T. Hossain, F. Khan, I. Adesida, P. Bohn, and T. Rittenhouse

University of Illinois at Urbana-Champaign, Urbana, Illinois

Nanoporous Silicon Carbide for

Nanoelectromechanical

Systems Applications

NASA/CR—2003-212198

April 2003

https://ntrs.nasa.gov/search.jsp?R=20030037184 2018-05-15T15:55:46+00:00Z
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T. Hossain, F. Khan, I. Adesida, P. Bohn, and T. Rittenhouse

University of Illinois at Urbana-Champaign, Urbana, Illinois

Nanoporous Silicon Carbide for

Nanoelectromechanical

Systems Applications

NASA/CR—2003-212198

April 2003

National Aeronautics and

Space Administration

Glenn Research Center

Prepared under Grant NAG3–2661
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The Propulsion and Power Program at

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Nanoporous Silicon Carbide For Nanoelectromechanical Systems Applications 

A major goal of this project is to produce porous silicon carbide (PSiC) via an 

electroless process for eventual utilization in nanoscale sensing platforms. Results in 

the literature have shown a variety of porous morphologies in SiC produced in anodic 

cells. Therefore, predictability and reproducibility of porous structures are initial 

concerns. This work has concentrated on producing morphologies of known porosity, 

with particular attention paid toward producing the extremely high surface areas 

required for a porous flow sensor. We have conducted a parametric study of electroless 

etching conditions and characteristics of the resulting physical nanostructure and also 

investigated the relationship between morphology and materials properties. Further, we 

have investigated bulk etching of SiC using both photo-electrochemical etching and 

inductively-coupled-plasma reactive ion etching techniques. 

I. Formation and Characterization of Porous SiC by electroless etching 

a. Formation 

Electroless etching is technically simple and inexpensive in comparison with 

anodic etching. However, several experimental parameters can be manipulated during 

electroless etching in order to produce the desired nanostructure. In this study, 

particular attention has been paid to the identity of the etchant, etchant concentrations, 

the polytype of the SiC wafer, the doping type and concentration in the wafer, the 

structural perfection of the SiC wafer, the presence of ultraviolet illumination during 

etching, and the presence of Pt as a catalyst during etching. 

Previous work conducted in this laboratory suggested the feasibility of forming 

PSiC by an electroless process. An etchant composed of HF and H2O2 was found to 

produce porous gallium nitride (PGaN). When the thin PGaN layer was completely 

etched away, it was discovered that the SiC substrate had also become porous. 

Therefore, the HF:H2O2 etchant was initially chosen in this study for the electroless 

formation of PSiC. However, experiments quickly showed that pure SiC wafers were 

etched very inefficiently by the HF:H2O2 etchant. Thus, new chemical etchants were 

investigated. 

Eventually, an etchant composed of a mixture of HF and K2S2O8 was found to 

produce highly porous PSiC consistently. Two different concentration ratios are 

currently used to yield different porous morphologies. In the “concentrated HF” etchant,

T. Hossain, F. Khan, I. Adesida, P. Bohn, and T. Rittenhouse

University of Illinois at Urbana-Champaign

Urbana, Illinois 61801

NASA/CR—2003-212198 1
a mixture of 14 M HF:0.075 M S2O8

2- is used. The other etchant concentrations used 

(“dilute HF”) are 0.50 M HF:0.15 M (saturated) S2O8

2-. 

The resulting porous morphology in SiC is highly dependent upon the polytype of 

the initial SiC wafer. To date, two different polytypes of SiC have been investigated: 

6H-SiC and 4H-SiC. 

Because the energy of the holes available from S2O8

2- reduction is not sufficient 

for direct injection into the valence band of SiC, ultraviolet irradiation is required during 

etching to form the porous morphology. In addition, sputtered Pt is necessary to 

catalyze the persulfate reduction and to act as a local cathode. The Pt areas also have 

the added benefit of acting as a sharp masking agent. Regions under the Pt remain 

unetched (Fig. 1), allowing for high-resolution patterning of porous structures. 

Figure 1. Plan view SEM image of a porous 

region (bottom of photo) next to unetched 

planar surface (top of photo) that was coated 

with 100 Å Pt prior to initiating the etching 

process. 

n-doped 6H-SiC. For short etch times (~10 min) with the concentrated HF 

etchant, a ridged porous morphology results (Fig. 2). For longer etch times (~60 min) 

with the concentrated HF etchant, strong porosity results that is still restricted to the top 

surface layer (Fig. 3). For short (~10 min) etch times with the dilute HF etchant, 

morphologies similar to figure 2 result. However, for long (~60 min) etch times with the 

dilute HF etchant, spongelike morphologies result that can extend deep (>100 μm) into 

the wafer (Figs. 4 and 5). 

