3C-Silicon Carbide Suspended Structures for
Harsh Environment Micro-Electromechanical
Systems
Samuel F Pfeffer-Matthews
School of Engineering, University of Warwick, Coventry, CV47AL, UK
Email: Sam.Pfeffer-Matthews@warwick.ac.uk
Abstract
The ambition of total integration of all variations of microelectromechanical systems (MEMS) (sensors, actuators, and structures)
has generated considerable research effort in recent years. The creation of
simpler fabrication methods helps support this goal and make it easier to
incorporate new materials into mass fabrication. This work investigates
the suitability of 3C-Silicon Carbide (SiC) MEMS devices and explores a
new cheaper fabrication process to create suspended structures for harsh
environment applications. In this process, structures are aligned to avoid
etch-resistive planes in making suspended regions while an underlying
silicon base remains to support the structures. This new technique is
demonstrated by forming suspended microwires, micro-nets, cantilever
beams, hall bars, spiderwebs and van der Pauw cross structures. This
suspension method aims to remove material that contains an underlying
misfit dislocation network which would otherwise provide the primary
source of electrical conduction in 3C-SiC grown on a silicon substrate.
Fabricated devices were observed using various microscopy techniques to
record the characteristics of 3C under suspension. A van der Pauw cross
was subsequently hall tested, and significant changes in the transport
properties of suspended 3C were observed at lower temperatures. This
work reports on the success of the new suspension method and illustrates
how 3C-SiC is a promising material for future suspended MEMS devices.
1
1. Introduction
Micro-electromechanical systems (MEMS) can sense, control, and actuate on the micro-scale. This
fast-developing technology has opened a whole new line of applications, and the total MEMS market is
forecasted to reach a value of US$18.2 billion by 2026 [1]. MEMS are widely employed in different
industries, including automotive, aerospace, natural gas extraction and space exploration, where hightemperature electronics are required [2]. In electric automotive applications, various locations provide
a wide range of operating ambient temperatures. For example, the coolant temperature can reach up to
120oC at 1.4 bar, the temperatures of wheel sensors and the transmission are around 150-200oC, and the
exhaust sensor can reach temperatures up to 850oC [3]. The aerospace industry is moving towards more
electric aircraft with the reduction or removal of hydraulic, mechanical, and pneumatic power systems
leading to an increase in the need for electrical sensors and actuators to improve the reliability of new
designs. In natural gas extraction, an electrical downhole gas compressor improves the output of gas
wells. It reaches an ambient temperature of 150oC since it is often installed close to the gas reservoir.
Such a system is expected to work reliably up to temperatures of 225oC with a lifetime of 5 years [4].
Furthermore, it is incredibly challenging to develop any form of electronics in space exploration. The
surface temperature on Venus can reach up to 460oC during the day but has an ambient temperature of
-140oC during the night [5]. Electronic equipment is required to work in harsh environments across an
extensive temperature range and with substantial thermal cycling in these applications.
In many engineering applications, large numbers of thermal solutions have been designed to cool
electronics in harsh environments and manipulate their operating temperatures. However, not only are
these solutions not effective in many scenarios, but they bring issues of undesired high costs, extra
weight, and extra volume. Moreover, there is often a lack of accurate thermal analysis for each
electronics component due to limited information on the actual operating environment and load cycling.
Thus, a failure of the cooling system can readily destroy a complete electronic design. It would be of
great benefit to have electronic components capable of enduring harsh environments themselves, as this
would reduce the upfront and operating costs of a system. Consequently, a significant amount of
research efforts has been focused on developing high-temperature electronics [6].
Silicon (Si) based systems are generally limited in their use for harsh environment applications due
to its narrow bandgap, ease of oxidation, corrosion, and degradation. Silicon has a limited operating
temperature range and is susceptible to being etched at high temperatures by reactive media, causing
decay in its mechanical strength [7]. Silicon Carbide (SiC) is recognised as a top material for
microfabricated sensors and actuators designed to replace Si-based devices in harsh environments due
to its excellent mechanical, electrical and chemical properties. Thinner drift layers and higher electrical
doping fields in SiC reduce a MEMS resistance compared to Si. Additionally, Higher thermal
conductivity, electrical field strength, and drift velocity significantly impact the size, efficiency, and
applications of SiC MEMS [8].
2
The developments in the 3C-SiC fabrication process and some commercial products based on 3C-SiC
have already shown the importance of SiC in high power and high-temperature devices for extreme
environments. The ability to produce 3C-SiC on Si facilitates the construction of SiC MEMS structures
using Si micromachining techniques, making 3C-SiC an exceptional material for power electronics and
MEMS devices that can work in harsh environments. However, one significant problem with devices
made from 3C-SiC materials is that large amounts of electrical leakage from material defects exists
which stops device action. To enable harsh environment MEMS, this problem can be tackled at a
processing level. This project proposes that leakage paths of extended defects can be isolated through
the freestanding suspension of 3C-SiC on Si.
One of the most common suspended MEMS components is the semiconductor membrane [9].
Fabrication of such membranes often starts from a silicon-on-insulator substrate which is subjected to a
combination of wet and dry etching processes. However, these membranes often become warped and
corrugated during fabrication [10]. This project thus suggests a new, cheaper and simpler fabrication
process to create suspended structures for harsh environment applications. In this report, 3C-SiC
suspended designs are fabricated using recently developed under etching techniques. These MEMS
structures are analysed through microscopy, electrical characterisation, and hall measurements over an
extensive range of temperatures. It is hoped that the manufacture and verification of these structures
shall form a baseline for the design and fabrication methods of new SiC micro-technology for harsh
environment electronics.
