Fabricating ultra - thin silicon nitride membranes suspended on silicon wafer

 VNU Journal of Science: Mathematics – Physics, Vol. 37, No. 1 (2021) 74-81 
 Original Article 
 Fabricating Ultra-thin Silicon Nitride Membranes 
 Suspended on Silicon Wafer 
 Pham Thi Hong1, Dang Huu Tung1, Nguyen Hai Anh2, 
 Dang Tuan Linh1, Nguyen Thi Thu Thao1, DinhThuy Hien2, 
 Nguyen Minh Hieu1, Nguyen Minh Hue3, Nguyen Tran Thuat1, 
 Nguyen Viet Tuyen1, Nguyen Quoc Hung1,* 
 1VNU University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam 
 2Advances Material Science and Technology, University of Science and Technology of Hanoi, 
 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam 
 3Department of Physics, Le Quy Don Technical University, 236 Hoang Quoc Viet, Hanoi, Vietnam 
 Received 03 March 2020 
 Revised 12 March 2020; Accepted 15 March 2020 
 Abstract: Ultrathin silicon nitride SiNx membrane suspended on a silicon wafer is a popular two-
 dimensional platform in MEMS applications. The unsupported membrane has a low thermal 
 conductivity, is electrically insulated, and very robust against mechanical impact. Remarkably thin, 
 it is difficult to fabricate and manipulate. Recently equipped with a dual chamber system for plasma 
 enhanced chemical vapor deposition (PECVD) and reactive ion etching, we calibrate it to deposit 
 silicon nitride Si3N4, silicon dioxide SiO2, and to dry etch these materials. Based on the superb 
 quality of Si3N4, we perform a through-wafer etch that creates suspended Si3N4 membranes. The 
 recipe is reliable and reproducible. We analyze the membrane’s chemical composition and optical 
 properties. Although created by PECVD, the membrane is so robust that it survives multiple 
 lithography steps. It extends our capability to study thermal transport at the submicron scale as well 
 as to fabricate micron size devices for MEMS applications. 
 Keywords: Power MEMS, Silicon nitride membranes, PECVD, silicon wafer. 
________ 
 Corresponding author. 
 Email address: hungngq@hus.edu.vn 
 https//doi.org/ 10.25073/2588-1124/vnumap.4518 
 74 
 P.T. Hong et al. / VNU Journal of Science: Mathematics – Physics, Vol. 37, No. 1 (2021) 74-81 75 
1. Introduction 
 Micro-Electro-Mechanical Systems (MEMS) often require qualities that a traditional silicon wafer 
can not meet. For example, in thermoelectric applications, the cold part of a micro-refrigerator should 
be isolated from the environment, while the hot part should stay thermalized with the surrounding. A 
platform that separates two regions of different thermal conductance is vital for the device performance 
[1-3]. Silicon nitride membranes have low stress, low thermal conductance, are electrically isolated, and 
thus, are an ideal platform for such applications. It is the best platform to study thermal transport at the 
submicron scale [4-7]. Not limited to micro-cooling applications, the membrane is transparent under the 
high energy electron beam and is widely used in the transmission electron microscopy community [8]. 
The same recipe to fabricate the membrane can also be employed to fabricate other structures [9], such 
as pressure sensors [10], RF switches [11], or atomic force microscope cantilevers [12]. 
 Extremely fragile, only certain labs possess the capability to fabricate suspended membranes. There 
are two main challenges. First, the high aspect ratio: 100 nm thin and a millimeter square suspended 
area make it so fragile that the yield becomes very low, which requires a low-stress material. Second, 
through-wafer etching is a demanding process that needs a long etching time in corrosive chemical 
solutions. The deposited dielectric thin film should be strong enough to survive such a process. Among 
dielectric materials, Si3N4 is the traditional choice due to its high quality as a low-stress material [13] 
and its compatibility with nano-fabrication processes. 
 Silicon nitride SiNx and silicon oxide SiOx are the two most important dielectric materials to use 
with silicon wafers. They can be used as passivation layers, isolating films, or piezoelectric materials, 
to name a few. They are mostly created from Chemical Vapor Deposition (CVD) method, with quality 
depends on the detail approaches: plasma enhanced CVD, low pressure CVD, hot wire CVD, or atomic 
layer deposition. Among these approaches, low pressure chemical vapor deposition (LPCVD) produces 
Si3N4 of high purity, low stress, and low price and can be used for mass production [14, 15]. However, 
it requires a high temperature and would destroy any pre-existed structure on the wafer. Depositing 
Si3N4 using LPCVD has to be the first step of the fabrication, which limits the popularity of the method. 
To lower such temperature barrier, the deposition in a plasma enhanced chemical vapor deposition 
(PECVD) machine occurs with the help of a plasma environment [16, 17]. The depositing temperature 
is often below 400 ℃ and is compatible with a wide range of processes. The lower temperature, however ... ded Membrane 
 P.T. Hong et al. / VNU Journal of Science: Mathematics – Physics, Vol. 37, No. 1 (2021) 74-81 77 
 We use a solution of 20% KOH to etch Si at 80 ℃, which results in an etching rate of 1.8 µm/min 
[21]. To ensure anisotropy etching, 10% isopropyl alcohol IPA is added to the solution. Typically, it 
takes more than 5 hours to etch through the 275 µm thick wafer. In this paper, the etching apparatus is 
set up for a single chip of 1.5 x 1.5 cm2. A full wafer etching system is under construction. 
