High throughput Copper Removal of Frontside Circuit Edit
H. Tanaka(aMichael Wong(bDavid Pan(bS. Kunihiro(cM. Sato(ca)FEI Company Japan Ltd, Shinagawa-Ku, Tokyob)
Thermo Fisher Scientific, Fremont, CA, USAc)
DENKEN Co, Ltd, Kitsuki-shi, Oita

Abstract: Circuit Edit (CE) is widely used for debug, characterization and prototyping in the IC industry. With the adoptionof stacked dies in modern IC and unique packaging, backside CE is becoming more common approach for CE [1],[2]. The
modern CE requires precise and gentle copper removal without exposing underlayer in the critical steps. The primary way toachieve the goal is the use of unique CE chemistry. The secondary option is to optimize recipe and utilize special beam scanningtechnique with bitmapping and exclusion mill, etc. On the other hand, frontside CE is still common in the IC industry. Itsapplication and requirement can be very different from backside edit. One of them is to be able to quickly remove RDL and/or
thick copper layer (~3um or thicker) because sometimes this step could take more than 50% of operation time of the whole CEjob. In this study, we present high throughput copper removal technique with water chemistry and low-keV beam (5kV).
Keywords: Circuit edit, Copper removal, Low-KV, Frontside CE, Water assisted etch.

1. Introduction
Circuit Edit (CE) is widely used for IC debug,
characterization and prototyping. Althoughbackside CE became more common technique,frontside CE is still needed because of several
reasons: 1) experience and knowledge required on
both of sample preparation and backside CE, 2)
system capability for backside CE, and 3) facility
to validate IC after completing backside CE.
Frontside CE application and requirements are very
different from backside. One of them is to be able
to quickly remove RDL and/or thick copper layer(~3um or thicker) included dummy layers because
sometimes this step could take more than 50% of
operation time of the whole CE job. It is a criticalissue of time management in the FA lab when theFIB system time is heavily spent on thick copper
removal. In this study, we present high throughput
copper removal technique with water chemistry andLow-KV beam (5kV). The technique helps
improve productivity on frontside CE.

2. Challenges of Copper removal
Since the adaption of Copper metallization inIC, Copper removal has always been challenging
due to several reasons [4]. 1) Grain structure. Thesegrains etch at different rates depending on grainorientation relative to ion beam. 2) No accelerated
removal of Copper like Halogen etching forAluminum. Copper etch solutions were to protect

the Inter Layers Dielectric (ILD) and therefore
etching process is slower. 3) Oxidizing Copper
makes it non-conductive. 4) Oxidizing SiO2 makes
it more sputter resistant. 5) Chemistry application.
In the report, we will demonstrate oxidizing
both Copper and SiO2 with water assisted etching
and low beam energy to reduce etching rate on SiO2
layer this time.

3. Proposed technique for thick copper
removal
3.1.

Low Beam Energy for reduced SiO2
damageGallium ion milling using low beam energy is
a common technique on TEM sample preparation.
It has not been common in CE applications because
the CE jobs demand the highest imaging resolution
and milling resolution. Therefore, CE FIB system isgenerally optimized with higher beam energy at30kV. Recently the low beam energy technique isadapted on backside CE workflow for advanced
technology nodes. It decreased Gallium penetration
into FinFET layers allowing the device still be
intact after fully exposes active region. So, the low
beam energy technique is expected to reduce
damage on SiO2 layer as well. We performed
1x1m milling into SiO2 layer with Dose2.0[nC/m2] at different beam energies (30, 16, 8and 5kV) and estimated SiO2 etching rate vsdifferent beam energies with cross section view.

Contact author: hideo.tanaka@thermofisher.com
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The lower beam energy decreases etching rate asshown in Table 1 and Figure 1.

