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Chip Scale Review July • August • 2017

[ChipScaleReview.com]

ionic contamination, which limits its

compatibility with CMOS processes.

Fo r i mp r o v e d s e l e c t i v i t y t o t h e

underlying materials, the use of a 2-step

etch sequence is implemented. The first

step is a fast etch using an HF/nitric

mixture, as described previously, to

tailor the profile [6]. This step typically

takes 1-2 minutes depending on the

thickness of the silicon. The amount

of silicon that is etched depends on

several factors. If the nonuniformity

of the silicon is radial in nature, the

initial etch process compensates for

the thickness variation by adjusting

the amount of time that the etchant

is dispensed on a pa r t icula r radial

portion of the substrate. This “dwell

time” can be controlled by adjusting

various process parameters such as the

tool arm scan speed, arm acceleration,

and the spin speed of the chuck upon

which the wafer is spinning during the

etching process.

Figure 4

illustrates

the hyperbolic motion prof ile of the

etch process that can be used to achieve

uniform dwell time over the entire

surface of the substrate, resulting in

greater uniformity post-silicon etch.

This allows the etch process to come

close r t o t he unde rly i ng mat e r ials

(t ypically to a silicon thickness of

2µm). If there is non-radial variation,

then the amount of silicon that needs

to remain for removal by the selective

etchant in Step 2 will depend upon the

total thickness variation (TTV). For

example, if there is a 3µm TTV across

the wafer, then 3+ µm worth of silicon

should be left after the first step for

the selective etch process. The etchant

is then changed to a selective etchant.

R e v e a l E t c h p r o v i d e s e x c e l l e n t

s e l e c t i v i t y t o e t ch s i l i c on i n t h e

presence of other materials [7].

Tabl e 1

shows d a t a on t he e t ch

r a t e s ob t a i n e d f o r s e ve r a l of t h e

materials that may be present during

the etch process. With the 2-step etch

s equence, t he f i n i sh i ng e t ch t ime

with the selective etchant can be kept

to a minimum. This is important, as

many of the selective etchants have a

slow etch rate, and bonding adhesives

u s e d i n t h e a d v a n c e d p a c k a g i n g

processes do not always hold up to

exposure to the chemistries involved

for long periods.

Anot he r fe at u r e of t he we t e t ch

system is the ability to stop the etch

when the silicon has been removed.

The single wafer wet etch approach

accomplishes t h is t h rough Veeco’s

WaferChek

®

process monitor, which

uses a charge coupled device (CCD)

camera and sof tware built i nto the

system to determine the end of the

process by monitoring color changes in

the substrate. As the wet etch system

removes the silicon, the ref lectivity

of light from the surface of the wafer

changes. A dip in the signal indicates

when the silicon has been completely

r emove d , a s s h ow n i n

F i g u r e 5

.

Depending on the materials that are

below the silicon and the selectivity

desired, a single etchant process may

a l s o b e u s e d whe r e t he HF/ n it r i c

mixture is used for the complete etch.

Examples of th is i nclude when the

underlying layer is a thick oxide or

pure polymer (PI or PBO).

A n e w e t c h f o r m u l a t i o n h a s

recently been developed by SACHEM,

called ST2011, which demonst rates

sig n i f icantly improved select iv it y

c omp a r e d t o Re v e a l E t c h a t t h e

expense of a lower silicon etch rate,

as shown i n

Table 1

. Determi n i ng

which selective etchant is appropriate

f o r a g i v e n a p p l i c a t i o n d e p e n d s

upon multiple factors, including the

ma t e r i a l s p r e s e nt i n t he pa ck age ,

the type of structures that are being

etched, and the extent to which the

bulk silicon layer is nonuniform, to

name just a few.

Figure 4:

Hyperbolic motion profile of the single-wafer etch process, which compensates for radial non-

uniformities resulting in more uniform silicon thickness.

Figure 5:

Endpoint detection.

Table 1:

Etch rates and selectivity of etch materials, including SACHEM Reveal Etch and new formulation, ST2011.