3
2. Background Research
2.1 Material Properties of 3C-SiC
More than 250 different polytypes of SiC exist [11], distinguished by differences in the stacking
sequence of the identical planes of silicon and carbon atoms. The basic structural unit consists of a
covalently bonded tetrahedron of a silicon atom surrounded by four carbon atoms (or one carbon atom
surrounded by four silicon atoms). The primary commercially available polytypes relevant to SiC device
applications are 4H-, 6H- and 3C-SiC. Out of these, 3C-SiC is the only polytype that can be synthesised
on Si substrates [12] because it is a stable low temperature polytype of SiC, while 4H- and 6H-SiC are
typically grown at temperatures above the melting point of Silicon (1400oC) [13]. This enables 3C-SiC
to be grown on large-area substrates which has led to it being the dominant polytype for MEMS
applications.
The high melting point and chemical inertness of 3C-SiC as well as the ability to selectively etch
away the Si substrate to form freestanding structures, gives this polytype a unique advantage over other
materials. Given its wide bandgap, the polytype has excellent thermal stability at high temperatures and
the high thermal conductivity of SiC permits rapid dissipation of heat which minimises localised heating.
3C-SiC exhibits a thermal conductivity 3 times higher than Si at 300K [14] and thus even highly
defective samples of 3C-SiC have unique applications in harsh environment MEMS [15].
Silicon carbide has a long history of being grown for a vast array of applications, starting with the
first LED, and then progressing to bulk wafer growth in the 1950’s [16], and finally in 1983 the
pioneering work of S. Nishino et al first demonstrated the growth of high-quality single-crystal 3C-SiC
on Si substrates [17]. Unlike the 4H- and 6H- polytypes, 3C-SiC can be heteroepitaxially grown on Si
substrates by Chemical Vapour Deposition (CVD) processes. This is possible because 3C-SiC and Si
have similar cubic structures with a lattice mismatch of around 20%. Heteroepitaxy overcomes the large
mismatch by carbonisation, which is used to form a thin 3C-SiC film directly on Si substrates, resulting
in a SiC/Si heterojunction that establishes good electrical insulation [18]. Batch fabrication is thus
possible since high quality and large area Si substrates are readily available at a low cost. A further
advantage of heteroepitaxy is that it provides control over the SiC membrane thickness and such that it
can be used as a platform for other heterostructures [9].
However, despite its very favourable material properties, 3C-SiC’s performance when used in making
high-temperature electronics is hindered by the presence of defects. The large mismatch in lattice
parameters and thermal expansion coefficients (of around 8%) between 3C-SiC and Si is blamed for
generating a high number of defects, such as misfit dislocations, twin and stacking faults at the interface
[19]. These defects alter the crystal structure of the samples and can modify the elastic properties of the
materials and cause large leakage current in power devices. Surface morphological defects, such as
triangular defects, are also created when epitaxial layers are grown at high C/Si ratio conditions. Such
4
triangular defects are shown on a sample of 3C-SiC in figure 1, indicating their size. While the exact
cause of these defects is not entirely known, they are generally thought to be formed because of technical
problems such as polishing damage or non-optimised growth conditions [20]. When SiC structures are
fabricated on technology killing defects such as these, they demonstrate a significant increase in leakage
currents and decreased breakdown voltages within power devices [21]. Furthermore, anti-phase
boundaries (APB) are important 2D extended defects affecting the properties of 3C-SiC grown on (001)
Si wafers. They are related to the formation of crystallographic domains. In literature APBs are also
called inverted-domain boundaries and they are the main defects responsible for the short circuit of
devices, under both reverse and forward bias polarisation [22]. This work reports that, through new
under-etching techniques, suspended structures of 3C-SiC could allow for the removal of the 3C-SiC/Si
interface, and with it much of the misfit dislocation network and APBs, thus limiting the electrical
leakage.
Figure 1: 500nm layer of 3C-SiC grown heteroepitaxially on Si substrate demonstrating
large triangular defects (post mesa etch)
2.2 Electrical Properties of 3C-SiC
Silicon Carbide has long been noted for its outstanding electrical properties, varying slightly between
polytypes. Compared to Si, SiC has a wider bandgap, a higher breakdown voltage, a higher saturation
drift velocity and a lower dielectric constant [13].
A complete comparison between the two is illustrated in Table 1. All the values in the table are
temperature dependent to a differing extent. The intrinsic carrier concentration 𝑛 has an exponential
5
dependence upon temperature due to electron-phonon coupling and is a significant quantity in hightemperature electronics as pn junction leakage currents in devices are generally proportional to 𝑛 [14].
Electrically active impurities in semiconductors are typically substitutional dopants, occupying
vacant lattice sites. Dopants for 3C-SiC include Nikel (n-type) and Aluminium (p-type) however,
undoped SiC is still typically n-type from residual nitrogen. For n-type 3C-SiC, Hall measurements have
yielded nitrogen activation energies from 50meV [23]. The fact that most dopant levels are deeper than
those in Silicon explains why a partial carrier freeze-out is observed in SiC at room temperature since
the thermal energy is only approximately 25.9meV at 300K.
Table 1 Comparison of measured electrical properties of 3C-SiC with Si [14]
Si
3C-SiC
1.12
2.4
0.3
> 1.5
Thermal Conductivity 𝑘 [W/cmK]
1.31
3.2
Intrinsic Carrier Concentration 𝑛 [cm3]
9.65× 10
Bandgap Energy 𝐸 [eV]
Critical Electric Field 𝐸 [mV/cm]
@𝑁 = 10 cm-3
Electron Mobility 𝜇 [cm2/Vs]
@𝑁 = 10 cm-3
Saturated Electron Velocity 𝜐 [107cm/s]
1.5 × 10
1430
800
1
2.5
The favourable measured electrical properties are the attractive features of 3C-SiC over metals when
it is doped with suitable impurities. These cause it to have excellent sensing properties even at very low
doping concentrations and hence why the material is often being used in MEMS applications [24].
However, the modelling of electrical parameters of 3C-SiC proves to be significantly more complex
than the other main polytypes [25]. This is because no 3C-SiC substrate exists, and all tested samples
tend to be heteroepitaxially grown on Si or 6H-SiC, resulting in significant lattice mismatches and high
defect densities. Additionally, given that the use of 3C-SiC is still in its infancy, there is little commercial
drive to establish 3C models. Currently, the most frequently used model to describe mobility
6
mechanisms and dependences in 3C-SiC is the Masetti model [26]. Figure 2 gives the carrier mobility
dependence in 3C-SiC against temperature for different doping concentrations; the solid lines
correspond to the Masetti model while dashed lines show the dependencies for the same concentrations,
utilising experimental data.