Table.2: Beam energy vs Milled depth per Dosewith water assisted etch.
Beam energy
[kV]
30
1685

Table.1: Beam energy vs Milled depth per Dose
without chemistry assistance.
Beam energy[kV]
30
1685Milled
depth[um]/Dose[nC/m2]
0.26
0.22
0.170.15

Milleddepth[um]/Dose[nC/m2]
0.08
0.060.040.03

Figure.2: Cross section view of etched SiO2 layer
with water assisted etch. From left to right, thebeam energies are 30, 16, 8 and 5kV.

3.2.

Water assisted etch
The Copper etching solution has beendeveloped along with the advanced technology
nodes since adaption of copper metallization.
Today, the chemistry application like CU2 and DX
delayering are more suitable for Copper etchingover Low-K dielectric layer than water assisted etch.
Water is oxidizer during copper removal and isdifficult to work on low-K dielectric layer.
However, it is still an attractive solution for thickcopper and/or RDL removal in frontside CE. Water
is an attractive solution as a precursor on copper
etching process for several reasons: 1) promoting
copper surface oxidation; 2) reducing thechanneling effect; 3) nontoxic and 4) easy to deliver
into the vacuum.
We tested 1x1m milling into SiO2 layer with
Dose 2.0[nC/m2] at different beam energies (30,16, 8 and 5kV) with water assistance and checkedcross section to see beam energy influence on SIO2
etching rate. As expected, the lower beam energy
with water clearly decreases etching rate as shownin Table 2 and Figure 2.

Here is summary of Low beam energy
with/without water etching on SiO2. Significantly
water assisted etch decreases the etching rate on
SiO2 layer as shown Figure.3.
0.6

Mill depth [um] per Dose

Figure.1: Cross section view of etched SiO2 withoutchemistry application. From left to right, the beamenergies are 30, 16, 8 and 5kV.

0.5
0.4
Without Water
With Water

0.3
0.2
0.1
0010203040
Beam Energy [kV]Figure.3: Comparison of SiO2 etching rate
with/without Water.
3.3.

Water assisted etch with high beam
current density
Beam current density is also a key to acceleratecopper removal and enhance productivity on CE. In
other way, it might impact SiO2 etching, even withthe same dose[nC/m2].

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To study beam current effect, we performed
10x10m milling into SiO2 layer with Dose2.0[nC/m2] at different beam current densities(~10, ~25 and ~90[pA/m2]). The SiO2 etch ratevs beam current density are as shown in Table 3 andFigure 4 & 5.
Table.3: Beam current density vs Milled
depth/Dose with water assisted etch.
b/c density
Milled[pA/m2]
depth[m]/Dose[nC/m2]
93
0.32
24
0.27
9.1
0.26

CE. The thickness of copper is 900nm. Navigate
over the target location. Typically, high beam
current is used to work for larger area and toaccelerate removal process. Once on destination,
tilt stage 45 degree to prevent discharge over thetarget then start to etch dielectric and expose
copper layer with XeF2. Exposed Copper layer
with 30x21.2m is shown in Figure 6. Bring tilt
stage back to 0 degree. Here we obtainapproximately 30x30m trench floor as shown inFigure 7.

Mill depth [um] per Dose

0.35

0.30
Figure.6: Copper layer exposure with angle 45
degree. Successfully reach thick copper layer.
0.25
02040
60
80100
Beam current density [pA/um2]
Figure.4; Beam current density vs Mill depth [m]per Dose.

Figure. 7: Obtain 30x30m trench floor.

Figure.5: Cross section view of 10x10m trenchfloor after performing water assisted etching withhigh beam current density (~90[pA/m2]).

4. High throughput thick copper removal
in Frontside CE
4.1.

Thick copper exposure

4.2.
Over etching for ILD
The trench floor is a mixed field with copperand ILD as shown 8. As illustrated in the previous
section, Water and Low-KV etching wouldn’t etchthe ILD much during thick copper removal. Needto perform over etching for ILD to maintainplanarity on the trench floor after completed thickcopper removal. This is the key to CE success rate.
Perform the over etching process based on dose ortime on the copper exposure process.

