We report the first study of gas phase silicon micromachining using pure bromine trifluoride (BrF/sub 3/) gas at room temperature. This work includes both the design of a new apparatus and etching characterization. Consistent etching results and high molecular etching efficiency (80%) have been achieved by performing the etching in a controlled pulse mode. This pure gaseous BrF/sub 3/ etching process is isotropic and has a high etch rate with superb selectivity over silicon dioxide (3000:1), silicon nitride (400-800:1) and photoresist (1000:1). Moreover, gaseous BrF/sub 3/ etching has also been demonstrated in surface micromachining process, where silicon nitride channels and membranes using polysilicon as the sacrificial layer have been successfully fabricated

Tai, Yu-Chong

Walsh, Ken

Wang, Xuan-Qi

Yang, Xing

Caltech Authors - Main

2D3.02 
Gals-Phase Silicon Etching with Bromine Trifluoride 
Xuan-Qi Wang, Xing Yang, Ken Walsh, and Yu-Chong Tai 
Department of Electrical Engineering, 136-93 
California Institute of Technology 
Pasadena, CA9 1 125, USA 
SUMIMARY 
We report’ the first !itudy of gas phase silicon 
micromachining using pure bromine trifluoride (BrF3) gas at 
room temperature. This work iricludes both the design of a new 
apparatus and etching characterization. Consistent etching 
results and high molecular etching efficiency (80%) have been 
achieved by performing the etching in a controlled pulse mode. 
This pure gaseous BrF3 etching process is isotropic and has a 
high etch rate with superb s,electivity over silicon dioxide 
(3000: I) ,  silicon nitride (400-E100: 1) and photoresist (1000: 1). 
Moreover, gaseous BrF3 etching has also been demonstrated in 
surface micromachining process, where silicon nitride channels 
and membranes using polysilic,on as the sacrificial layer have 
been successfully fabricated. 
Keywords: Bromine Trifluoride, Gas-Phase Silicon Etching 
INTRODUCTION 
Bulk silicon micromachiniing in MEMS often uses wet 
chemical and plasmaheactive ion etching. Wet chemical 
etchants yield high selectivity but cause surface tension effects 
during drying which can induce direct mechanical damage and 
surface stiction of fragile structures. Plasmaheactive ion 
processes solve many of these problems with the caveat of 
limited selectivity over silicon dioxide and nitride. It is 
therefore advantageous to have a pure gas-phase silicon etchant 
that is selective and tension-free. It has been reported that 
many fluorine-containing interholagens will etch silicon 
spontaneously in the vapor phase including xenon difluoride 
and bromine trifluoride [ 1,2,3]. However, the use of pure BrF3 
gas for silicon micromachining has not been reported before. It 
is nevertheless attractive because BrF3 has a high vapor 
pressure at room temperature, which implies ease of use and 
high etching rate. In this work, we then report our effort of 
exploring gas-phase BrF3 silicon micromachining and conclude 
from our results that the BrF3 etching is simple, fast, aaslcahle, 
and repeatable. 
ETCHING CHEMISTRY AND THEORY 
At room temperature, BrF3 is in the form of a colorless to 
gray yellow liquid with a vapor pressure of approximately 7.0 
Torr. The etching mechanism of BrF3 with silicon is believed to 
be the same as that of other fluorine-containing interholagens 
reported in [ 1,4] described by the following sequence of steps: 
( I )  nondissociative adsorption of gas-phase species at the 
surface of the solid etched; ( 2 )  dissociation of this absorbed 
gas; (3) reaction between adsorbed atoms and the solid surface 
to form an adsorbed product molecule (SiF4); (4) desorption of 
the products molecule into the gas phase. Here, the etching of 
silicon on the surface is believed to be fluorine atoms and the 
volatile products are SiF4 [3] and Br2 as shown in equation (1): 
(1) 
Assuming the etching process is chemical reaction limited 
and the BrF3 concentration is uniform in the reaction chamber, 
the number of BrF3 molecules in the reaction chamber, N, can 
be expressed as, 
4BrF, + 3Si = 2Br, + 3SiF, 
dN 4 - - -  - - RA x n,, 
dt 3 
where R is the silicon etching rate in &minute, n,i is the atomic 
density of silicon A is the silicon opening area of the etching 
