a batch atomic layer deposition approach for advanced DRAM dielectrics
Owyang and Larry Bartholomew,
drive toward sub-100-nm feature sizes places extreme performance demands
on semiconductor manufacturing processes and equipment. Chemistry and
hardware challenges are being addressed through the development and
implementation of novel processing techniques such as atomic layer deposition
(ALD) and the incorporation of exotic materials using designer chemicals
such as high-k dielectrics for films. The stringent control of film
properties drives the development of advanced chemical-delivery techniques,
control hardware, and the choice of materials.
fabricate memory cell capacitors in advanced DRAM applications, it is
critical to develop materials with a higher dielectric constant than
that of Al2O3. At the 45-nm
node, materials must be able to provide 100% conformal step coverage
into deep-trench capacitors with
aspect ratios approaching 80:1. In addition, they must have high thermal
stability and very low leakage. Finally, they must offer reasonably
high throughput to achieve cost-effective manufacturing.
study presented in this article focuses on efforts by Aviza Technology
(Scotts Valley, CA) and a global device manufacturer to use a batch
ALD tool to deposit high-dielectric-constant HfxSi1–
xO2 films. Research at Aviza using
a single-wafer platform has shown that HfxSi1–xO2
can meet the technical requirements of advanced DRAM applications. However,
depositing the films using a single-wafer tool is not cost-effective
because to minimize leakage, the films must be processed using a long
recipe time to achieve step coverage and compositional control. To overcome
this limitation, Aviza transferred its proprietary coinjected HfxSi1–
xO2 process from the single-wafer
platform to the Verano 5000 batch ALD system, improving throughput and
I. Materials for DRAM capacitor dielectrics and their dielectric
listed in Table I, several potential candidate materials can be used
to manufacture DRAM capacitor films with higher dielectric constants
than Al2O3. Of these films, HfxSi1
– xO2 is attractive because of its high thermal
stability and chemical compatibility with silicon and polysilicon.1
Although the dielectric constant of HfxSi1–xO2
at a 50% Hf:(Hf + Si) ratio is not much higher than that of Al2O3,
the films can be made silicon- or hafnium-rich by changing the relative
ratio of the hafnium and silicon precursors. Thus, their dielectric
constant and thermal stability can be modified.
shown in Figure 1, by increasing the hafnium content of Hf:(Hf + Si)
films, a higher dielectric constant approaching that of pure HfO2
can be obtained. By increasing their silicon content, the films' dielectric
constant decreases and they become more like SiO2.2
With a higher silicon content, the films' thermal stability improves.
1: Estimated dielectric constant versus the concentration of hafnium
HfxSi1–xO2 films offer potential
advantages for improving device electrical performance, they are being
evaluated for production applications in advanced device technology
nodes below 90 nm. Hence, hafnium silicate compounds may replace Al2O3
in next-generation DRAM dielectrics.
versus Batch ALD Processing
of its low residual carbon content and high deposition rate, tetrakis-ethyl-methyl-amino
hafnium (TEMAHf) is the preferred hafnium precursor for ALD hafnium
silicate films. However, these hafnium liquid precursors have lower
vapor pressure than trimethyl aluminum (TMA), the typical Al2O3
precursor.3 Because of their low vapor pressure, hafnium
precursors require longer chemical pulse times in the process chamber
to achieve sufficient chemical concentration and saturate the wafer
surface. If surface saturation is not achieved, the thickness uniformity
of the film is degraded, as illustrated in Figure 2.
2: Pulse times required to achieve depletion and saturation on monitor
and patterned wafers processed on an ALD system dispensing TMA precursor.
An extra chemical dose is required to saturate a patterned wafer
surface with substantial topography.
single-wafer tools, if the chemical-delivery system cannot compensate
for the longer pulse times by injecting a higher-concentration dose
of a precursor, the need to achieve film saturation and uniformity results
in longer ALD process cycle and total recipe times, drastically reducing
wafer throughput. Figure 2 shows the TMA pulse times required to achieve
different amounts of surface saturation on two different types of wafers.
The pulse time shown for flat monitor wafers also applies to patterned
wafers with low-aspect-ratio structures, such as transistor gates. For
DRAM deep-trench structures, a longer pulse time is required for the
chemical precursor to reach the bottoms of deep trenches and achieve
complete surface saturation. As indicated in Figure 2, it takes substantially
more precursor pulse time to achieve either partial depletion or full
conformal deposition to the bottoms of the trenches on patterned wafers
than either partial depletion or full saturation on monitor wafers.
this case, excess saturation of a flat monitor wafer does not increase
the deposited film thickness, since in true ALD processes, proper purging
before the oxidizer pulse removes excess precursor, yielding no more
than a monolayer of film with each process cycle. By the same token,
if the chemical-delivery system deposits too much precursor, the increased
time required to purge it decreases throughput. Furthermore, the use
of very-high-concentration doses of precursor, especially those with
high sticking coefficients, causes more than a single-monolayer ALD
reaction on the wafer surface.