The following illustrates workflow for thickcopper removal with 30x30m area in frontside

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Figure.10: Etch through copper and expose ILD
layer partially. Here is endpoint on thick copper
removal at beam energy 30kV.
4.4.

Figure.8: Complete over etching process. Readyfor thick copper removal.
4.3.
Thick copper removal at 30kV
Increase beam current at 47nA to acceleratecopper removal as shown Figure 9.

Thick copper removal with Low-kV

Continue Copper removal with H2O after
reduced beam energy 5kV and beam current at13nA as shown in Figure 11. The endpoint is theblack and white contrast on live milling image.
Complete copper removal without underlying
exposure as shown Figure.12. The low beamenergy at 5kV process takes for 8mins. The total
removal time over 900nm thickness isapproximately 10mins.

Figure.11: Perform copper etching with water andlow-KV.
Figure.9: Switch higher beam current at beamenergy 30kV for thick copper removal.
Perform thick copper removal process withhigh beam current at beam energy 30kV. Onceetched through copper layer and expose ILD(SiO2) partially as shown Figure 10, here is theendpoint and then reduce beam energy to 5kV. It
takes about 1min.

Figure.12: Completion of Copper removal without
underlying exposure.
Obtain the tilt image on the trench floor to
make sure underlayer intact. This is shown inFigure 13.

The 41th NANO Testing Symposium (NANOTS2021), 25–27 Oct. 2021c⃝The
Institute of NANO Testing73be done within 90mins. Without low kV &Water assisted etch, the RDL removal over
40x20m area will take approximately 2-3
hours and the whole edit will take 3-4 hours.
The technique enhances more than 60%
productivity.

5. Conclusions
Figure.13: Trench floor validation with tilt image.
Expose underlying on central trench floor with
15x15m. Expose the copper layers as shown
Figure 14. The result demonstrates frontside CE
capability.

This work shows utilization of low beam energy(5kV) and Water (oxidizer) assisted etch on frontside
CE. It allows to decrease etching rate on SiO2 layerand also worked underneath thick copper and/or RDL
layer. Therefore, the technique is able to use higher
beam current than general CE process during thickcopper and/or RDL removal process. This work also
demonstrates the improvement of frontside CE
productivity. The removal process is accelerated aby≥50%.

Acknowledge
The authors would like to thank Miyuki Sato ofDENKEN corporation, Ltd. for the help in
validating the thick copper removal technique onreal cases repeatedly.

References
Figure.14: Underlying exposure on central trench
floor with 15x15m after completed thick copperremoval.
We validate the technique on two practical
frontside edit as shown in the following.
(1) Cut Metal 3 layers on 9 stacked layers
underneath ~3m thick copper dummy layers.
With low kV & Water assisted etch, the thickcopper dummy removal took approximately
15mins. Whole CE job was completed
approximately 3-4hours. Without low kV &Water assisted etch, the RDL removal over
40x20m area will take approximately 2-3hours and the whole CE job will take 7-8 hours.
The technique improves approximately 50%
productivity .
(2) Tie Metal 2 to Metal 2 and cut Metal 2 on 3
stacked layers underneath ~6m thick RDL.
With low kV & Water assisted etch, the RDL
removalover40x20m
area
took
approximately 20mins. The whole CE job can[1] Michael Wong, Oleg Sidorov, Or Hamison, et al.
ISTFA2020: Influence of Dose Delivery of Low-Beam
Energy Gallium on FinFET Device[2] Hideo Tanaka, Chun-Cheng Tsao. ESREF2020:
Reliable Optical endpoint technique on Si trenching forbackside circuit edit
[3] Chad Rue, Randall Shepherd, Roy Hallstein & RickLivengood, 2007 ASM International: Low KeV FIB
Applications for Circuit Edit.
[4] TR Lundquist, T Malik, R Jain, D Renard, VV Makarov.
European FIB User group 2007: Issues to be address for
improved Circuit Edit Productivity.

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