Figure 2: 3C-SiC Carrier Mobility dependence on Doping Concentration from 10-1000K [26]
At lower temperatures, carriers within the 3C-SiC move more slowly, so there is more time for them
to interact with charged impurities throughout the crystal structure. Therefore, with a decrease in
temperature, one observes a decrease in mobility as impurity scattering increases. At higher
temperatures, lattice vibrations cause a decrease in mobility with increasing temperature. The total
mobility is derived from the sum of these contributions [27]. Such trends shall be vital to compare with
characterisation measurements conducted in this report.
7
2.3 Process Methods for 3C-SiC MEMS
Effective fabrication routes play key roles in the realisation of 3C-SiC MEMS. The advantageous
material properties of SiC form a cornerstone for SiC MEMS as well as fabrication challenges. Thus,
unlike well-established Silicon microfabrication techniques, there is still a strong requirement for
efficient and cost-effective micromachining processes for SiC MEMS [12]. Suspending the 3C-SiC
layer of a MEMS device allows for the isolation of leakage paths of extended defects and is something
which has been briefly explored in current literature.
MEMS device process methods often employ a secondary material that does not contribute to their
structure but acts as a sacrificial material in the manufacturing flow. Zawawi et al. [28] created a
suspended SiC diaphragm for use in a MEMS microphone to detect poisonous gasses. The 3C-SiC film
was epitaxially grown on both the top and bottom of a Si substrate. The top, polished side was designated
to be the acoustic diaphragm, while the unpolished layer and the Si substrate acted as a sacrificial
material to be removed. First, a photoresist layer was spun onto the top surface before being baked twice
to harden the resist. Using a mask aligner, it was then exposed to UV light for 35 s to pattern and soften
the area of the photoresist under the mask. The unpolished 3C-SiC was removed using a reactive ion
etching (RIE) process, and the suspended structure was created by back-etching part of the Si substrate
by immersing it into Potassium Hydroxide (KOH). This back etching process took approximately 15
hours at a temperature of 80oC. This paper illustrates a prevalent processing route for 3C-MEMS;
however, while the final devices were successful, the etching of the backside of a wafer takes a
significant amount of time. Instead, micromachining from the front surface is the favoured route to
incorporate MEMS alongside CMOS and other devices, where processing consists of defining the
structural boundaries of the device active layer and then removing the material from under this layer
[29].
One method of fabricating suspended structures without back-etching is through dry etching of the
top surface using a combination of isotropic and anisotropic etch steps to fabricate a device [19].
However, this requires multiple expensive processing steps to create different mask layers that require
high temperatures.
A cheaper method involves utilising cheap wet etchants that selectively remove sacrificial layers due
to their high etch selectivity [30]. Anisotropic etchants such as tetramethyl-ammonium hydroxide
(TMAH) or potassium hydroxide (KOH) have been used to create suspended structures by an etch
against the etch-resistive {111} planes in Si(001) substrates [29, 31, 32]. This more straightforward and
cheaper method has been verified using Germanium, Silicon and Gallium Nitride devices. Still, there is
no evidence in the literature to date of the fabrication of 3C-SiC devices using this process. 3C-SiC is
extremely chemically resistive against KOH and TMAH and so is a potentially excellent material for
this type of micromachining.
8
2.4 Electrical Characterisation of SiC Devices and Hall Effect Measurements
The Hall Effect provides a simple method of accurately determining the electrical resistivity, carrier
mobility and carrier density in semiconductor devices. The Lorentz force provides the underlying
physical principle of the Hall effect: when an electron travels perpendicular to an applied magnetic field,
it is deflected by the Lorentz force; this causes a voltage to build up perpendicular to the current and the
magnetic field, as illustrated in figure 3 [33].
Figure 3: Illustration of the Hall Effect [59]
This traverse voltage is known as the Hall voltage 𝑉 and it has magnitude equal to:
𝑉 =
𝐼𝐵
𝑞𝑛𝑡
Where 𝐼 is the current, 𝐵, is the magnetic field, t is the sample thickness, and 𝑞 is the charge on one
electron. In many cases, one uses sheet density 𝑛 = 𝑛𝑡 instead of bulk density. One thus obtains the
equation [33]:
𝑛 =
𝐼𝐵
𝑞 |𝑉 |
Thus, by measuring the hall voltage from known current and magnetic field strength values, one can
determine the sheet density of charge carriers in semiconductors.
The van der Pauw method is a common technique to measure a semiconductor device's resistivity. It
employs a four-point probe placed around the structure's perimeter, allowing it to provide the average
resistivity where other methods only provide a linear resistivity in the sensing direction [34]. Given that
both sheet density and mobility are involved in the sheet resistance 𝑅 , one can determine the charge
carrier mobility within a semiconductor from the equation:
𝜇=
1
𝑞𝑛 𝑅
9
2.5 Recent Applications of 3C-SiC MEMS
3C-SiC MEMS are being used in large amounts of new applications and devices. Advanced SiC
sensor systems have been developed to detect pressure, acceleration, and radiation, particularly in high
temperature and high-pressure environments. Being a more robust material than Si, SiC shows superior
tribological properties and thus can be used to realise micromechanical devices such as micromotors
and microresonators. Furthermore, the wide bandgap in 3C-SiC makes it useful for a wide range of bioengineering opportunities.
In the drive to miniaturise oscillator components, MEMS resonators are excellent candidates to
replace quartz crystals [35]. 3C-SiC has been used as an excellent structural material when operating in
a harsh environment due to its robustness, chemical inertness, and temperature stability. SiC MEMS
resonators can also act as filters over a wide tunable range and thus can be used to replace filter banks
in multiband communication systems and wide-band tracking receivers [36, 37]. The frequency stability
of SiC resonators is comparable to quartz crystals and due to its favourable electrical and material
properties, 3C-SiC has been frequently used for developing a variety of resonating structures.
