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17

Chip Scale Review March • April • 2019

[ChipScaleReview.com]

Understanding the impact of silicone contamination on

semiconductor package performance

By Jaimal Williamson

[Texas Instruments]

he de t r i me n t a l ef fe c t of

polysiloxanes, or silicones,

and t hei r cont r ibut ion to

delami nation and cont ami nation at

various material interfaces dates back

decades. Their impact is as ubiquitous in

microelectronics as the widespread use of

copper interconnects. For example, it is

reported in past literature [1] that as little

as 10 parts per million (ppm) of silicone

adsorbing from the atmosphere on an

electrical contact was sufficient to degrade

contact performance. As described in the

cited literature, the catastrophic effects of

silicone contamination are notorious and

well documented across multiple industry

sectors and applications [2,3].

This article examines atomic mass unit

distribution of a wide range of silicone

oligomeric compounds (based on a standard

silicone encapsulant packaging material),

including a series of analytical techniques

t o unde r st and mat e r ial outga ssi ng

behavior during cure. Specific analytical

tools like thermogravimetric analysis/

Fourier transform infrared spectroscopy

(TGA-FTIR), direct insertion probe –

mass spectrometry (DIP-MS), time-of-

flight secondary ion mass spectroscopy

(ToF-SIMS), and X-ray photoelectron

spectroscopy (XPS) were used in tandem as

complementary techniques to study silicone

outgassing behavior and contamination

adsorbed on semiconductor-based copper

(Cu) lead frame package and silicon die

surfaces. Cure profiles were compared and

simulated using TGA-FTIR and DIP-MS

to determine qualitative and quantitative

amounts of silicone cont ami nat ion

outgassing. Surface-sensitive ToF-SIMS

and XPS techniques were used to analyze

the adsorbed silicone contamination on the

aforementioned chip and package surfaces

post cure.

To elucidate chemical information of the

silicone contaminant, DIP-MS data was

overlapped between mass-to-charge ratio

(m/z) and time (on the x-axis) to understand

the mass unit (degree of polymerization)

and the specific time the silicone fragment

outgassed as a function of cure temperature.

Chemist r ie s of si l icone f r agment s

generated from DIP-MS were compared

qualitatively to chemical structures of

silicone species found on the die surface

from the ToF-SIMS analysis. XPS was used

to compare atomic concentration of silicone

contaminant adsorbed on the Cu lead

frame surface based on the binding energy

associated with siloxanes and silica as a

function of the cure condition. The synergy

obtained from the use of multi-analytical

instrumental techniques was essential

to determine the effect of cure temperature

and time on silicon contamination. Results

directed a path for the optimization of

the semiconductor package assembly

process flow.

Silicones are a universal choice for

original equipment manufacturers (OEMs)

and outsourced semiconductor assembly

and test (OSAT) sites as materials to support

die attach, glob top, lid attach, and general

encapsulation for an array of semiconductor

package applications. Because silicone

chemistry is impervious to moisture and

thermally stable across a wide temperature

range, it is an attractive material for a myriad

of microelectronic applications. However,

formulating a silicone material with ideal

physical and chemical properties, such as

a high degree of chemical inertness and

resistance to extreme temperatures, leads to

consequences in a semiconductor package

assembly environment. For example,

enabling material versatility in silicone

formulations can be made polydisperse,

where many short chain oligomers of

varying mass unit can outgas at different

periods during the cure profile. In addition,

solvents added for rheological control can

exacerbate the outgassing phenomenon

of silicones contaminating onto adjacent

surfaces and components.

For these reasons it is important to fully

characterize the impact of cure conditions

in terms of the ramp time, terminal

temperatures, and dwell times in relation to

the outgassing effect. This article focuses

on using TGA-FTIR, DIP-MS, ToF-SIMS

and XPS to study the silicone oligomeric

fragments outgassing during cure to

enable a path to mitigate the magnitude

of adsorbed silicone contaminants on

adjacent silicon and the lead frame surface

that impact adhesion. The information

extracted is used to improve chip to

package integrity (i.e., chip and lead frame

adhesion to mold compound) and optimize

package assembly process flow.

T

Figure 1:

TGA thermogram plot showing the weight of a silicone material from a generic cure profile up to 150°C.