# Etch Defect Characterization and Reduction in  Hard-Mask-Based Al Interconnect Etching

Authors: Hong-Ji Lee; Che-Lun Hung; Chia-Hao Leng; Nan-Tzu Lian; Ling-Wu Young; Tahone Yang; Kuang-Chao Chen; Chih-Yuan Lu
Journal: International Journal of Plasma Science and Engineering
 (2008)

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## Abstract

This paper identifies the defect adders, for example, post  hard-mask etch residue, post metal etch residue, and blocked etch metal island and investigates the removal characteristics of these defects within the oxide-masked Al etching process sequence. Post hard-mask etch residue containing C atom is related to the hardening of photoresist after the conventional post-RIE ashing at275∘C. An in situ O2-based plasma ashing on RIE etcher was developed to prevent the photoresist hardening from the high-ashing temperature; followed wet stripping could successfully eliminate such hardened polymeric residue. Post metal etch residue was caused from the attack of the Al sidewall by Cl atoms, and too much CHF3 addition in the Al main etch step passivated the surface of Al resulting in poor capability to remove the Al-containing residue. The lower addition of CHF3 in the Al main etch step would benefit  from the residue removal. One possibility of blocked etch metal island creating was due to the micromasking formed on the opening of TiN during the hard-mask patterning. We report that an additional TiN surface pretreatment with the Ar/CHF3/N2 plasmas could reduce the impact of the micromasking residues on blocked metal
etch.


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## Body

## 1. Introduction

As the dimension of devices continues to scale
down, the Al patterning by dry etching becomes challenging due to insufficient
amount of photoresist to leave for etch rate fluctuation [1]. A damascene
process is the highly potential approach to avoid the metal dry etching
problems [2]. However, it is known that switching Al to W or Cu involves
additional investments, both in terms of equipments and process development
efforts [3]. Since Gabriel et al. [4] and Stojakovic  and Ning [5] adopted hard
mask to pattern Al in the dry etching process, it is worth exploring Al dry etching
with hard mask in our 75 nm nonvolatile memory technology and beyond. With developing the TEOS hard-mask-based Al
patterning in our study, the etch defectivity is regarded as leading edge
challenge in the manufacturing line. As illustrated in Figure 1, the defect
types, referred to as “post hard-mask etch residue,” “post metal etch residue,”
“blocked etch metal island,” “particle,” “bridging,” “particulate bridging,”
and “corrosion,” were found at the early stage of development. Interestingly,
the major adders of post hard-mask etch residue, post metal etch residue, and
blocked etch metal island were particularly high in the oxide-masked Al
patterning but few in the conventional resist-masked Al patterning. From the
observations of scanning electron micrographs as shown in Figure 1, these three
defect types pose a tangible and substantial yield risk due to their subtle
physical characteristics and high density on wafer. Hence, it is important to
understand the behavior of these defect adders within the Al etching process
sequence.Examples of defect adders within
the oxide-masked Al etching process sequence.
(a)(b)(c)(d)(e)(f)(g)This paper describes the identification of the
foregoing defect adders. Among these defect adders, post hard-mask etch residue
is commonly observed after the oxide hard-mask patterning followed by resist
ashing and wet stripping. Such defect formation is found strongly related to
the ashing process temperature. Post metal etch residue is found to be
concentrated near the semi-iso metal sidewall damage and does not appear to be
randomly distributed; the defect is demonstrated to be the attack of the metal
sidewall by Cl atoms. One possibility of blocked etch metal island creating is
the fine micromasking formed on the opening during the hard-mask patterning;
the micromasking would cause blocked metal etch resulting in metal island
during the Al etching. The reduction of these defect adders optimizingO2 plasma ashing sequence, moderating the addition of CHF3
gas in the Al etching, and using additional TiN surface pretreatment prior to
the TiN/Ti breakthrough step was reported.