Electrostatic, electrothermal and piezoelectric actuation methods have been employed to create SiC
MEMS oscillators, mixers, and filters.
The SiC pressure sensor is one of the main application areas of SiC MEMS as it is used to extend the
usefulness of diaphragm-based sensors to harsh environments. Pressure sensor signals can be produced
through various methods, including piezoelectric, piezoresistive, capacitive, and strain gauges. Most
current literature focuses on the MEMS capacitive pressure sensor because it provides higher
measurement sensitivity, decreased temperature sensitivity, and reduced power consumption over its
piezoresistive counterparts [8]. MEMS pressure sensors manufactured using silicon carbide are more
robust than their silicon equivalents and can operate at temperatures up to 500oC. 3C-SiC MEMS have
been developed to monitor performance inside gas turbines which ensure a reduction in degradation and
wear of components [38]; a pressure sensor, an anemometer and a temperature sensor have been
integrated into a singular SiC MEMS structure for use on Venus missions [39]; and NASA is currently
investigating the design of engine-compatible SiC smart sensors for their propulsion systems [40].
Additionally, SiC-based nanotubes have recently gained much attention. The SiC nanotubes have higher
reactivity than the carbon nanotube and the boron nitride nanotube due to SiC’s high polarity nature.
Thus, they serve as a better sensor to detect CO2 and NO2 [41].
SiC biomaterial plays an essential role in the field of bio-engineering due to its excellent density and
thermomechanical properties. The wider bandgap increases the sensing capacity of the material, its
chemical inertness prevents the material from corrosion, and its mechanical properties can be altered by
changing the sintering additives. For instance, the advantageous properties of boron and carbon doped
SiC come from the lack of grain-boundary phases, making it a promising candidate material for
10
biomedical implants [24]. The 3C-SiC polytype has also shown excellent in vitro compatibility with
fibroblasts, skin cells and neural cell lines [42] and has led to the development of biocompatible glucose
sensors [16].
The arising issue of the energy crisis and pollution has attracted much of current literature towards
using hydrogen as a future renewable energy source. The current industrial production of hydrogen emits
large amounts of greenhouse gases. Still, new solutions such as photocatalytic water splitting technology
are very successful in the existing literature [43]. Several materials have been used for photocatalytic
water splitting and are still being investigated. Unfortunately, most semiconductor materials that enable
efficient photoelectrolysis easily suffer from corrosion in water [44]. To create efficient photocatalysts,
the semiconductor chosen must have a bandgap between 1.23 and 3.1 eV and the valence and conduction
bands need to satisfy the thermodynamic requirements for oxidation and reduction [45]. Thus, 3C-SiC
has been seen to be the perfect candidate that meets the requirements while showing excellent physical
and chemical properties [16, 24].
Finally, 3C-SiC has seen its use in wastewater management, a growing challenge today. The oily
waste from different industries pollutes the soil, water, and air. While many techniques have been
utilised to remove emulsions, these techniques come with high operating costs and low efficiency in
treating stable emulsion. Instead, membrane filtration offers high efficacy and moderate costs and is
much more compact than current treatment methods [46]. The development of SiC membranes has seen
an increase in device lifetime and higher porosity in structures [47].
11
3. Fabrication and Testing of Suspended Structures
3.1 Structure Design
Many different structures were designed for this project, each based on those currently seen in many
modern sensors. Some structures were also fabricated to investigate the behaviour of 3C-SiC under
suspension. Many of the devices to be hall tested included large metal contacts, which acted as supports
for many of the suspended membranes. The square contacts further allowed for better etching profiles
when forming a mesa layer.
Seven van der Pauw crosses were designed, four 200μm in diameter and three 350μm in diameter,
varying in thickness from 5μm to 50μm. While traditionally used for resistivity measurements, Van der
Pauw crosses have also been shown to provide more than three times the sensitivity of standard resistor
sensors as they utilise the high-accuracy four-wire resistance measurement method [48]. The van der
Pauw cross thus shows excellent promise in MEMS sensing devices that demand even smaller
transducer sizes. The structure’s resistance is independent of its size and depends only on its thickness.
This, combined with the cross’ simple geometry, enables significant miniaturisation of the sensor’s size
[49].
The use of a 6-contact Hall bar improves the accuracy of hall measurements of 3C-SiC. A current is
applied across the Isource and Idrain and the Hall voltage arising due to a magnetic field perpendicular to
the surface is measured as shown in Figure 1. The four-terminal resistance of the central channel is then
measured by current biasing the device and measuring the potential difference between V1 and V2, thus
neglecting the contact and lead resistances.
Figure 4: Six-Contact Hall bar Design. Labelled with measurement variables
Two spiderwebs of varied sizes were suspended. Such constructs are optimal for nanomechanical
resonators used in micro-sensors. When vibrating in the megahertz range, almost no energy is lost
outside of the spiderweb since the vibrations move in a circle within the structure and never touch the
outside. This property leads to excellent isolation from external noise [50]. The designs used in this
12
project were taken from the previous work of Shah et al. [29], which verified their fabrication using
Germanium. All features of the spiderweb are rotated and patterned off of the surface <110> directions
while the frame is anchored to the substrate. This misalignment allows for the under etching of the web
while the frame resists the TMAH etching process. Metal is lined along the longest legs of the web to
investigate their strength to see whether future devices can be placed upon the centre of the suspended
webs.
SiC nanowire devices were designed to study their suspension behaviour. By lining some of these
structures with metal on either side, it was hoped that one would be able to observe the effects of the
metal on the SiC and whether it caused them to curl downwards. Cantilever beams were also added to
test whether the 3C-SiC could support its own weight upon suspension. Many of these individual
cantilevers were lined with metal support structures to investigate the type of stresses that the metal
would apply to the 3C-SiC.
Finally, three different flexible suspended micro-nets were fabricated using a new tessellating design.
