L.Lachal
1 , S.Auclair
1
,
P.Lavios
1 , JM.Finet
1 , R.Assidi
1 , M.Klein
1 , M.H. Vaudaine 1 ,
F.
Delaguillaumie
1
, C.Marcoux
1
, P.Andreucci
1
, H.Grange
1
, A.Berthelot
1
, R.Garcia-
Santana
1
, F.Lasbordes
1
, E.Lajoinie
2
, O.Louveau
2
and J.Chiaroni
1
1
Minatec CEA-Leti, 17 rue des Martyrs 38054 Grenoble, France
2 STMicroelectronics, 850 rue Jean Monnet, 38926 Crolles, France laurent.lachal@cea.fr
Due to the devices downscaling, the HF vapor release process, one of the most effective techniques for MEMS release, becomes more challenging regarding stiction. An initial compatibility study shows that many metals are compatible excepted Ni and Cu. Then a design of experiment (DOE) was performed on thermal oxide wafers to obtain the parameters for a free stiction process.
Nevertheless, current MEMS release processes can not be used on
NEMS structures due to more aggressive shape factor. The solution to adapt current HF vapor MEMS release process to
NEMS consists in the reduction of the etchrate, reducing the water layer on the surface during the etch step and thereby stiction occurrence. For very challenging structures, reducing the etch rate is not sufficient to avoid stiction, heated processes have then to be used .
The evolution from Micro Electro Mechanical Systems (MEMS) to Nano Electro
Mechanical Systems (NEMS) technology, due to CMOS co-integration requirement and performances enhancement, requires a drastic decrease of device characteristic dimensions (from 10 µm to 100 nm). Hence, release process becomes more and more critical in regards with stiction, stiffness and material compatibility issues. HF vapor is known to be an effective technique for the microstructure dry release, limiting the presence of water on surface [1] and, subsequently, avoiding stiction. The aim of this study is to find a free adhesion process with an acceptable compatibility level to be used on NEMS structures.
Experimental
Processes were performed in a batch HF vapor etch tool manufactured by Semitool
Inc. A simplified flow diagram of the system is shown in figure 1. The system operates at ambient pressure and uses a patented [2] HF/alcohol vapor process. Vapors are generated from liquid sources using an inert carrier gas flowing through a patented [3] vapor generator. There are three N
2
delivery lines, one goes through the HF generator, one goes through the IPA generator, one through nothing. These three lines gives the total flow which is a constant in regards of etch rate uniformity. So the ratios of HF or IPA that will be given are in total flow percentage. Mass Flow Controllers (MFCs ) allow calibrating proportions of HF and IPA vapors in the etchant vapour mix.

The etchant vapor is delivered to a high density polyethylene (HDPE) chamber which can accommodate wafer sizes up to 200mm in diameter and load sizes from 1 to 25 wafers. Wafers are slowly rotated within the process chamber during vapor delivery to promote vapor distribution and uniformity of etching. The carrier design is widely open to allow vapor access to the wafers with minimal flow disruption. Different carriers allow to process wafer from 100 to 200mm diameter. The etch process was characterized on 200mm silicon wafers with various films, including thermal oxide, PECVD undoped silica glass films obtained with TEOS and Silane precursor (hereafter designated as USG-
TEOS and USG-Silane), silicon nitride and various metals. Film measurements were carried on an ellipsometer for oxide etch rate and by resistivity and SEM observations for metals. SEM evaluations were performed to characterize the stiction.
Figure 1: Schematic representation of HF vapor tool
Results and discussion
Materials compatibility with HF vapor
First of all, a compatibility study was performed with different oxides and metals with a 65% HF ratio process, which correspond to a standard MEMS release process.
Different oxides, used as sacrificial buried layer for NEMS release, were studied to use the right oxide for each NEMS applications. Etch rate results of dielectric materials are presented in table 2. The highest etch rates are obtained for the two PECVD oxides
(TEOS and silane precursors) as they have the lowest density. However, their etch rates are closer to those of thermal oxide with a post deposition annealing, which allows to remove impurity such as carbon, over-stoichiometric oxygen and nitrogen. Thus, thermal budget of NEMS wafers is a key parameter of release process.
Table 2: etch rate of various oxide

