Critical Thickness Threshold in HfO2 layers
Pascal Besson1, a, Virginie Loup2, Thierry Salvetat2, Névine Rochat2,
Sandrine Lhostis1, Sylvie Favier1, Karen Dabertrand1, Vincent Cosnier1
1
2
STMicroelectronics, 850 rue Jean Monnet, F-38920 Crolles, France
CEA-DRT - LETI/DTS - 17, avenue des Martyrs 38054 Grenoble CEDEX 9, France
a
pascal.besson@cea.fr , pascal.besson@st.com ,
Keywords: High k, HfO2, Threshold, Thickness, Crystallized
Introduction
Wet etch ability of HfO2 high k layers in diluted HF solutions is very attractive to achieve an
efficient selective removal with respect to silicon consumption and metal gate compatibility for
advanced technology node. Unfortunately in most cases HfO2 layers can not be etched in a wet
mixture after the gate stack formation and dry etch. This behavior is now clearly correlated to the
monoclinic crystalline phase [1;2]. This phase can be either an “as deposited” state in the case of
high temperature deposition mode such as MOCVD, or a post integration state related to added
thermal budget as for ALCVD. In this study, the HfO2 etch rate with a diluted HF/HCl chemical
solution was tightly monitored as a function of both ALCVD and MOCVD deposition modes, under
different annealing conditions and different deposited thicknesses.
Experimentals
200mm p-type Si (100) substrates were prepared in an immersion wet bench using the DDC
sequence [3] providing an ozone based 8 Å chemical oxide [4]. The HfO2 layers were grown either
by ALCVD at 350°C with HfCl4 solid source or by MOCVD with Hf(OtBu)2(mmp)2 precursor and
O2 at various deposition temperatures from 410°C up to 550°C. The sample thickness varied from
1nm up to 8nm in both ALCVD and MOCVD cases. Annealing steps were performed either into a
RTP 850 furnace under N2 ambient at atmospheric pressure. Finally, the various layers were etched
in HF mixture with HCl addition to maintain a constant pH whatever the HF concentration in the
range [0.1%, 50%]. ATR FTIR provided data on amorphous or crystallized trends of the different
samples. Surface haze measurement on a KLA Tencor SP2 tool was used to monitor the HfO2
surface in the as deposited state and after each etching step. The haze data were correlated to AFM
measurement on a Digital Instrument Dimension 3100 system.
Results and discussion
By monitoring MOCVD layers etch rate in a 0.5%HF-1%HCl mixture, a strong dependence on
initial thickness as on deposition temperatures (430°C, 450°C, 500°C and 550°C) is evidenced. For
each deposition temperature samples exhibit a thickness threshold above which a near zero etch rate
is observed while a very fast one is measured below. Figure 1 evidences this threshold by plotting
the residual thickness as a function of the initial deposited thickness after 180s etch time in the
previously described HF mixture. There is not dependence of the threshold with the etching mixture
concentration. This behavior is directly linked to the presence or not of the monoclinic crystalline
phase into the high k layers. Figure 2 shows ATR FTIR spectra obtained on 430°C and 450°C
deposited high k films for thickness values very close to the critical thresholds, 90-100Å and 50-5965Å respectively. The easily etchable 90 and 50Å samples are well related to an amorphous material
ATR spectra while the non etched 110 and 65Å samples clearly exhibit the 780cm-1 peak
corresponding to the monoclinic crystalline phase [5]. The “time at temperature” which drives the
samples towards the monoclinic phase and which is directly related to the deposited thickness could
be proposed as a simple explanation for the thickness threshold in MOCVD layers. Nevertheless, by
monitoring the etch rate on a single 550°C MOCVD wafer with a non uniform initial thickness in
the range [35Å, 40Å] the threshold is also evidenced. Indeed on such wafer surface the wet etch
results in a phobic state in the 35Å center area while the edge area still presents the initial 40Å film.
In this case the wafer received the same process time at 550°C, the initial non uniformity has been
explained by a flow effect.
Regarding ALCVD HfO2 data on figure 3, the “as deposited” 350°C samples present high etch rates
whatever the initial thickness is, when up to 8nm where the crystallization begins avoiding a full
layer removal. Nevertheless, after a 600°C 15min anneal the same initial deposited thickness
dependence than previously seen on MOCVD is exhibited with a threshold at 28Å. No etch occurs
above while below it etch rates are a little bit lower than on “as deposited” layers. Here again,
obviously no deposition thermal effect can be considered between a 28 and 30Å samples. The
behavior is more related to the crystallization onset after 600°C anneal.