Figure 2. Plan view SEM showing the ridged 

porous structure resulting from short etch 

times in 6H-SiC wafers with either 

concentrated or dilute HF etchant. 

NASA/CR—2003-212198 2
Figure 3. Plan view SEM image showing 

the strong planar porosity (restricted to top 

surface layer) that results from etching 6HSiC in the concentrated HF etchant for long 

etch times. 

Figure 4. Plan view SEM image showing 

the spongelike porosity that results from 

etching 6H-SiC in dilute HF etchant for long 

etch times. 

Figure 5. Cross-sectional views of the spongelike morphology in 6H-SiC that has been 

etched with the dilute HF etchant for long times. The top surface of the wafer is at the 

top of each photo. 

NASA/CR—2003-212198 3
At this point in time, the depth of the porous layer into the wafer is still difficult to 

control. Current results indicate that the structural perfection of the SiC starting material 

plays a major role in how deep the pores propagate into the wafer. However, 

dislocations, micropipes, stacking faults, etc. appear to vary greatly across the face of 

commercially-available 2” SiC wafers. Initial X-ray diffraction (XRD) experiments have 

been conducted to link crystallographic perfection of the starting material with resulting 

porous morphology. 

n-doped 4H-SiC. The 4H polytype of SiC yields a distinctly different morphology 

when exposed to the same etchant conditions as the 6H wafer. Unfortunately, 

predictability of morphology for the 4H polytype is much poorer than that for the 6H-SiC. 

The resulting morphology appears fairly independent of the concentration of etchants 

used. For short etch times (~10 min) a ridged porous structure similar to the 6H 

polytype is obtained in 4H wafers (cf. Fig. 2). For longer etch times (~60 minutes), 4H 

wafers often yield a cellular structure with nanofingers, but a highly porous spongelike 

structure may also be obtained (Fig. 6). 

Figure 6. Plan view SEM images showing the different morphologies that can develop 

when 4H-SiC is etched for long times (~60 min) in either the concentrated or dilute HF 

etchant. In the left photo, a cellular structure with fingers has developed, while in the 

right photo a more spongelike morphology has formed. 

In the 4H wafers etched to date, no submerged porosity has been achieved. The 

etchant has only formed a surface layer of pores ≤ 25 nm deep. For this reason, 

electroless etching of 6H polytype wafers will initially be investigated further for the 

development of high surface area sensor applications. 

In addition, electroless etching has not resulted in porous structures for either 

semi-insulating 6H-SiC or p-doped 4H-SiC. The surface of these wafers remains 

primarily planar. The HF/S2O8

2- etchant does, however, decorate the dislocations, 

micropipes, and scratches on the surface of the wafer. 

NASA/CR—2003-212198 4
Because the high surface area characteristic of porous materials is one of the 

drivers for their application in sensing platforms, the specific surface area of SiC will be 

an important parameter in applications such as flow-sensing in the interior of hightemperature combustion chambers. Although there is wide variation with the specific 

processing conditions, literature results for porous silicon (PSi) indicate surface areas of 

approximately 100 m2

/g. Surface areas for several PSiC samples formed in this work 

were measured via BET isotherms and were ca. 50 m2/g. 

b. Optical Characterization of Porous SiC 

Various optical properties of PSiC have been investigated in this study. In the 

literature, porous silicon has shown a blue-shifting of its photoluminescence after the 

formation of a porous nanostructure. In light of this, cathodoluminescence (CL) and 

photoluminescence (PL) studies were conducted to look for similar changes in 

luminescence. Also, Raman studies were conducted in order to characterize the porous 

morphology, as has been reported in the literature for porous silicon. 

Cathodoluminescence was conducted to explore the possibility that the bandgap 

of PSiC would be blue-shifted and intensified, as it is in PGaN, possibly leading to 

efficient blue or ultraviolet emitters. However, results of this study have shown that no 

blue-shifting of the peak CL occurs for either porous 6H-SiC or porous 4H-SiC. 

Furthermore, porous 6H-SiC shows only slightly more intense CL emission, while 

porous 4H-SiC shows slightly less intense CL emission as compared to the bulk 

substrate. 