 Figure 2. Fabrication process for Si3N4 membranes: (a) cleaned silicon wafer. (b) Depositing Si3N4 on both side 
 of the chip. (c) Spin coating photoresist on both sides of the wafer. (d) Opening the etching window on the 
 backside of the wafer using photolithography. (e) Dry etching Si3N4 in SF6 plasma. (f) Removing photoresist 
 using acetone. (g) Wet etching in KOH solution. 
 The recipe to fabricate the suspended Si3N4 wafer can be summarized as the diagram in Figure 2: 
first, the silicon wafer is diced into 1.5 1.5 cm chips, and dip in HF for 30 s. Si3N4 thin films are 
deposited on both sides with identical parameters. Because KOH etches photoresist, the back-side Si3N4 
acts as a mask during this etching step. Photolithography patterns the backside of the chip with circles 
of 600 µm diameter. We then use reactive ion etching to open the SiNx windows from the backside. The 
chip is wet etched in KOH through this etching window until it is stopped by the top SiNx layer. 
 Optical properties of the SiNx thin films are measured using a spectroscopic ellipsometer. This 
measurement indirectly measures through the ellipsometric angles. It is necessary to create a model for 
our sample to deduce the sample parameter like thickness and optical constant. We used amorphous 
model to fit our SiNx. Here, the refractive index n and the extinction coefficient k of the material are 
described by these following equations: 
 2
 A(E − Eg)
 B .E + C E > E
 n(E) = 0 0 and k(E) = { E2− B.E + C g 
 E2− B.E + C
 0 E ≤ Eg
Where: 
 A B2
 B = ( − + E . B − E2 + C) 
 0 Q 2 g g
 A B
 C = [(E2 + C). − 2.E .C] 
 0 B g 2 g
 1
 Q= .√4C − B2 
 2
 Here, B0, C0, and Q are obtained from fitting parameters A, B, C. Eg is the band gap energy, A is a 
parameter depending on the dipole matrix squared and describe the strength of the extinction coefficient 
 2 2
peak. B = 2E0 and C = E0 + Γ0 are expressions of physical parameters. The experiment data must be 
fitted with the model of the sample before deducing the sample parameters [22]. This model allows us 
to calculate the band gap Eg of the SiNx thin films. We optimize the fabrication conditions for Si3N4 thin 
film based on this value. 
78 P.T. Hong et al. / VNU Journal of Science: Mathematics – Physics, Vol. 37, No. 1 (2021) 74-81 
3. Results and Discussion 
 Figure 3. (a) The band gap of SiNx deposited at different 퐹푅 3/퐹푅푆푖 4 measured using spectroscopic 
 ellipsometry. Si3N4 is obtained at 퐹푅 3/퐹푅푆푖 4=16. (b) Energy dispersive x-ray spectrum (EDS) of the sample 
 showing Si and N component. Cu peaks are an artifact from the copper tape used to immobilized the sample. 
 It is important to deposit SiNx with the correct ratio such that the stoichiometry is Si3N4, the most 
thermodynamically stable form. Our thin film’s band gap and its optical properties are obtained from 
fitting experimental ellipsometry spectra with models using the above-mentioned amorphous dispersion 
relation for the active layer. Figure 3 shows the dependence of the SiNx band gap on the flow rate ratio 
FRNH3/FRSiH4. The data with flow rate ratio from 0 to 9 is taken such that the flow rate of NH3 is kept 
constant at 80 sccm while the flow rate of SiH3 is changed from 10 sccm to 5 sccm. For flow rate ratio 
from 9 to 18, the flow rate of SiH4 is kept at 10 sccm, and the flow rate of NH3 is changed from 20 to 80 
sccm. It is clear that the increase of FRNH3/FRSiH4 resulted in an increase of band gap of SiNx. From 
this result, SiNx deposited with FRNH3/FRSiH4= 80 sccm/ 5 sccm has band gap of 4.394 eV, the closest 
to the band gap of Si3N4 [23]. This result is reconfirmed with an energy dispersive X-ray spectroscopy 
measurement EDS (data not shown). 
 The etching rate of Si3N4 using ICP RIE is an important parameter. Figure 4 shows the dependence 
of the depth of the wells as measured from the profilometer on RIE etching time. The SF6 plasma first 
etches the Si3N4 film. Upon completion, Si etching continues. These rates of the two processes are 
corresponding to two slopes in Figure 4. The etching rate of SiNx is 0.8 nm/s and the etching rate of Si 
is 9 nm/s with 50 sccm SF6 gas, 0.3 Torr pressure, capacitive coupled plasma power of 20 W, and 
inductive coupled plasma power of 200 W. 