A close-up of the repeating design is shown in Figure 2, in which an inner rectangular hole is rotated at
45 degrees. When this structure is TMAH etched, the outer perimeter makes up the border of the planes
which shall be under-etched. This design would investigate the potential of creating large flexible
membranes of 3C-SiC and observe their durability during the fabrication process.
Figure 5: Tessellating design used to form suspended micro-nets [58]
13
Figure 6: Complete Mask Design used to fabricate Suspended Structures
3.2 Fabrication
3C-SiC was epitaxially grown on a 525 μm thick Si (001) substrate through reduced pressure chemical
vapour deposition in an LPE ACIS M8 reactor. N-type doping with nitrogen was obtained in the top
100nm of 3C-SiC by introducing N2 into the reactor chamber (Figure 7 Step 2). These films were used
for half of the samples fabricated, while the other half consisted of undoped 3C-SiC.
A 1.8 μm S1818 positive photoresist layer was spun onto the samples at a rotation speed of 500 rpm
for 10s, followed by a second spinning at 4000 rpm for 30s. The samples were then baked at 100 oC for
2 minutes to harden the photoresist. The metal layer of the mask shown in Figure 6 was then used in an
MA8 mask aligner, and the samples were exposed to UV light to soften the area of the photoresist under
14
the mask. The exposed samples were then immersed in MF-319 developer for 40s to remove the softened
photoresist.
Following a hydrofluoric acid (HF) clean to remove any oxides, 150 nm of Nickel (Ni) was deposited
onto the samples using an electron beam evaporator. Any unwanted Ni was lifted-off as the photoresist
was removed in an acetone bath, leaving square metal contacts (Step 3). These contacts were annealed
in a furnace at 1000oC for 2 minutes in order to stabilise them and reduce their resistance.
The lithography process was then repeated in an identical manner using the mesa layer of the
photolithography mask. 350 nm of Aluminium Oxide (Al2O3) was evaporated onto the samples to form
an etch mask, patterning each individual structure onto the sample (Step 4). A Reactive ion etching
process (RIE) using Sulphur Hexafluoride (SF6) and Argon (Ar) etched the patterns into the 3C-SiC at
an etch rate of 250 nm/min (Step 5). Any remaining Al2O3 was cleaned off each sample by immersing
it in HF for 30 minutes.
Bulk samples which were not suspended were also fabricated using an identical process. These were
to be used as a control to compare with the suspended structures.
Finally, the samples were etched in a 25 wt% TMAH bath at 80oC for 100 minutes to suspend the
structures (Step 6). Surface-orientated <100> features are completely under-etched while surfaceorientated <110> features leave various etch resistive planes. The under-etching rate of this process was
found to be approximately 0.6 μm/min. Once suspended, it was important to ensure that the structures
would not break. One potential risk derived from the fact that the surface tension of water left on the
sample after washing could pull on and destroy suspended 3C-SiC. To avoid this, samples were
immersed in isopropanol which displaced any water remaining underneath the structures. The samples
were left to naturally dry for a significant period to allow the isopropanol to evaporate away.
Figure 7: Summary of fabrication process to create Suspended Structures
15
3.3 Transmission Line Measurements
The contacts used for the suspended structures are ideally ohmic and with little contact resistance.
Ohmic contacts have linear current-voltage characteristics and should not inject minority carriers into
the substrate [51]. With the annealing processes utilised in this project, one would expect the Nickel
contacts on 3C-SiC to have a resistivity in the range of 10
to 10
Ω 𝑐𝑚 [52, 53].
Transmission line measurement (TLM) is a technique used in this project to determine the contact
resistance between the metal and the 3C-SiC involving the design of a series of metal-SiC contacts
separated by different distances. Probes were applied to pairs of contacts, and I-V curves were produced
across different contacts over a range of -2 to 2 V. The resistance of each pair of contacts was derived
from these curves. The total resistance calculated is a sum of the contact resistance of the first contact,
the contact resistance of the second contact and then sheet resistance of the 3C-SiC material between
them.
𝑅 = 2𝑅 + 𝑅
A typical arrangement for a TLM test pattern is shown in Figure 8. Several measurements are made
at different lengths along the TLM design, and a linear graph of resistance versus contact separation is
plotted. The y-intercept of this line gives twice the contact resistance.
Figure 8: Typical TLM design. A single rectangular (blue) semiconductor region has
the same sheet resistance as the contact areas of the suspended structures. An
array of contacts (grey) is formed over the doped region with various spacings [54]
In this report, contact resistance measurements were taken on both doped and undoped samples of
3C-SiC. Within each TLM, there were seven 400μm wide contacts spaced at distances from 11μm to
86μm.
16
Calculating the contact resistivity 𝜌 provides a standard quantity to compare the quality of contacts.
However, despite knowing the physical area of the contacts, this is often not the same as the effective
contact area. This is because current does not flow uniformly into a contact, but instead, current
crowding occurs at the edges. This phenomenon is displayed in figure 9: as one moves away from the
contact edge, the current drops off exponentially with a characteristic length 𝐿 known as the transfer
length [51]. While the current flow remains uniform through the semiconductor, it is not uniform when
flowing into the contacts, and thus we can’t use the physical length 𝐿 and width 𝑊 to determine the
contact area.
Figure 9: Current Crowding Phenomenon observed within a TLM Design [54]
We can calculate the effective contact area using the transfer length 𝐿 . It is equal to the average
distance a charge carrier travels in the semiconductor beneath the contact before flowing up into it. It is
given by:
𝐿 =
𝜌
𝑅SiC
The effective area of the contact can thus simply be treated as 𝐿 𝑊. The contact resistance is then
[54]:
𝑅 =
𝜌
𝑅 𝐿
=
𝐿 𝑊
𝑊
And the total resistance measured is thus equivalent to:
𝑅 =
𝑅
(𝐿 + 2𝐿 )
𝑊
Hence, on a graph of contact separation versus resistance, the x-intercept gives twice the transfer
length 𝐿 which can be used to find the contact resistivity.