sample. In [l], the BrF3 etching rate is shown as a function of 
silicon substrate temperature and fitted to the Arrhenius 
equation in the form of, 
N 
(&minute) (3) 
V 
where E, is the activation energy that has a value of -6.4 
kcal/mole for BrF3 at room temperature, k is the Boltzmann 
constant (1.987~10” kcal/mole/K). In our case, T is 300K, V is 
2220cm3, the initial value of N is 7.76~10”. 
At 1.0 Torr gas pressure and room temperature, Eq. (3) 
gives a maximum etching rate of 4.13pmlmin of our system. 
By combining Eq. (2) and Eq. (3) and solving the first-order 
differential equation, the number of BrF3 molecules in the 
reaction chamber, N, is found to be an exponential function of 
time. Then, we can convert N to the silicon mass loss as a 
function of etching time, M(t)  
R = 1.16 x lo-’’ - T”ZExp(-E,  I k T )  
M ( r )  = M ( W )  [l - ..(- t,] (4) 
(5) 
V 
= C- A (second) 
C is a constant and has a value of 8 . 7 8 ~ 1 0 - ~  s/cm for our 
system. From Eq.(5) we see the time constant is inversely 
proportional to the silicon exposed area A .  For example, for 
the opening area of 3cm2, Eq. (5) gives a time constant of 65 
seconds. 
ETCHING APPARATUS AND OPERATION 
Our etching system shown in Figure 1 and Figure 2 consists 
of a reaction chamber, a vapor reservoir, nitrogen purge, xenon 
dilution and a vacuum subsystem. We control the individual 
gas flow by switching the corresponding valve. Pressure in the 
reservoir and reaction chamber are monitored by a Baratron 
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2D3.02 
and a pressure gauge. The construction materials are stainless 
steel and brass. For safety reasons, the whole system is set up 
inside a fume hood with plenty of ventilation. Etching 
experiments are conducted in the pulse instead of continuous 
BrF3 flow etching for the following two reasons. First, 
continuous flow of BrF, vapor through the system etches 
silicon too fast which makes the etching difficult to control. 
Second, continuous BrF3 flow has a low etching efficiency. 
Unconsumed BrF3 vapor will enter the pump system and cause 
safety concerns and pump failure. We start our etching by 
loading the wafefs in the reaction chamber. Both the reaction 
chamber and vapor reservoir are pumped down below 10 
mTorr. The chamber separation valve is then closed, and a 
fixed amount of BrF3 is introduced into the vapor reservoir. 
The chamber separation valve is then opened to allow BrF3 
vapor to enter the reaction chamber. Enough time is given to 
make sure all the BrF3 vapor inside the reaction chamber is 
consumed. Finally, the chamber separation valve is opened and 
both the chamber and reservoir are pumped down. The above 
procedures then defines one pulse of the etching. At room 
temperature and 6 Torr pressure in the vapor reservoir, the 
number of BrF3 molecules in one pulse is calculated to be 1.29 
Chamber 
BrF, Xe 
Figure I :  Schematic of the Etching System 
Figure 2: Picture of the Etching System 
x 10 -4 mole. When balanced with the reaction chamber, there 
will be 1.1 x mole of BrF3 in the chamber and theoretically 
up to 2.3 mg of silicon will be etched away. 
SAMPLE PRE TION 
Silicon etching experiments are conducted on silicon dice 
with edges covered by hard baked photoresist, which is shown 
later to have a very low etching rate. To remove the native 
oxide on the silicon surface, all the samples are etched in 
buffered hydrofluoric acid for 10 seconds followed by 
deionized water rinse and nitrogen drying before etching. 
Samples are then loaded into the reaction chamber and 
evacuated to 10 mTorr. Samples are kept in the vacuum for at 
least 10 minutes before starting the first BrF3 pulse. Our 
experiments show that Nz drying is enough to produce reliable 
and repeatable etching, while baking the sample at high 
temperature is not necessary. Acetone cleaning should be 
avoided. Otherwise, a white polymer like film will form on the 
silicon surface when acetone residue is exposed to BrF3 vapor 
and this white film will stop further etching. 
The etching depth and the undercut are measured under a 
microscope with a calibrated focus. The thickness of the mask 
