S10-2
Challenges of leading-edge technology in plasma processing
先端プラズマプロセス技術の課題と展望 1
Masaru Izawa, Naoki Yasui
伊澤 勝, 安井尚輝
Kasado Design & Product Div., Hitachi High-Technologies Corp.
794 Higashitoyoi, Kudamatu, Yamaguchi Pref. 744-0002, Japan（株）日立ハイテクノロジーズ・笠戸 〒744-0002 山口県下松市東豊井794
Dimensions of semiconductor devices shrink into nanometer range. In this area, more accurate surface
reaction control is required in plasma etching. Time modulation (TM) of discharge and gas injection wasinvestigated to improve etched profile. It was found that etched profile (vertical, selectivity, wiggling) wasimproved in critical etching process. Integrated process tool with PE-CVD was investigated for MRAM
patterning. It was found that deposition and oxidation were suppressed at the sidewall.

1. Introduction
As dimensions of semiconductor devices shrink
further into nanometer range, transistor
performance is improved not only by scaling andadopting boosting technology but also byadopting new channel material and structure. It isalso required to reduce power consumption inmobile devices. In logic devices, 3D-trasistor(FinFET) was adopted to improve switchingproperty. Also, stacked Flash memory cells(3D-NAND) and non-volatile memory (e.g.
MRAM) are investigated to replace a portion ofcurrent memory devices. When fabricating thesedevices, more accurate and higher aspect ratio
patterning is required. For example, in logic
device fabrication, fin width reaches 3.7 nm at the
5nm node, where nanowire FET and Ge/III-V
channel materials are other candidates [1]. Thus,
required accuracy will reach atomic level in theetching process. To realize nm scale etching, newetching process functions are necessary to
suppress wiggling, roughening, and degradationof etched layer as well as improve selectivity. At
the same time, reasonable productivity(through-put, stability, repeatability) is required
for mass production. In the 90’s somesophisticated functions were already investigatedsuch as pulsing control, atomic layer control,etc,[2-8] however were not adopted at that time.
As devices shrink these functions seems tobecome useful.
In addition, etching of new material is requiredin emerging devices. For MRAM fabrication, notonly reduction of deposition at sidewall of pattern
in etching, but also encapsulation under vacuumis required to reduce degradation at the interface.
To meet these requirements, plasma etch tool
was improved continuously. In particular, timemodulation (pulsing of RF bias, discharge, andgas) technologies and power control of wafer RF
bias seems to be key technologies for accurate
profile control as well as a selection of gas
chemistry.
In this paper, we investigated time modulation(pulsing) technologies in directed self-assembling(DSA) mask etching and fin etching. Also thispaper showed integrated process including
etching and PE-CVD for MRAM patterning.
2. Time Modulation (TM) Technology
To improve trade-off between selectivity andetch profile, including iso-dense differences, TM(pulsing) technologies were investigated. TM bias
(wafer RF bias pulsing) technology has alreadybeen used in semiconductor fabrication [7]. In nm
scale etching, pulsed discharge and gas pulsing
technologies [2] are candidates to improve etched
profile.
Fig. 1 shows schematic view of microwave
ECR etching apparatus and pulsing technologies(DTM: Dual Time-Modulation) [9]. Here, ECRMicrowave（2.45GHz）

Pulsed Microwave

ONPlasma
Wafer

OFFTM(pulsing)-BiasWafer
wafer
RF biasONOFF

Fig. 1. Schematic view of microwave ECRetching apparatus and DTM .

(a)

(b)

(c)

100nm

Fig. 2 Top view image after DSA mask etching.
(a)

(b)

Fig. 4 Cross-sectional TEM image of MRAM etching.

Fig. 3. Cross sectional SEM image after fin etching.

developed with an integrated PE-CVD toencapsulate the etched profile under vacuum.
Fig. 4 shows cross-sectional TEM image ofetching and encapsulation. Here, MTJ stackcontained Ta electrode at the top and bottom. Taperangle was about 80 deg. Deposition and oxidation
were not found at the sidewall due to suitable gas
chemistry.

plasma was designed to have good affinity topulsing discharge. By using the DTM, wiggling
reduction was investigated in DSA mask etching.
As shown in Fig. 2 (a), wiggling occurred in acontinuous wave (CW) plasma. On the contrary,
wiggling was improved dramatically by using DTM
technology (b). Here, O2 gas flow was adjusted tosimilar etching rate as (a). Finally, it was found thatwiggling was eliminated without decreasing of etchrate (c). By using DTM, ion incident flux andactivated O radical flux are suppressed even at low
gas pressure condition. We considered wiggling
was improved because the stress of the polystyrene
surface did not increase.
Gas pulsing technology was investigated for fin
etching [10]. Etching progress and sidewall
protection steps were changed cyclically without
interruption of the discharge. As shown in Fig. 3,
etched profile using gas pulsing (b) is more verticalthan that of continuous mixed gas injection (a). Inaddition, thickness of remained hard mask and etchfront flatness were also improved.

4. Summary
Plasma etching technology has been widely usedin VLSI fabrication. This technology has been
innovated by adopting new functions althoughsome functions were proposed in 90’s. In this paper,
TM (pulsing) technologies (pulsed discharge andgas pulsing) were usable functions for nm scale
device fabrication. For MRAM etching, a clean
sidewall was achieved by using a specialized etch
tool with under-vacuum encapsulation.
Atomic level precision etching is required infuture device fabrication where etched material,
etch stop layer, and structure become more
complicated. Accurate surface reaction, reductionof stress and damage (both of physical andchemical) with reasonable through put is required
continuously in dry etching process. In addition,
wet cleaning less after etching may be required asshown in MRAM patterning because removal ofsacrifice layer becomes not acceptable ordifficult.

3. Etching Technology for Non-volatile material
EMCP (Electro-Magnetically Coupled Plasma)etch tool was designed to enable long-term stability
for non-volatile materials etching [11]. MRAM
etching was investigated by using EMCP because
typical MTJ etch by-products of Fe and Co are
non-volatile.
In MTJ-MRAM etching, sidewall control isrequired to avoid chemical degradation of magnetic
properties and to remove conductive deposition
which causes electrical short. In addition, the MTJ
seems to be degraded by air-exposure after etching.
To overcome these issues the process tool wasReferences[1] ITRS 2013.
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