## 2. Experiment

A blanket metal stack structure used in this study was
deposited as follows (from bottom): PEOX/Ti/TiN/Al-0.5% Cu/Ti/TiN/TEOS. Then, an
organic antireflective film was coated on top of TEOS. The line patterns
printed by using an ASML PAS-5500/700C DUV scanner with 420 nm resist. The hard-mask
etching was carried out on an AMATMxP+ capacitively-coupled plasma
dielectric etch system (see Figure 2(a)) with CF4/CHF3/Ar/N2 chemistry
followed by oxygen plasma ashing (SA-2000, PSC) at 275C° for 20
seconds and EKC265 wet stripping. EKC265 (EKC Technology Inc.) is a semiaqueous organic mixture formulated to remove
resist residue generated after etch process. After the oxide hard mask was patterned, the remaining metal
stack was etched on a Lam Research Alliance 9600PTX low-pressure, high-density,
inductively coupled-plasma metal etch system (see Figure 2(b)). The starting
metal etch baseline consisted of three substeps, for example, the TiN/Ti breakthrough
(BT) step with Cl2/BCl3/CHF3/N2 gas ratio
(10/2/3/2), the Al main etch (ME) step with Cl2/BCl3/CHF3/N2 gas ratio (12/10/3/2), and the over-etch (OE) step with
Cl2/BCl3/CHF3/N2 gas ratio
(12/14/3/2). The defect adders on wafer were inspected with a KLA-Tencor 2351
pattern inspection tool and a JW-7555S scanning electron microscopy (SEM). The
chemical composition of the defect adder was analyzed using energy
dispersive X-ray spectroscopy (EDS).Schematic
diagrams of etchers used in this study.
(a)
Magnetically enhanced reactive ion etcher (MERIE)(b)
ICP etcher


## 3. Results and Discussion

## 3.1. Characterization and Reduction of Post Hard-Mask Etch residue

A typical  TEOS dielectric as hard mask was etched onMxP+ with the following conditions: pressure (200 mt); power (600 W)CF4/CHF3/Ar/N2 gas ratio (4/2/15/2), and 40% OE after the TEOS endpoint was
detected. Figure 3 shows the SEM micrograph of TEOS that
determines the sequent Al interconnect dimension, interestingly, keeping the backbone residues on top of the hard-mask structure. The
residue was identified mainly containing C atom from the observation of its EDS
spectrum (see Figure 3(c)). The data support the contention that the residue is
related to the hardening of photoresist during the oxygen plasma ashing
process; followed wet stripping is not effective in removing such hardened
polymeric residue. According to the report of Gillespie et al. [6], it is
helpful to reduce the hardened photoresist by lowering the ashing temperature.
In this respect, we proposed an in situ O2-based plasma ashing process
on MxP+ etcher (cathode temperature: 15C°) to prevent the
photoresist hardening from the conventional post-RIE ashing at 275C°.
As seen in Figure 4, post hard-mask etch residue on top of TEOS could be
eliminated by optimizing in situ O2-based plasma ashing with rf
power of 150 W for 12 seconds followed by EKC265 wet stripping.(a), (b) SEM
micrographs of TEOS after etching. The backbone residues were found on top of
TEOS, containing C atom identified by its EDS spectrum (c).
(a)(b)(c)(a), (b) SEM
micrographs of TEOS after etching with in situO2-based plasma ashing followed
by EKC265 wet stripping.
(a)(b)