High k
Threshold
430°C
100Å
MOCVD
450°C
500°C
60Å
45Å
550°C
38Å
As Dep
80Å
ALCVD 350°C
600°C 15min N2 Atm
28Å
Table I: critical thickness threshold measured by ellipsometry for different HfO2 types
The wet etch rate measurement demonstrates a relevant information about high k layer state: mainly
the existence of a critical thickness threshold at which the monoclinic phase transition occurs, as
presented in table 1. However, the unique thickness decrease monitored by ellipsometry can not
provide a full characterization of the high k layer removal, especially for samples just below this
critical threshold. This is a major issue as the high k thickness integrated for a suitable EOT in an
advanced CMOS process flow is mainly in the 20 to 30Å range. Indeed, these values are very close
to the 28 and 38Å thickness thresholds observed for annealed ALCVD and MOCVD 550°C samples
respectively. Focusing on a restricted thickness range around the threshold, ATR FTIR
characterization is performed in parallel with the thickness measurement after 7min30sec and 15min
etch time in a 0.1%HF/HCl mixture. Figure 4 shows the HfO2 peak evolution as a function of its
thickness. Initially, only the wafer A with 41Å high k layer exhibits a clear monoclinic crystalline
trend with two well define adsorption bands at 780cm-1 and 675cm-1. Wafers B and C with 36 and
31Å respectively present a similar large and poorly defined peak relative to an amorphous structure.
After the etch steps, the wafer A spectrum remains unchanged confirming the stable thickness
value. The HfO2 peak intensity of wafers B and C decrease in line with the thickness loss.
Nevertheless, when the residual thickness values reach 9Å from 36Å initial thickness, the two peaks
of the monoclinic phase appear again on the FTIR spectra while the SiO2 peak shifts from 1228 to
1213 cm-1. Moreover, on the wafer C the SiO2 peak of the oxide under-layer disappears but a small
peak at 675cm-1 seems always present.
As the HfO2 monoclinic phase is not sensitive to HF mixture at room temperature, some crystallized
residues stay on the surface. For these HfO2 layers below the critical threshold, the surface residues
could be considered as part of crystallized HfO2 initially embedded in an amorphous matrix. Using
TXRF the residual hafnium atoms concentrations on the surface of wafers A, B and C are: 2.5E14,
1.4E14 and 1.75E13 for residual thicknesses of 41, 9 and 0Å respectively. Far from ellipsometric
values FTIR and TXRF data suppose a large amount of residual HfO2 on the surface expected as
being in crystallized structure. AFM measurements reveal a smooth high k surface on wafer A. But
wafer B exhibits a very huge roughness as if the residual crystallized HfO2 was under columnar
form with a height equivalent to the nominal deposited thickness. Investigating the other high k
types the strong dependence of the residual hafnium surface concentration with the initial deposited
thickness is verified in figure 5. When the initial thickness is lower than 20Å the residual hafnium
surface concentration is lower than the Low Limit Detection of the TXRF (LLD value = 2.45E10
at/cm²). From 20Å to the critical threshold the residual hafnium concentration always appears to be
high and more correlated to the initial deposited thickness than the residual thickness, even when
the monitored residual thickness is equal to 0. For initial deposited thicknesses very close to the
critical threshold, the ellipsometric data can present within a same wafer different values from 0 to
few Å (<10Å) correlated with the nominal thickness range (<2Å). In these cases AFM pictures in
table II always evidence the columnar aspect with just a little change in residues density depending
on the residual value.