layers before and after the etching is determined using a 
Nanospec thin-film thickness measurement instrument The 
roughness of the silicon surface is measured using a surface 
profiler (Tancor a-step 200). 
ETCHING EXPERI 
Bulk Silicon Etching 
Bulk silicon etching experiments are conducted on silicon 
chips with circular openings. The diameter ranges from 4 pm to 
4 mm. Different masking materials such as thermal oxide, 
LPCVD silicon nitride, hard baked AZ4400 and AZ1518 
photoresist are used as the mask materials to study the etching 
rate and etching selectivity. 
First, we measure the etching rates of samples with 800 pm 
circular windows at various gas pressures with a constant pulse 
duration of 10 minute. Figure 3 shows the vertical etching 
depth as a function of the number of purses at the different BrF3 
pressures. The etching rate ranges from 10 pdpulse  up to 140 
350 
300 
250 
E 
2 200 
5 
3 150 n 
2 loo 
3 50 
0 
0 1 2 3 4 
Number of Pulses 
Figure 3: Etching Rates at Different BrF3 Pressure 
TRANSDUCERS '97 
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1506 
2D3.02 
2 
h 
E" 
3 
2 
- 1.5 
* I  
0.5 
0 
Figure 4: Silicon Mass Loss vs. Pulse Duration Time for 
Different Exposed Silicon Areas 
pdpulse, which correspond to a average etching rate range 
from 1 pm/min up to 14 p d m i n  for this particular opening. 
Next, at a constant BrF3 pressure of 1 Torr, we study the 
etching at different pulse duration. The pulse duration time is 
varied from 20 seconds to 20 minutes. Figure 4 shows the mass 
of silicon etched away versus pulse duration time with different 
window opening areas. The samples with different openings are 
tested separately and for every data point, we use a new silicon 
sample. 
Figure 4 shows that not onlly the etching rate but also the 
system etching efficiency is a function of exposed silicon area. 
We define the system efficiency as the ratio of the silicon mass 
etched by our system to the theoretical silicon mass etched 
assuming all the BrF3 is consunned by silicon, which is 2.3 mg 
in our case. The maximum efficiency from Figure 4 is 80% for 
the 4 inch bare silicon wafer (1610 cm'). 
Third, we investigate the etching rate as the function of the 
opening area. Circular openings with diameters from 4 pm to 4 
mm are used. Please note that all the samples are etched in the 
same loud. We choose 1 Torr BrF3 pressure and 15 minutes 
h 
v 5.
eo 
C 
c 
0 
.I 
5 
1 10 100 1000 
Circular Opening Diameters (pm) 
Figure 5: Vertical and Horizontal Etching Depth vs. Circular 
Opening with Diameters of 4pn,  8pm, I O W ,  20pm, 4 0 m ,  
70pn, 1 OOpm, 2 0 0 p , 4 0 0 p ,  8 O O p ,  2mm, and 4". 
50 -- 
- 
40 -- 
a 
5 
8 30 
z 
- 
P -- 
2 
i 20 -- 
IO -- 
J I  
0 
0 2 4 6 8 10 12 
Number of Pulses 
Figure 6: Vertical Etching Depth vs. Number of Pulses with 
Different Circular Opening Sizes 
pulse duration. The etching depth and undercut of these 
samples versus the sample opening area are plotted in Figure 5. 
We noticed from Figure 5 that the etching depth of the 
40pm to 200 pm samples are about 50% more than the larger 
samples. For the samples with diameters under 40 pm, the 
smaller the opening the less silicon is etched. To confirm the 
data, another series of experiments are done with the same type 
of samples using 500 mTorr, and 15 consecutive pulses 
(lmin./pulse). All the samples are etched at the same time. The 
results are shown in Figure 6. Compared to the last experiment, 
although BrF3 is refreshed 15 times to achieve the similar 
Silicon /Poly-Si Fredpamds 
Figure 7: Surface Micromachined Channels 
Figure 8: Surface Micromachined Membranes 
TRANSDUCERS '97 
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Chicago, June 16-19, 1997 
1507 
2D3.02 
5” t 
I 
0 2 4 6 8 10 
“ h e r  of Pukes 
Figure 9: Etching Length in su$ace Micromachined Channels 
vs. Number of Pulses 
etching depth, the etching depth differences of the various 
samples have not been reduced significantly. 
Etching Selectivity 
Checkerboard openings of 800pm X 800pm on a silicon 
wafer with several masking materials are tested to study the 
etching selectivity. The etching selectivity of silicon to other 