 With a proper Si3N4 thin film and a correct etching rate, we fabricate the suspended Si3N4 membrane. 
Figure 5 shows optical images for some of our Si3N4 membranes. Miller indices create the plane in a 
silicon crystal as {100} correspond to the front view, {110} to edge view, {111} to vertex view. In the 
wet etching with KOH corrode silicon in {100} direction much faster than {111} direction as in the 
diagram shown in figure 5 (a). Wet etching in KOH is well-executed that selective rate at different 
crystal orientation is clearly demonstrated. The tilt angle as seen from the back side in Figure 5c is an 
exact 54.7 degree, agree with previous literature [24]. Furthermore, a circular photolithographic mask 
results in an octagonal shape in Figure 5b. 
 P.T. Hong et al. / VNU Journal of Science: Mathematics – Physics, Vol. 37, No. 1 (2021) 74-81 79 
 Figure 4. The dependence of the Si3N4 well depth on RIE etching time. The Si wafer is etched at a much faster 
 rate upon the completion of the Si3N4 layer. Here, the plasma power is 20 W at pressure 0.03 Torr and the SF6 
 gas flow is 50 sccm. 
 Figure 5. (a) Cross section diagram of the suspended Si3N4 membranes. The thickness of Si3N4 is 100 nm, while 
 Si wafer is 275 µm, (b) Top view of the Si3N4 membrane prepared with round photolithography mask, (c) 
 Bottom view of Si3N4 membranes after wet etching by KOH 20% solution, (d) and (e) ultra large membrane of 
 different rectangle sizes. 
 In this work, we produce membranes of three sizes: 1000 µm 300 µm, 600 µm 300 µm and 200 
µm 200 µm. To account for the etching angle, the photographic mask dimensions are 1400 µm 700 
µm, 1000 µm 700 µm, 600 µm 600 µm, respectively. The yield is listed in Table 1. Apparently, the 
larger the windows size, the lower the yield. It is worthwhile to note that a window’s size of 1 mm is a 
macroscopic scale that only required in special applications. To have a higher yield, it is better to 
fabricate membranes with a size less than 100 µm. 
 The silicon nitride membranes have an atomically flat surface as shown by the scanning electron 
 -
micrograph and atomic force micrograph in figure 6. Clearly, Si3N4 thin films are not corroded by OH 
ion during the wet etching process. The membrane is compatible with other nano fabrication process. 
Together with the low thermal conductivity and mechanical durability, it is an ideal platform for a wide 
range of applications. 
 Table 1. A list of membranes fabricated in this work. The Si substrate is 275 µm thick. The Si3N4 membrane is 
 100 nm thick. 
 Shape Dimension Figure Number of membrane made Yield 
 Octagonal 177.8 µm 177.8 µm 5b 14 51.9 % 
 Rectangle 282 µm 590 µm 5d 3 50 % 
 Rectangle 284.3 µm 977 µm 5e 1 16.7 % 
80 P.T. Hong et al. / VNU Journal of Science: Mathematics – Physics, Vol. 37, No. 1 (2021) 74-81 
 We have employed this membrane for two research directions in our group. In one application, we 
deposit conducting wire on the membrane and measure thermal conductivity using 3ω method [25]. 
Once the value for Si3N4 is known, we can determine the thermal conductivity of any thin film deposited 
on top [26]. Thus, it is a universal platform to measure thin film thermal conductivity. In another 
application, we fabricate thermoelectric cooler using Peltier effect in BiTe and SbTe [27], the most 
popular pair of the thermoelectric material. The hot part of the cooler is placed on the bulk Si wafer, 
while the cold metallic part is placed on the membrane. Due to low thermal conductivity, the cold part 
is isolated from the environment, and thus ensure the superiority of this cooler. This cooler is compatible 
with nano-fabrication and could control temperature locally at the micron scale. 
 Figure 6. (a) Scanning electron micrograph and (b) atomic force micrograph of the suspended membrane 
 showing an atomic flat surface. The thickness scale on the AFM image is on the order of 1 nm. 
4. Conclusions 
 Using plasma enhanced chemical vapor deposition and wet etching method, we successfully 
fabricated ultra-thin Si3N4 suspends membranes of various size and shape. Si3N4 thin films were 
deposited on silicon wafer with FRNH3/FRSiH4= 80 sccm/ 5sccm at 317 ℃ at 7 W power. The etch rate 
of SiNx and Si are 0.8 nm/s and 9 nm/s, respectively with the following dry etching condition: 50 sccm 
SF6 gas, 0.03 Torr pressure, CCP of 20 W, and ICP of 200W. For 275 m thick Si wafer, the etching 
time in KOH is about 5 hours. We successfully fabricated the Si3N4 membranes with different sizes, and 
the biggest size is 1000 µm 300 µm. The capability to self-support the suspended membrane has open 
new research direction for us, especially in power-MEMS. 
Acknowledgments 
 We acknowledge the support from the National Foundation for Science and Technology 
Development through Grant Number 103.02-2015.79. Samples were fabricated and measured at the 
Nano and Energy Center and Faculty of Physics, VNU University of Science. 
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