longer residence times to enable the precursor to reach the bottoms
of the trenches at concentrations that do not overdose the top surface,
it is impossible to achieve 100% step coverage. Consequently, using
low-vapor-pressure hafnium precursors to meet the step-coverage requirements
for DRAM deep-trench structures with aspect ratios of >50:1 requires
pulse times lasting several seconds, not just milliseconds as in the
case of TMA. For single-wafer ALD systems, such long pulse times are
impractical for manufacturing. At a typical 7-nm film thickness, throughputs
for planar low-aspect-ratio patterned wafers are in the double-digit
range while throughputs for DRAM wafers with deep trenches having high
aspect ratios are in the low-single-digit range.
ALD systems can afford longer pulse times than single-wafer systems
by processing many wafers at a time. To accommodate the increased surface
area of multiple wafers in a larger-volume process chamber, pulse times
are increased enough to satisfy the residence times required for the
precursor to penetrate to the bottoms of deep trenches. The tool discussed
here processes 50 wafers simultaneously, enabling the use of long pulse
times so that the precursors can achieve full patterned-wafer surface
saturation and step coverage while maintaining production-worthy throughputs.
Figure 3 compares the throughput of a single-wafer system and a batch
system used to process planar and deep-trench wafers.
3: Relative wafer throughputs for single-wafer and batch ALD tools
as a function of wafer topography. The batch tool improved throughput
by 300–400% for flat wafers and 200–300% for patterned
difficulty posed by the longer precursor pulse times used to deposit
hafnium silicate compounds is that the ratio of hafnium to silicon must
remain the same throughout the entire pulse to yield a conformal film
of consistent composition at the top and bottom of the trench. Step
coverage in deep-trench structures is a function of precursor dose and
residence time. The scanning electron microscope (SEM) image in Figure
4 illustrates a deep-trench structure with an aspect ratio >50:1
in which an HfxSi1–xO2 film has
achieved conformal step coverage.
coverage and compositional control of a compound film will not be attained
if one precursor depletes relative to the other during the chemical
pulse. For chemical-delivery systems that use an inert carrier gas through
liquid bubblers, the vapor pressure of the TEMAHf precursor is insufficient
to deliver a high-enough continuous dose because the consumption rate
is higher than the vapor regeneration rate as a result of bubbler depletion.
Thus, during an extended chemical pulse, the ratio of TEMAHf to TEMASi
may decrease from the beginning to the end of the pulse.
4: SEM image showing conformal step coverage of HfxSi1–xO2
in a deep-trench structure with an aspect ratio >50:1. The deposited
thickness was increased to highlight the step coverage.
depletion, an issue for single-wafer systems that attempt to saturate
deep-trench structures, is magnified in batch HfxSi1–xO2
processing, where much more chemistry must be injected onto the wafer
to cover much more surface area. While single-wafer systems may be able
to prevent depletion of dual precursors at differing rates at the expense
of throughput, batch ALD processing requires additional advanced chemical-delivery
techniques to achieve uniform precursor injection and evacuation in
the process chamber. Furthermore, rapid chamber evacuation and chemical
purging are critical in ALD processing to maximize throughput and prevent
minimizing process chamber volume, the ALD system under investigation
reduces the purge time between chemical pulses. Dual vertical injectors
positioned at the wafer edge inject metal-organic precursor and oxidizer
alternately while establishing the true cross-flow gas dynamics required
to achieve uniform within-wafer and wafer-to-wafer film thickness, film
composition, and conformal step coverage. Film-compositional control
is achieved using independent dual liquid injectors for the hafnium
and silicon precursors, a method that is not subject to the vapor-
pressure-depletion effects associated with bubblers or vapor-draw techniques
used for long pulse times. The dual-liquid-injector technique allows
the desired precursor dose to be injected during the desired pulse time,
even if the pulse time is extended. Hence, the residence time required
for the precursor to reach the bottom of high-aspect-ratio structures
can be satisfied at an acceptable throughput.
demonstrate film-thickness uniformity within a 50-wafer batch and the
repeatability of the process, four consecutive runs were performed on
the ALD batch system under the same HfxSi1– xO2
process conditions. Figures
5a–d plot the spectroscopic ellipsometry results for 10 300-mm
bare silicon monitor wafers spaced throughout each 50-wafer batch. Table
II summarizes the thickness and refractive index (RI) uniformity
achieved for each of the four runs using 49-point mapping with a 3-mm
edge exclusion. Among all 40 monitor wafers measured, the worst individual
within-wafer thickness uniformity was ±3.19% by range, or 1.62%
σ/mean. The worst individual within-wafer refractive index uniformity
was 0.87% σ/mean.