3.4 Hall Measurements
The aim of using Hall measurements and the Van der Pauw technique is to determine the sheet carrier
concentration by measuring the Hall voltage 𝑉 . From these measurements, one can calculate many
properties of the suspended structures, such as carrier mobility and sheet resistance, and thus compare
17
them to their bulk equivalents. A current is applied through opposing contacts on a structure, and the
Hall voltage is measured across the remaining pair of contacts.
In this project, a Hall Kit was used, photographed in figure 10, which enabled testing over a range of
temperatures from 300K down to 40K. Such testing would enable the observation of carriers being
“frozen out” to see what other underlying transport mechanisms could be prominent within the 3C-SiC.
Figure 10: Hall Kit, School of Engineering, University of Warwick, Coventry
Each sample is loaded into the central chamber before a turbopump brings it down to a near-vacuum.
Helium is then pumped through the chamber to adjust the temperature to the desired point before Hall
measurements are taken through the control units pictured. The entire process is monitored and
controlled using LabView scripts.
Initially, a given 4-contact structure (figure 11) is checked for internal consistency and measurement
repeatability. A current is applied through the sample through two in-line leads (1 to 2 or 3 to 4), and
the voltage is measured across the remaining two contacts. By rotating the sample contacts and switching
the current direction, eight voltage measurements give eight resistance values. One checks for
measurement repeatability by seeing if measurements of, for example, 𝑅
,
and 𝑅
,
are consistent
given current reversal should yield the same resistance measurements. Then, the sheet resistance of the
sample can be determined from two characteristic resistances [33]:
𝑅 = (𝑅
,
+𝑅
,
+𝑅
,
+𝑅
,
)/4
𝑅 = 𝑅
,
+𝑅
,
+𝑅
,
+𝑅
,
/4
18
The van der Pauw equation [34] then gives the sheet resistance as:
𝑒
+𝑒
=1
which can be solved numerically for 𝑅
Figure 11: Sample 4-Contact van der Pauw Cross
for Hall Testing
Hall measurements with a magnetic field yield carrier concentration and mobility values as described
earlier in this report. A magnetic field B is applied through the structure; a current is passed through two
opposing contacts e.g. 𝐼 ; and a hall voltage is measured across 𝑉 . Once again, through rotating the
sample and switching the direction of the field, eight measurements of hall voltages are recorded, which
are used to determine the transport characteristics of the device.
There are a few practical aspects to be considered when conducting these measurements. Firstly, the
quality of ohmic contacts on each sample is of great importance and the wires which bond them to the
testing board. While this project verifies the use of Nickel contacts with transmission line measurements,
the quality of wire bonds made to each contact is difficult to quantify and may lead to some offset hall
voltages. To control this problem, two sets of hall measurements are required for both positive and
negative magnetic field directions. Secondly, the uniformity of the sample is essential for accurate
results, and thus only samples of the highest quality were chosen to be hall tested. Finally, to avoid
thermomagnetic effects due to a non-uniform temperature, all samples were mounted onto copper blocks
before being placed into the hall kit. This ensured a consistent temperature distribution across each
sample tested.
19
4. Results and Discussion
4.1 Verification of Suspended Structures
Upon completion, photos of each suspended structure were taken through a polarised optical
microscope. White light interferometry was utilised to observe the suspension properties of specific
structures. Additionally, SEM inspection of tilted samples provided insight into the repeatability of the
suspension process and the strain imparted by the Ni on each structure. This section of the report
explores the studies of each device after suspension.
4.1.1 Van der Pauw Crosses
The fabrication of each VdP cross was very successful, with the suspension of even the smallest
crosses. Figure 12 a) illustrates the high quality of the structure’s edges; it is seen how the Ni contacts
served as a self-aligning etch mask to provide sharp edges for each structure. When viewed under the
SEM, one observes hexagonal shapes beneath each contact, each showing the remaining Si that anchors
the structure to the underlying substrate. By tilting the sample, the remaining plinths of Si are also visible
towards the top of each contact. This verifies the success of the TMAH at etching along the crystal
planes of the Silicon.
A contour map of this device is given in figure 12 c) and shows how the suspended structure is flat,
showing very little bowing. Each cross can support its weight and does not sink in the middle. However,
the contacts here are seen to “flop” at the edges where the underlying silicon anchor no longer supports
them. To ensure there is enough Si under each contact, future devices would need to be fabricated with
a more significant consideration over the length of time each sample is left in the TMAH bath. Larger
VdP crosses may also require larger contacts so that the SiC can be completely under-etched while still
ensuring that there shall be enough anchoring silicon under each contact.
20
Figure 12: Suspended van der Pauw Cross (200x10m) a) Optical Microscope b) SEM c) Contour Map
4.1.2 Suspended Spiderwebs
The larger of the two spiderwebs designed was successfully suspended with metal tracks running
through to the centre of the design so that it can be used as a potential integration platform. Where parts
of the frame have been destroyed, the spiderweb isn’t supported and has sunk slightly, as shown in figure
13 c), yet the remaining structure is seen to be suspended with no bowing. Under SEM, one observes
that the SiC on this particular structure is missing on one of the legs of the web (seen in the bottom left
of figure 13 b)). The metal lining is strapping the structure together here, and the web still maintains
structural stability despite the gap in SiC. Such a feature could be used to one’s advantage in future
designs, applying this to all legs of the spiderweb to isolate the SiC entirely. Testing would be required
to see if suspension is possible with just metal straps on all anchor points of the web. It is likely that
better adhesion methods for the Ni would also be needed for a successful design utilising this new
feature.
It is worth noting that there were adhesion issues with the alumina etch masked used in the fabrication
process. Namely, the lift-off processes failed to produce clean patterning for RIE etching. This meant
that the smaller spiderweb designed (4m) was not successfully fabricated. Furthermore, remaining
alumina stuck on samples acts as an insulator when in the SEM, leading to distortion in some photos
and measurements taken on the machine. This is further discussed in the following sections.