Some sensing modalities have been proposed in which sensing is based on the 

reduction of PL emission upon immersion in the test medium. PL investigations in this 

study have yielded results similar to literature reports from anodically-etched PSiC. PL 

from the bandgap region of wavelengths does not appear to intensify in the porous 

samples. Changes in the PL in this region of the spectrum are apparently due to local 

changes in the doping, defects, etc., and are unrelated to the development of porous 

morphology. 

PL from the broad visible peak (500-700 nm) of porous 6H-SiC appears to redshift slightly (up to 20 nm) and become more intense in the porous samples. Intensity 

increases of 3x are common. The largest increases in PL intensity are derived from the 

band at 486 nm. The PL at this wavelength increases as the porosity of the sample 

increases. PL increases of 50x are achievable via electroless etching of SiC (Fig. 7). 

Raman investigations were conducted in an attempt to quantify the degree of 

porosity non-destructively. In a porous structure, the lattice symmetry is disrupted and 

coupling of phonon states with different K values occurs, leading to a broadening and 

shift in the location of the Raman bands. Previous work has shown that the longitudinal 

optic (LO) mode of PSi tends to broaden asymmetrically and red-shift as nanostructures 

NASA/CR—2003-212198 5
PL Intensity (arbitrary units) 

Wavelength (nm)

Figure 7. Photoluminescence spectrum demonstrating the increase in intensity of PL 

emission centered at 486 nm as a result of porous morphology. The top spectrum is 

from an unetched piece of 4H-SiC wafer; the bottom spectrum is from a piece of the 

same 4H-SiC wafer that has been etched in the concentrated HF etchant for one hour, 

resulting in a spongelike porosity. 

are formed. However, in the literature, there are no reports of similar red-shifts in the 

location of the LO mode in PSiC formed via anodic etching. Likewise, in the electroless 

etching used to form the PSiC in this work, the LO mode was not found to shift with the 

development of porosity. In addition, the LO mode does not exhibit any significant 

broadening as the structure becomes porous. These results point to the extreme 

stability of PSiC, and indicate that the porous layer does not introduce significant strain 

to the wafer. 

NASA/CR—2003-212198 6
II. Bulk Etching of 6H-SiC 

In the fabrication of some types of sensors, it is necessary to perform bulk 

etching to form thin films directly or to etch isotropically for eventual release of thin films. 

We have initiated investigations of both wet and dry etching of SiC for these 

applications.

a. Wet Etching 

In this study, the experiments were carried out on single-crystal n-type 6H-SiC 

wafers. A two-layer metal film of Ti(100A) followed by Pt.(2200A) was used as an etch 

mask and the etch patterns were defined photolithograpically using a lift-of process. 

The SiC sample was place into a Teflon electrochemical cell, along with a Pt wire 

counter electrode and a saturated calomel reference electrode. Dilute HF was 

employed as an etching solution. The SiC sample was illuminated by UV light for an 

hour and was biased at different voltages. The UV light was focused down to a 2 cm2 

spot with a power density of 125 mW/cm2. The schematic drawing of the PEC etching 

system is shown in Figure 8. 

UV illumination 

Ameter

Voltmeter 

Pt Cathode Sample 

Dilute HF 

Figure 8. Schematic of etching apparatus

As indicated above, the etching process consisted of two-steps. The SiC sample 

was first anodized to form a deep porous layer and the layer was subsequently removed 

by thermal oxidation followed by an HF dip. To illustrate the two-step etching, SEM 

studies were performed before and after the anodized layer was oxidized and etched 

off. The surface of the anodized SiC became porous as shown in Fig. 9, however, we 

have not studied the porosity engendered by anodic etching at this point. The anodized 

PSiC samples were thermally oxidized in a steam ambient for 4 hours at 1150 0

C. 

NASA/CR—2003-212198 7
Under these conditions, SiC oxidized to form SiO2. A short HF dip removed the oxide 

and produced etched features as shown in Fig. 10. 

Figure 9. Anodized SiC surface (After PEC etch) 

Figure 10a. SiC etch profile at 3.2mA Figure 10b. SiC etch profile at 4.2mA 

 (3.13 μm etch depth) (3.91 μm etch depth) 

Figure 10c. SiC etch profile at 6.1mA Figure 10d. SiC etch profile at 7.4mA 

 (7.77 μm etch depth) (12.6 μm etch depth) 

NASA/CR—2003-212198 8
The maximum etch rate obtained was approximately 2000A/min. Etch depth as 

a function of etch current is presented in Fig. 11. It showed etch depth increased 

significantly with etch current. Further increases in etch current (bias) will be 

investigated. 

