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Chip Scale Review March • April • 2017

[ChipScaleReview.com]

Furthermore, these IPDs can also be

directly integrated into higher functioning

RF systems-in-package (SiP) devices,

leading to smaller packaging, lower power

consumption, and increased bandwidth.

In this article we will present an overview

of PSGs for the electronics packaging

industry with a focus on the integration of

integrated passive devices (IPDs) for the

RF industry. We will present on our recent

advances in the monolithic integration of

high-Q inductors and capacitors and several

IPDs that have target performance up to

20GHz. We will present both modeling and

measurement data for several devices.

What are photosensitive glasses?

Photosensitive glass ceramics (PSGs)

were first identified and explored in the

1950s and were initially discovered by

Dr. Stanley Donald Stookey at Corning.

Corning first commercialized photosensitive

glass ceramic products under the trade

name CorningWare in the 1950s. The initial

products focused on high-temperature

stability materials for household cooking

and included dishes and stovetops.

Photosensitive glasses belong to the

lithium–aluminum–silicate family [6] with

impurities of metal oxides that greatly

contribute to the photostructurability of

these glasses. This class of materials is

capable of existing in both an amorphous

glassy state and a crystallized ceramic state

(crystalline-phase lithium metasilicate)

within the same substrate. PSGs are

characterized by their ability to selectively

pattern ceramic features in the bulk glassy

material through lithography.

In recent years, PSGs, with their unique

3D structuring ability, have been explored

for a variety of technical applications [7]

including microfluidics, optoelectronics,

and more recently, led by 3D Glass

Solutions, RF IPDs and systems-in-

packages (SiPs). The primary PSG that we

use is APEX

®

Glass.

Process approach

The manufacturing of 3D structures in

PSGs is accomplished through a patented

3-step manufacturing process. The first

step in processing an IPD wafer is to

expose the glass using a lithography

mask to create through-glass features

(e.g., through-glass vias (TGVs) and

through-glass capacitor plates). This

is accomplished using a chrome-on-

glass mask placed directly onto the glass

wafer, without photoresist, and exposed

to 310nm UV light (

Figure 1a

). During

this step, photo-sensitizers in the glass

undergo a redox exchange initiated by

the UV light.

I n t he s e cond s t ep , t he g l a s s i s

b a k e d a b o v e i t s g l a s s t r a n s i t i o n

temperature (

Figure 1b

), where mobile

ions surround the exposed regions,

converting the previously exposed glass

into a nano-crystalline ceramic phase.

After the bake step, the exposed pattern

has been converted into ceramic, going

all the way through the glass wafer.

Unexposed regions of the wafer remain

in the original glassy state.

In the third processing step, the

wa f e r i s e t c h e d i n a d i l u t e d a c i d

(

Figure 1c

), preferentially etching the

Glass-based SiP solutions for

high-performance/high-frequency RF filters

By Jeb Flemming, Roger Cook, Tim Mezel, Kyle McWethy

[3D Glass Solutions]

igh-Q RF filters in a small

cost-effective form factor are a

key enabler for today’s mobile

electronic devices. For applications above

3GHz, traditional RF filter materials such as

piezoelectrics and ceramics fail to provide

the necessary performance metrics to enable

compact, low-power devices. Therefore,

new materials are being pursued to meet the

market demand for the production of smaller

high-performance integrated passive device

(IPD) RF filters including bandpass filters,

diplexers, and duplexers among others.

Due to the ever-increasing demand

for wireless data access and mobile

devices performance, the FCC has

recently designated three new frequency

bands for 5G applications: 4.9-5.8GHz,

27.5-29.5GHz, and 37-40GHz [1]. At

these frequencies, acceptable material

choices are reduced further, and by

default, impose stricter requirements

on manufacturing options to meet the

required performance.

Glass has been touted as a very good

substrate for RF applications including

5G frequencies by a number of authors,

including multiple articles from the Georgia

Tech Packaging Research Center [2-5],

which has done extensive research on glass

substrates for electronic applications. The

main attributes of using glass for an RF

substrate are: 1) better material properties

at RF frequencies; 2) decreased surface

roughness for fine line redistributions; and 3)

ability to manufacture in large formats (wafer

and panel) to meet industry cost targets.

Photosensitive glass-ceramic (PSG)

materials are a class of materials that

offers all of the benefits of glass but have

some additional beneficial attributes

that regular glasses do not offer. These

benefits include: 1) the ability to transfer

patterns directly to glass with a standard

photolithography step; 2) the ability to

create small, precise, features at high

densities; and 3) the opportunity to

integrate IPDs such as high-Q inductors

and capacitors into a single substrate.

H

Figure 1:

APEX

®

glass processing steps.