Figure 13: Suspended Spiderweb (12m) a) Optical Microscope b) SEM c) Contour Map
21
4.1.3 Cantilever Beams and Nanowires
Figure 14: Suspended Cantilever Beams without Ni a) Optical Microscope b) SEM c) Contour Map
d) SEM at larger magnification showing edge profiles
The suspended cantilever beams provided insight into the effects of Ni placed onto each sample as
well as the suspension properties of the 3C-SiC.
Plain suspended beams without Ni are shown in figure 14, which illustrate the strength of the SiC
under suspension. Only the longest beams bend down slightly at the ends, while shorter cantilevers are
almost perfectly flat. Figure 14 d) shows a larger magnification of the cantilever beams and reveals that
their profile is not completely clean and contains many rough edges. While such small features are
difficult to perfect in a university cleanroom, one potential improvement to the lithography process
would be to add an additional bake to the photoresist after it has been developed. This would cause the
photoresist to reflow after patterning, smoothing out the edges of features at risk of causing potential
resolution losses.
Cantilever beams lined with Ni exhibited different suspension properties. The metal is seen to implant
tensile strain on the SiC, pushing it apart. This thus causes it to bend downwards, as observed in figure
15. The same observations are also seen on many of the contacts of other structures. Further research
could analyse the quantitative amount of stress imparted onto the SiC and the bending strength of these
22
MEMS beams. This would require using a mathematical cantilever model and large deflection beam
theory to compare theoretical images with the ones seen under the SEM [55].
Figure 15: Suspended Cantilever beams lined with Ni a) Optical Microscope b) SEM
Another interesting observation was made with cantilevers that were not patterned off the surface
<110> directions, unlike those already displayed. In theory, the TMAH should not have under-etched
these structures, yet one sees their suspension in figure 16. A potential explanation is that the triangular
defects within the material have enhanced the etching process such that features have been under etched
where they shouldn’t have been.
Figure 16: Cantilever Beams that are not patterned off the
silicon crystal planes yet have still been suspended
Finally, nanowires were designed and lined with Ni to observe their behaviour. While the wires were
successfully suspended and showed no bowing in the SiC, it was hoped that the metal on such designs
would cause them to fold in on themselves or exhibit other interesting properties. However, this was not
the case as seen in figure 17 where the wires remain entirely flat.
23
Figure 17: Suspended nanowires
4.1.4 Suspended Micro-nets
All sizes of the micro nets were suspended successfully, with no deformities seen in the SiC. The
mishappen contacts of the tilted sample under SEM further demonstrates the stress imparted by the Ni
onto the SiC. Despite this, these membranes show great promise for use in flexible electronics.
One advantage exhibited by the micronets is their resistance to cracks and other defects. A crack
formed in this design does not propagate through the entire structure but is terminated at a subsequent
hole. This phenomenon is shown in figure 19 in which a small crack from one edge is soon terminated,
and the device holds its structural integrity.
Figure 18: Suspended Micro-net (24m) a) Optical Microscope b) SEM c) Contour Map
24
Figure 19: Crack propagation through a suspended micro-net
4.1.5 Further Reflection on the Fabrication Process
A few things could be improved upon within the fabrication process. When depositing Ni onto samples
for contacts, a lack of adhesion was observed between the Ni and the 3C-SiC. This led to many structures
becoming unusable quite early on in their assembly. While the current process utilises an electron beam,
magnetron sputtering is another way to create very dense contacts with better adhesion. Sputtering is a
plasma-based coating method that generates a magnetically confined plasma near the surface of the
metal. Positively charged ions from the plasma collied with the negatively charged Nickel, and atoms
of Ni would be ejected onto the sample [56]. It provides a much more uniform coating of the sample
than electron-beam evaporation and is thus a potential solution to a lack of contact adhesion.
Furthermore, it was noted that when samples coated with Alumina (Al2O3) were left for an extended
period of a few weeks, the alumina would crystalise and become stuck to the samples. Using acetone,
traditional lift-off methods would not allow the alumina to be removed, nor would it be fully etched
away when immersed in HF. While it is not entirely known why this occurred, one can assume that
changes in temperature in the winter period may have altered the adhesion properties of the alumina and
photoresist left on the samples.
25
4.2 Transmission Line Measurements
All Nickel contacts displayed Ohmic behaviour, as shown in figure 20. Interestingly, there were some
considerable heating effects observed in the 3C-SiC. One observes that contacts spaced further apart
saw a more significant decrease in resistance at larger currents than those closer together.
8
I-V Curves for Different Pairs of Contacts
10-3
Contact Separation: 86
Contact Separation: 11
m
m
6
4
Current (A)
2
0
-2
-4
-6
-8
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
Voltage (V)
Figure 20: Transmission Line Measurements, I-V Plots of Different Pairs for Contacts on a Doped Sample
A plot of resistance versus contact separation is given in figure 21. The undoped sample displayed
larger amounts of resistance due to its fewer number of charge carriers. Table 2 lists the calculated
resistivities of each set of contacts using the method outlined earlier in this report. The values for
resistivity lie within the expected range for Nickel contacts on 3C-SiC, which utilise the annealing
process; the contacts are thus justified for use within the MEMS structures.
26
Resistance of Pairs of Contacts on Doped and Undoped Samples
650
600
Resistance ( )
550
500
450
400
Undoped sample
N-doped sample
350
0
10
20
30
40
50
60
70
80
90
Contact Separation ( m)
Figure 21: Transmission Line Measurements, Plot of Total Resistance Measured vs Contact Separation on Doped
and Undoped Samples
Table 2: Contact Resistivity of Ni Contacts on Doped and Undoped Samples
N-doped Sample
Contact Resistivity [Ω cm2]
8.57 × 10
Undoped Sample
506.0 × 10
The doped sample exhibits a much lower contact resistivity as it exhibits a smaller Schottky barrier
to the charge carriers that pass through it. For better contact resistance, future MEMS designs could
further dope the 3C-SiC directly under the areas of Nickel to give a much better Ohmic contact.