## 3.2. Characterization and Reduction of Post Metal Etch Residue

After the TEOS hard mask was patterned, the remaining metal
stack was etched on Alliance
9600PTX. Figure5(a) shows the cross-sectional SEM
micrograph of Al interconnect etched using high CHF3
(15 sccm) flow rate during the ME step in the starting etch baseline; the
Al profile was tapered and notch-free. Subsequently, the defect inspection in
die-to-die mode revealed the defect adder density around 3.98 defects/cm2 in average, in which the major defect type is post metal etch residue (78% of
total defect adders). Upon SEM review, the residues
are found to be concentrated near the semi-iso Al-damaged sidewall and do not
appear to be randomly distributed. From the analysis of EDS spectrum (see Figure 6) attained
from a representative residue adder, the defect contains Al, F, Cu, Si,
and Cl atoms. The data indicate that the residues are related to the attack of
the Al (0.5% Cu) sidewall by Cl atoms, and too much CHF3 addition
in the ME step could passivate the surface of Al resulting in poor capability
to remove the Al-containing residues. As expected in Figure 5(b), the extent of
Al residue was decreased with decreasing CHF3 additive gas (6 sccm) in
the ME step; the ratio of Al residue to total defect adders was suppressed from
78% to 2.7%. The reduction of the CHF3 flow rate in the ME step
can effectively improve the post metal etch residue issue, but it will also
result in less passivant on the metal sidewall to protect the Al bottom corner
from the attack of Cl atoms. As shown in Figure 5(b), a slight bottom notching
was observed on the Al corner. Therefore, there is a tradeoff between Al
notching elimination and residue removal.Figure 5
(a) SEM micrograph of Al
profile etched using 15 sccm ofCHF3 during the ME step. The major
defect type is post metal etch residue (78% of total defect adders). (b) The
ratio of post metal etch residue to total defect adders was decreased to 2.7%
at the lower CHF3 flow of 6 sccm; however, there is
a slight Al bottom notching observed from physical profile. Defect density (Do)
was expressed as means ± standard deviation (σ) in counts/cm2. Each
etch condition was tested on three wafers.Figure 6
EDS for analyzing post metal etch residue. The
residues contain Al, F, Cu, Si, and Cl atoms (solid line). The dashed line
indicates TEOS background.


## 3.3. Characterization and Reduction of Blocked Etch Metal Island

Blocked etch metal island is identified as a tangible yield risk in the oxide hard-mask-based
Al etch manufacturing. SEM micrograph of metal island defect (see Figure1(c))
revealed that it is a block of etched metal stack causing metal short between
metal lines. The defect level on the wafers in Figure 7 showed 15 metal island
adders in average and 21 metal island adders at maximum by implementing the starting
metal etch baseline. There are several possibilities causing blocked metal etch
such as the micromasking residue (e.g.,  TixOyCz) formed on
the surface of TiN during the TEOS hard-mask etching with carbon fluoride chemistry [7] or the particulate flaking on the wafer from
the etch chamber wall [8]. The possibility of blocked metal etch from the
micromasking was investigated to lessen by adding a TiN surface pretreatment
with Ar/CHF3/N2 gas ratio (12/2/1) prior to the TiN/Ti BT step on
metal etcher. As shown in Figure 7, the reduction of metal island adder was observed
using the etch recipe with additional TiN surface pretreatment, inspecting six
metal island adders in average and 10 metal island adders at maximum. The
results reveal that the additional TiN surface pretreatment prior to the TiN/Ti
BT step on metal etcher can reduce the impact of the micromasking on blocked
metal etch significantly. Note that the particulate flaking
from etcher wall was minimized by performing a preventive maintenance clean on
the etch chamber in this study.Figure 7
Trend of metal island defects versus metal etch
recipe. The open circle symbol (∘) presents metal etch using the conventional
recipe without TiN surface pretreatment; the solid triangle symbol (▴) presents metal etch with TiN surface pretreatment
prior to the TiN/Ti BT step.


## 4. Conclusions

The etch defect investigations in this work are
summarized as follows.(I)
Post hard-mask etch
residue containing C atom is found strongly related to the hardening
of photoresist after a275C° ashing process. An in  situ O2-based
plasma ashing on RIE etcher  is developed to prevent the photoresist  hardening
from the high-ashing temperature. The result reveals that such hardened
polymeric residue on top of TEOS can be
eliminated by optimizing in situ O2-based plasma ashing with rf
power of 150 W for 12 seconds followed by EKC265 wet stripping.(II)
Post metal etch  residue is
demonstrated to compose of Al, F, Cu, Si, and Cl atoms. The data indicate that
the residue is  related to the attack of the Al (0.5% Cu) sidewall by  Cl atoms,
and too muchCHF3 addition in the Al ME step can passivate the surface
of Al resulting in poor capability to remove the Al-containing residue. To
lower the CHF3 additive gas from 15 sccm to 6 sccm in the Al ME step
will greatly improve the residue remaining issue.(III)
One possibility of
blocked etch metal island creating is the fine micromasking formed on the
opening of TiN during the hard-mask
patterning. A TiN surface pretreatment with theAr/CHF3/N2 plasmas can
be implemented prior to the TiN/Ti BT step to reduce the impact of the
micromasking on blocked metal etch.



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*Source: 154035-2008-09-23.xml*