6: Mean film thickness, wafer-to-wafer thickness variation, within-wafer
thickness uniformity, and refractive index values of HfxSi1–xO2
film for each batch of 50 wafers processed using the ALD batch system.
uniformity for all 40 wafers was ±2.50% for thickness and ±0.29%
for refractive index. Batch-to-batch repeatability was excellent, as
indicated by the minimal increase in wafer-to-wafer thickness variation
over any single batch. Figure 6 plots the mean thickness, within-wafer
thickness uniformity, and the mean refractive index for each batch.
Also shown is wafer-to-wafer thickness uniformity, which was derived
from the mean thicknesses of the 10 wafers measured in each batch. Mean
within-wafer thickness uniformity was 1.12% σ/mean, or ±2.45%
range. Batch-to-batch thickness repeatability was ±0.23% range,
and batch-to-batch refractive index repeatability was ±0.05% range.
7: Relative wafer-per-hour throughput and per-wafer TEMAHf and TEMASi
chemical consumption for batch versus a 100-Å single-wafer
process. At the same 100 Å, the batch process improved throughput
by ~450% over the single-wafer process while consuming one-third
the amount of TEMAHf and one-eighth the amount of TEMASi.
addition to the throughput improvement achieved by running the HfxSi1–xO2
process on a batch instead of a single-wafer tool, chemical consumption
decreased significantly. Figure 7 shows relative 300-mm wafer-per-hour
throughput and per-wafer chemical consumption for TEMAHf and TEMASi
batch processing at various film thicknesses normalized to a 100-Å
single-wafer process. The process conditions for both the single-wafer
system and the batch system were set to obtain similar conformal step
coverage results in advanced DRAM deep-trench structures. At the same
100-Å thickness, the batch process improved throughput by ~450%
while consuming only 33% of the TEMAHf and 12% of the TEMASi per wafer
compared with the single-wafer process.
films are of interest to semiconductor device manufacturers because
they can cover severe topographies such as deep-trench capacitors in
advanced DRAM structures as well as more-planar gate-stack structures
in low-leakage logic applications. These applications require materials
with a higher dielectric constant than Al2O3.
ALD processing has significant advantages over single-wafer processing
because it enables higher throughput and significantly lower chemical
consumption, reducing the cost of production manufacturing. The technical
design of the batch ALD system delivers liquid precursors that yield
good within-wafer film-thickness uniformity. The batch-to-batch repeatability
studies described in this article demonstrate that the total wafer-to-wafer
thickness variation of ±2.5% across four consecutive batches was
only marginally higher than the worst within-batch wafer-to-wafer thickness
variation of ±2.44% range. Finally, the use of independent direct
liquid injection for dispensing TEMAHf and TEMASi precursors ensures
that compositional control of the dielectric properties of the bulk
film and the proper ratio of hafnium to silicon can be maintained.
authors would like to acknowledge Carl Barelli, Yoshi Okuyama, S. G.
Park, Chris Tousseau, Jay DeDontney, and Bryan Ford from Aviza Technology
for their technical assistance, and Hood Chatham, also from Aviza, for
his discussions on the topics discussed in this article.
Wilk, RM Wallace, and JM Anthony, "High-k Gate Dielectrics: Current
Status and Materials Properties Considerations," Journal of Applied
Physics 89, no. 10 (2001): 5243–5275.
Senzaki et al., "Atomic Layer Deposition of Hafnium Oxide and Hafnium
Silicate Thin Films Using Liquid Precursors and Ozone," Journal
of Vacuum Science and Technology A 22, no. 4 (2004): 1175–1181.
Hausmann et al., "Atomic Layer Deposition of Hafnium and Zirconium Oxides
Using Metal Amide Precursors," Chemistry of Materials 14, no.
10 (2002): 4350–4358.
Owyang is director of ALD product management at Aviza Technology
(Scotts Valley, CA). Before joining the company, he served as a senior
strategic enabling engineer at Intel, where he introduced high-k materials
into development. Before his tenure at Intel, Owyang held technology
development positions at LSI Logic and Philips Semiconductor. He holds
five patents related to process technology. He received a BS in chemistry
from the University of California, Berkeley. (Owyang can be reached
at 831/439-6405 or email@example.com.)
Bartholomew is a principal process engineer at Aviza Technology.
After beginning his career at Watkins-Johnson, he worked for Silicon
Valley Group's thermal systems division and ASML's thermal division.
Bartholomew has 26 years of experience in atmospheric pressure CVD and
ALD process development for production applications. He holds six patents
and has two patents pending. He received a BA in physics from the University
of California, Santa Cruz. (Bartholomew can be reached at 831/439-6313
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