27
4.3 Hall Measurements
Hall measurements were carried out on a doped suspended van der Pauw cross (200x20μm) and its
bulk equivalent from 300K down to 40K. A clear trend of increased conductivity with an increase in
temperature is shown in figure 22. However, one observes that the graph is split into three sections, each
Resistivity (
Resistivity (
cm)
cm)
representing seemingly different transport characteristics.
Figure 22: Resistivity vs Temperature for Bulk and Suspended VdP Cross from 40-300K
Both the bulk and suspended samples exhibit similar trends at the lowest temperatures as the
resistivity falls with temperature. The rise and fall in resistivity from 160K to 230K is more pronounced
in the suspended sample, although both samples show similar resistivities at temperatures closer to
300K.
In the bulk sample, it is clear from figure 23 that charge carriers begin to “freeze out” as the
temperature decreases to the point where there are a negligible number of intrinsic electron-hole pairs.
The suspended sample shows a similar carrier concentration at 300K but drops a lot sooner. It then sees
a significant jump in carrier concentration from 200K to 190K. This large change is not erroneous,
indicated by the small spread of data at these temperatures, but indicates a considerable transition in the
transport mechanisms within the 3C-SiC.
28
Carrier Concentration (cm-3)
Carrier Concentration (cm-3)
Figure 23: Carrier Concentration vs Temperature for Bulk and Suspended VdP Cross from 40-300K
A similar trend is also seen at the same temperatures where the suspended cross exhibits a drop in
mobility. The bulk sample sees minor falls and rises in mobility as the temperature decreases, suggesting
three separate modes of transport taking priority in each section.
Bulk VdP Cross: Mobility vs Temperature
106
105
105
104
104
Mobility (cm2 V-1s-1)
Mobility (cm2 V-1s-1)
106
103
102
103
102
101
101
100
100
10 -1
40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300
Temperature (K)
Suspended VdP Cross: Mobility vs Temperature
10-1
40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300
Temperature (K)
Figure 24: Carrier Concentration vs Temperature for Bulk and Suspended VdP Cross from 40-300K
29
Samples were doped with 𝑁 = 10
cm-3 within the top 100nm of the 3C-SiC layer. Using doped
layers removes the temperature dependence that arises from freeze out of intrinsic carriers. At room
temperature, many intrinsic carriers from throughout the sample and underlying substrate contribute to
the current, but as the temperature decreases and “freezes them out”, only the highly doped layer of SiC
contributes [57]. When comparing this trend between the bulk and suspended crosses, one assumes a
uniform layer thickness throughout the current path; however, this may not be the case. Changes within
the structure’s thickness could contribute towards changing transport mechanisms as the temperature
decreases.
The suspension of the 3C-SiC layer removes the SiC/Si interface and with it, much of the misfit
dislocation network caused by lattice mismatches. However, the resistivity between bulk and suspended
samples is quite similar at the lowest temperatures. This would suggest that some of the dislocation
networks remain and completely isolate the dislocations' electrical conduction. Further work would be
required to locate the part of the network which remains and further understand why it wasn’t eradicated.
The mobility graphs against temperature show that, generally, carriers move slower as the
temperature decreases. The “jump” in mobility could be explained by the fact that the remaining defects
within the 3C become electrically active at specific temperatures. The change would thus be caused by
the ability of carriers to interact with many of the charged impurities found in these defects, causing a
sudden drop in mobility as they become entrapped. This could also explain an increased carrier
concentration in those areas as exhibited by the samples at the same temperature.
Suspended VdP Cross: Hall Voltage vs Temperature
10 -2
Hall Voltage (V)
10 -3
10 -4
10 -5
10 -6
40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300
Temperature (K)
Figure 25: Hall Voltage vs Temperature for Suspended VdP Cross from 40-300K
30
Another important observation to note is that the hall voltage of the suspended VdP cross significantly
increased at lower temperatures (figure 25). This supports the use of 3C-SiC sensors in cold
environments, potentially offering a cheaper alternative to current sensor designs capable of operating
at liquid helium temperatures.
No literature to date has recorded the hall measurements of suspended 3C-SiC, and this work has led
to the discovery of changing transport mechanisms at lower temperatures. Greater study of the
suspended structures is required to discover precisely what defects remain within the 3C and their effects
on these varying transport properties. Through deep level transient microscopy, one could work out the
location of defects at different points and verify the temperatures at which each defect is electrically
active. Additionally, conductive atomic force microscopy could provide surface maps of the structures
coupled with I-V data to locate various defects. Both techniques would provide greater insight into why
this project has observed changing transport properties within each sample of 3C-SiC at lower
temperatures.
5. Conclusions
The nature of 3C-SiC MEMS devices has been investigated in this report to evaluate their use in harsh
environment applications. A new simple method has been developed to fabricate suspended 3C-SiC
devices using anisotropic wet etching techniques. Fifteen different designs were manufactured to
observe the effects of the material under suspension, and no significant bowing or deformities were
found. The contacts of each suspended device were verified through transmission line measurements
before a single van der Pauw cross was hall tested from 300K down to 40K. Results indicated that
different transport mechanisms within the 3C-SiC structure took precedence at various temperatures,
with large jumps in mobility seen at 200K. It was suggested that the remaining defects within the 3C
became electrically active at these temperatures and entrapped charge carriers.
A few adjustments could be made to the fabrication process for future designs to ensure better
adhesion with metal deposited onto the sample. Furthermore, a more in-depth look into remaining
defects in the suspended structures and hall testing more designs could provide a better explanation as
to the cause of the changes in transport characteristics observed in this report. Quantitative stress analysis
of the cantilever beams fabricated would also lead to a more detailed insight into the effects of Ni
deposited on suspended 3C-SiC.
With the knowledge from this project, it is suggested that 3C-SiC is a promising material for
suspended MEMS devices and that new suspended membranes can be fabricated utilising these new
anisotropic etching techniques.
31
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7. Appendices
7.1 Process Workflow Sheet
Figure 26: Process workflow sheet summarising new fabrication process for suspended 3C-SiC MEMS
35