After total or partial etching, residues are observed for any type of silicon nitride and only for USG-Silane. These residues are characterized by a white powder on the wafer surface after HF vapour etching. Water rinse, which is not allowed after NEMS release, and oxygen downstream plasma at 250°C are efficient to remove this powder and, thus, residues. Characterizations, 1-before etching, 2-after etching and 3-after O
2
downstream plasma treatment at 250°C, have been achieved with Fourier Transformed Infra-Red
(FTIR) (Fig. 3) in order to study the composition of these residues. The Si-O-Si mode and the Si-OR mode, which are oxide typical modes, are observed before etching. Two new modes, characteristic of amine, appear after HF vapour etching: one at 3323 and one at 1428 cm
-1
. Finally, amine characteristic peaks are totally removed by the following O
2 treatment. Similar tests were performed on thermal oxide. No amine was detected after etching. Thus, for oxide, the presence of amines is specific to the silane etching.
Concerning nitride, similar results are obtained as for silane. Amines are removed with a 250°C annealing.
Figure 3: Silane and LPCVD nitride Fourier Transformed Infra-Red spectra
Different metals usually used in microelectronics were also tested on patterned wafers.
TiN (fig4.a), AlCu, AlSi (Fig. 4b), W and Ti are fully compatible with HF vapor process as no consumption was measured for W, 10Å/hour etchrate for AlSi, 20Å/hour for TiN,
50Å/hour for Ti. Ni (Fig. 4d&e) is clearly etched as cracks can be seen at low magnification (fig 4e) and no measure could be performed after etching. Even if no consumption can be measured for Cu (fig. 4c), morphological characterizations, with
SEM and AFM analysis, show a modification of the surface.

Figure 4 : SEM analysis of a) AlSi, b) Cu c) Ni integrity
HF Vapor working zone definition
We have studied thermal oxide etch rates and wafer to wafer non-uniformity (N.U.), for IPA ratio = 10%, as a function of HF ratios (Fig. 5). It highlights three characteristic zones:
A stiction zone for a HF ratio from 50% to 100%. Due to a very high ratio of HF species in the chamber, lots of water, which is the by-product of the etching reaction, is generated. Thus, water on the wafer surface can not be desorbed leading to “similar to wet” mechanisms, explaining very high thermal oxide etch rate.
A vapor zone for a HF ratio lower than 40%. There is less HF species in the chamber, leading to a slower rate of the chemical reaction and water by-product.
Water can then easily desorb from surface, avoiding MEMS stiction.
An instability zone for a HF ratio from 40% to 50%, characterized by an increase of N.U.: the two mechanisms “similar to wet” and vapor occur at the same time on the wafer surface.
Figure 5: Etchrate and N.U. = f (HF ratio) with IPA ratio=10 %

Process has been optimized to perform the maximum etch rate in the MEMS working zone. Release of an accelerometer (fig. 6) show good performance in term of stiction for
MEMS devices. However, this optimized MEMS process does not achieve acceptable release for NEMS as show in figure 7.
Figure 6: Release of an accelerometer with and without stiction
Process has to be adapted to each NEMS devices in order to perform a sufficient throughput, with a maximum etch rate, and prevent stiction, by reducing the amount of water on the wafer surface. Therefore the “NEMS working zone” is on the very left part of the diagram shown in figure 5.
Figure 7: Adhesion of NEMS with MEMS standard release process
NEMS HF vapor release process optimization
Designs of experiment (DOE) were performed in order to determine the input parameters impact on the etch rate and to optimize the process by minimizing device stiction and maximizing throughput.
The parameters for the NEMS working zone were first defined. For this targeted window, HF ratio has to be included between 5 and 40% and IPA ratio between 30 and
95%. Results indicate that the total flow do not act on the etchrate. The key factor for the etchrate is the HF ratio: the higher it is, the higher the etch rate (fig.8). The IPA ratio act in the opposite way, reducing the etch rate. For etch rate under 40Å, the IPA ratio do not have anymore any impact (left part of Fig.8).

