald’s team at UCSB has developed

titanium wafer fabrication processes and

various devices, including metal RF-

switches, using titanium wafers [9,10]. Ti

fab processes such as deep reactive ion

etching (DRIE), wafer-to-wafer bonding

and more traditional film deposition and

photolithography have been developed. The

same photoresist spray coating processes

that allow metal films, and hence, runners

to be patterned over the 10- to 100-micron

steps of a silicon MEMS cap wafer can be

applied to a metal cap wafer.

Figure 2

shows examples of metal

wafers that have been patterned to form

N

Figure 1:

a) Ruptured silicon pressure sensor

diaphragm; b) Thin-film stainless steel pressure

sensor; c) Thin-film steel pressure sensor welded to the

machined package with a threaded fluid connection.

Figure 2:

Metal wafers with patterned through via,

bond pad opening, and fluid port.