materials is found to be greater than 3000: 1 for LPCVD silicon 
dioxide, and 1000 to 1 for hard baked AZ 4400 and AZ1518 
photoresists. The etching rate of silicon nitride depends on the 
quality and silicon concentration of the nitride layer and results 
in a selectivity range of 400:l to 800:l. In the case of most 
metals, like aluminum, copper, gold, and nickel, BrF3 forms a 
passivated non-volatile metal fluoride layer on the metal 
surface and further reaction is stopped. The selectivity over 
these metals is greater than 1000 to 1. 
Polysilicon Sacrificial Layer Etching 
As shown in Figure 7 and Figure 8, surface micromachined 
channels and membranes with silicon nitride as the structural 
layer and polysilicon as the sacrificial layer are also released by 
this BrF3 etching process. Figure 9 shows that no obvious 
etching rate difference is found among the channels with 
different widths. A total length of 300 pm has been etched into 
the channels with 10 etching pulses. Circular membranes have 
similar etching rates as the channels. 
DISCUSSION 
Eq. (5) shows that the etching time constant z is inversely 
proportional to the surface area. Our bulk etching experiment 
results in Figure 4 indicate such a qualitative relationship. 
Since the BrF3 etched silicon surface is quite rough, the 
effective silicon surface area is larger then the mask opening 
area A. 
From our bulk silicon etching experiments shown in Figure 
5 and Figure 6, we notice the local loading effects for large 
openings (greater than 200pm diameter) and aperture opening 
effects for small openings (under than lOpm diameter). We 
believe these two effects are caused by the formation of local 
BrF3 depletion region. 
We can qualitatively explain the diffusion limited effects by 
comparing the circular opening radius with the mean free path 
L of the gas molecules. When the opening radius r is close to L, 
the diffusion front profile can be represented by a semi- 
spherical surface with radius r shown as opening A in Figure 
10, which has a surface area of 2n r2. If r >> L, as in the case of 
opening B, it has a surface area of m2, which means the number 
of BrF3 molecules reach the silicon surface per unit area 
without any collision with other gas molecules is half of the 
case A. This explains why the etching depth per pulse in B is 
half of A as shown in Figure 5. In the case of opening C, after 
etch down a depth of d, when d > 2r, and r << L, the opening 
that allows BrF3 molecules get into it is reduced compare to the 
case of r similar to d. thus reducing the etching rate. 
Y 
I I I I I  
L 
df 
A B 
Figure IO: Local Loading Effect and Aperture Opening Effect 
CONCLUSIONS 
We have designed and characterized a simple BrF3 silicon 
isotropic room temperature etching system for both bulk and 
surface micromachining. Consistent high etching rate is 
achieved with using various masking materials. The etching 
rate in pulse mode is dependent on gas concentration, reaction 
pressure, pulse duration, pattern opening area and effective 
surface area. 
ACKNOWLEDG 
The authors would like to thank Trevor Roper, Larry Begay 
for help with the system setup and Tomas Tsao, Charles 
Grosjean with helpful discussions. 
[ l ]  D. E. Lbbotson, J. A. Mucha and D. L. Flamm, 
“Plasmaless Dry Etching of Silicon with Fluorine- 
Containing Compounds” J. Appl. Phys. %(lo), Nov. 
[2] F. Chang, R. Yeh, G. Lin, P.B. Chu, E. Hoffman, E. J. J. 
Kruglick, K. S. J. Pister, M. H. Hecht, “Gas-Phase Silicon 
Micromachining with Xenon Difluoride”, SPIE Vo1.254 1, 
pp.117. 
[3] U. Kohler, A. E. Guber, W. Bier, M. Heckele, “Fabrication 
of Microlenses by Plasmaless Isotropic Etching Combined 
with Plastic Moulding”, Sensor and actuatiors A,53, 
1984, pp. 2939-2942. 
(1996), pp.361-363 
[4] H. F. Winters and J. W. Coburn, “The Etching of Silicon 
with XeFz Vapor.” Appl. Phys. Letter, Vo1.34(1), pp.70- 
73. 
1508 TRANSDUCERS ’97 1997 lnternafional Conference on Solid-state Sensors and Actuators 
Chicago, June 16-19, 1997 


Gas-phase Silicon Etching With Bromine Trifluoride

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Gas-phase Silicon Etching With Bromine Trifluoride

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