Figure 8 : Etch rate Vs IPA and HF ratios
Due to a lack of etching species when reducing the etchrate lower than 15 Å/mn (i.e. reducing the HF ratio under 10%), there is a quick increase for within wafer nonuniformity on wafer up to 15 %. Thus, lowest etch rate processes are not recommended.
Release of e-nose were performed with these low etch rate processes. No stiction and good compatibility with materials was demonstrated (fig.9).
Figure 9 : nanometrics e-nose after HF vapor release
Temperature impact on NEMS HF vapor release
Even with the minimum etch rate, we observed release process limitation with very challenging devices (Fig. 11a), due to their very aggressive shape factor (length/width =
50). A complementary way to succeed in reducing the sticking issue is required. One of the main solutions consists in increasing the temperature of the wafers during the release process. This can be done by heating a part of the gas carrier injected in the chamber.
Indeed, this enables an easier water desorption on wafer surface [5] limiting the capillarity force effect. The etch rate dependence with temperature was studied (Fig. 10).
For two different HF ratio, the etch rate decreases when increasing the temperature, due to the HF species diminution on the wafer surface, facilitating water desorption.

Figure 10: Thermal oxide etch rate function of wafers temperature
5µm long and 100 nm wide arrays release was achieved with a 32°C heated optimized process. Fig. 11b indicates that no array is stuck on the surface or against another array as for room temperature process (fig.11a). Therefore there is an improvement with the same process by heating the wafers during the release process.
These results confirm that the increase of temperature reduces the amount of water adsorbed on the surface. This water comes on one hand from the atmospheric humidity, which is easily adsorb on the hydrophilic surface of native oxide, and on the other hand from the reaction by-product during etch. By heating the wafers, water coming from the reaction is more easily desorbed from the wafer surface and water coming from the atmosphere is reduced, the both reducing the stiction phenomenon. a) b)
Figure 11: 100 nm*5µm beams after HF vapor etch at room temperature (a) and at 32°C (b).
Conclusion
The migration from MEMS to NEMS brings new challenges for devices process because of the size decrease. This is the case for the release etch, which is one of the key point of MEMS & NEMS manufacturing.
First, compatibility with several metals and etchrate with different oxide were tested.
With USG-Silane and silicon nitride PECVD films, residues were observed after etching.
FTIR measurements indicate that these residues are made of amines and can be removed with oxygen plasma or annealing. For metals, Ti, AlCu, AlSi, W and TiN show a good compatibility with etching process.
A study was then performed to define the working zone for MEMS release. For high
HF ratio, there is a runaway of the chemical reaction, leading to a water increase on the

surface and by the way to the stiction of the systems. Thus the MEMS working zone was defined for HF ratio under to 40% for constant IPA ratio.
However, stiction was observed on smaller structures when using this MEMS release process, stressing out the need of optimization for NEMS release. A DOE was performed to obtain the parameters for slower etchrate. Results indicate that the keyfactor is the HF ratio and that for very slow etchrate IPA ratio has no incidence .
For very challenging structure, reducing the etchrate is not sufficient because of very aggressive shape factor. Heated processes were then successfully tested on patterned wafer. Heating the wafers help the liquid produced by the chemical reaction to desorb more easily from the wafer surface.
Acknowledgments
This work was partially supported by the French National Research Agency (ANR) through Carnot funding. This research was carried out at the CEA-Leti MINATEC facilities.
References
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2.
U.S. Patent 5,954,911 and U.S. Patent 6,319,841
3.
U.S. Patent 6,126,734
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C.S. Lee et al, J. Of Electrochem. Soc., 143, 1099-1103 (1996)
5.
Y. Fukuta et al, Jap. J. of Appl. Phys., 42 , 3690-3694 (2003).