ALCVD “As dep”
Initial Thickness = 80Å
Initial Thickness = 82Å
Residual Thickness 0Å
Residual Thickness = 5Å
MOCVD 450°C
Initial Thickness = 62Å
Residual Thickness = 10Å
MOCVD 450°C
Initial Thickness = 60Å
Residual Thickness = 0Å
MOCVD 550°C Wafer B
Initial Thickness = 35Å
Initial Thickness = 36Å
Residual Thickness = 0Å
Residual Thickness = 5Å
Table III: AFM pictures of 3 HfO2 types just below the critical threshold after etch and range on a same wafer/each type
Based on all these results the behavior of high k layers around the critical thickness threshold can be
summarized as follow: The layers can be etched with HF solutions but the etch rate is strongly
impacted by the crystallized compounds density. The crystallized compounds density increases
linearly with the initial deposited thickness up to a critical threshold where the layer is fully
crystallized. The “true” critical threshold can be higher than the ellipsometric one presented
previously. Indeed a small part of amorphous HfO2 can be still present even if the ellipsometric data
remain stable after a wet etch. This point can be suspected by using the haze measurement on a SP2
tool on wafers just around the ellipsometric threshold. Figure 6 presents the haze intensity as a
function of the etch time and the residual thickness. Wafer B haze evolution occurs in line with the
removal of the amorphous part and the roughness increase seen by AFM. Wafer A with only 2Å
loss reveals also a haze increase, not seen by AFM, which could be related to the removal of a small
amorphous part between the crystallized structures. For samples with thicknesses more clearly
below the threshold, the isolated crystallized parts included inside the amorphous matrix are likely
removed by lift-off effect. This may explain the lack of residual hafnium concentration for layers
below 20Å as checked by TXRF.
Conclusion
A specific chemical behavior of HfO2 layers in the 15, 40Å range currently use for CMOS
applications is demonstrated. Working on this behavior can help the crystallization mechanism
understanding and provide more explanation about the origin of the residues currently seen after the
gate etch.
References
[1] Karen Dabertrand, et al, EMRS-2006, Symposium
[2] Shinji Fujii, et al, Applied Physics Letters 86 (2005)
[3] F.Tardif, et al, UCPSS, Belgium (1996), p 175.
[4] S. Petitdidier, et al, UCPSS, (2002)
[5] N.Rochat, et al. Physica Status Solidi (c), 1 (2003)
0,13
160
140
430°C
Initial Thickness
450°C
After First Etch Step
500°C
After Second Etch Step
ATR Absorbance [a.u.]
Residual Thickness [Å]
0,11
0,1
120
100
0,12
41Å
0,09
0,08
36Å
25Å
550°C
80
0,07
0,06
9Å
0,05
60
31Å
40
0,04
0,03
19Å
0,02
20
0Å
0,01
0
0
0
20
40
60
80
100
120
140
160
1300
1200
1100
900
800
700
600
Wavenumber [cm-1]
Initial Thickness [Å]
Fig.1. Threshold for different deposition temp. of
MOCVD HfO2 layers as a function of initial thickness
Fig.4. 550°C MOCVD HfO2 ATR FTIR spectra
evolution as a function of the residual thickness
0,25
1,00E+15
50
Hf concentration [at/cm²]
450°C 59Å
0,2
450°C 65Å
0,15
430°C 90Å
430°C 110Å
0,1
0,05
1,00E+14
ALCVD 350°C
+ 15 min Atm
600°C Anneal
40
SAMPLES
ABOVE
THRESHOLD
ALCVD 350°C
As Dep
1,00E+13
30
1,00E+12
20
1,00E+11
10
TXRF LLD
0
1300
1200
1100
1000
900
800
700
1,00E+10
600
0
10
15
-1
Wave number [cm ]
25
30
35
40
3
48
105
2,5
15
y = -0,43x+22
2
R = 0,981
y = -0,28x+26
R2 = 0,992
y = -0,49x+27
R2 = 0,996
y = -0,60x+21,5
R2 = 0,9916
0
45
0
10
20
30
40
50
60
70
80
90
30
y = -0,12x+77
2
R = 0,99
y = -0,23x+49
R2 = 0,98
15
Residual Thickness [Å]
40
90
60
Wf A
Wf B
Wf C
32
2
1,5
24
-
1
16
0,5
8
0
0
0
100
200
300
50
Fig.5. Hafnium atoms surface concentration as a function of
the initial and residual thickness
30
75
45
400
500
600
700
Etched Time [sec]
Fig.3. 350°C ALCVD HfO2 etch rate in HF/HCl
mixture as a function of the initial thickness. White
squares and black triangles are “as dep” and “600°C
15min annealed” respectively
0
200
400
600
800
0
1000
Wet Etch Process Time [min]
Fig.6. Haze intensity evolution as a function of etch
time and residual thickness
Wide Channel Oblique Incidence Haze
Intensity [a.u.]
Residual Thickness [Å]
20
Initial Deposited Thickness [Å]
Fig.2. ATR FTIR spectra of 450°C and 430°C
MOCVD HfO2 layers as a function of initial thickness
120
Residual Thickness after HF Etch [Å]
MOCVD
550°C
450°C 50Å
Absorbance [a.u.]
1000