 

0

2

4

6

8

10

12

14

2468

Etch Current in mA

Etch Depth in micrometer

 Figure 11. Etch depth as a function of etch current 

b. Dry Etching 

Owing to its inert chemical nature and high bond energy, SiC substrates are 

resistant to common wet chemical etchants, making dry etching studies necessary for 

the fabrication of free suspending SiC cantilevers. This makes it essential to do 

comprehensive studies on etching characterization of SiC to find etching conditions 

suitable for various device fabrication needs. For example we need highly anisotropic 

etching to define the dimensions, length, width and thickness of the SiC cantilever. On 

the other hand we need isotropic etching condition to undercut the SiC to form the selfsuspending cantilever. 

Dry etching techniques mostly depend on plasma assisted etching. Plasma 

assisted etching proceeds via a combination of physical and chemical mechanisms. 

The dominant mechanism is usually determined by the choice of partial pressure ratio of 

the reactive or inert gases. It is also affected by other accompanying plasma 

conditions. In the case of inductively coupled plasma reactive ion etching (ICP-RIE), 

these would be gas flow rates, chamber pressure, bias voltage and ICP coil power. By 

using different combinations of the above mentioned conditions the substrate can be 

etched by (a) physical sputtering – purely physical removal of the material by energetic 

ions of the gas molecules; (b) chemical plasma etching – neutral radicals formed in the 

plasma reacting with the substrate to form volatile compounds; (c) ion enhanced 

chemical etching – energetic ions either damage the surface to enhance the rate of 

NASA/CR—2003-212198 9
surface chemical reactions or they help enhance the removal of the chemical byproducts from the surface. Etching conditions in which physical sputtering mechanisms 

or ion enhanced chemical etching is dominant tend to be anisotropic. While etching 

conditions in which chemical plasma etching is dominant are isotropic in nature. 

Figure 12. Etch rate of SiC vs. pressure

We have extensively studied ICP-RIE etching conditions for SiC. Figure 12 

shows etch rate of SiC with respect to chamber pressure for different ICP coil powers 

using SF6/O2 gas chemistry. Increasing pressure increases the density of reactive 

species present in the chamber. However, it also decreases the mean free path, 

thereby affecting the energy with which the ions impinge on the substrate and the 

average lifetime of the ions before they recombine. Increasing the ICP coil power 

increases the density and the 

energy of the ions overcoming the 

reduction in etch rate due to the 

reduced mean free path and 

lifetime of the ions. Keeping the 

bias voltage value at –300 V, when 

the ICP coil power was increased 

from 500 to 900 W, the optimum 

chamber pressure increased from 

4 to 8 mT. At –300 V bias voltage, 

900 W ICP coil power, and 8 mT 

chamber pressure, an etch rate of 

1.05 μm was achieved for SF6/O2

gas mixture. This is the highest 

etch rate achieved for SiC using a 

commercial ICP-RIE system. 

Figure 13 shows the proposed fabrication sequence for the self-suspending SiC 

cantilever. It will begin with deposition of Indium-Tin-Oxide (ITO) mask pattern on the 

SiC wafer followed by anisotropically etching SiC. Next, a thin film of ITO will be 

sputtered over the entire wafer. We will then blanket etch the SiC wafer to etch back 

the ITO mask. The last step would be to isotropically etch SiC to release the free 

standing cantilever. 

 SiC Wafer 

1) Deposit ITO etch mask 2) Etch SiC 3) Deposit conformal ITO layer 

 4) Etch back ITO mask 5) Undercut SiC using ICP-RIE 6) Cross-sectional view of 

self-suspending beam 

 Figure 13. Process for achieving self-suspended SiC films by dry etching

NASA/CR—2003-212198 10
Presently we are optimizing the dry-etching process to undercut SiC. Figure 14 

shows an undercut SiC structure using Ni mask. The structure was undercut by 2.6 μm 

in 150 minutes giving an undercut rate in excess of 1 μm/hr. Conditions to achieve high 

undercut rate will be investigated. 

 

 

 Figure 14. SiC etched using ICP-RIE in SF6 showing undercut. 

III. Summary 

 Several key accomplishments have been achieved during FY02 for this grant, 

including: 

• determination of an electroless etchant and conditions capable of producing 

PSiC 

• demonstration of precise patterning of the porous morphology 

• quantification of surface area; results have been obtained with PSiC that are 

comparable to that of the more mature porous silicon technology 

• characterization of PL and Raman for various porous morphologies, serving as a 

basis for nanoelectromechanical systems investigations 

• bulk etching of SiC using electrochemical etching, oxidation, and HF dip 

• characterization of ICP-RIE etching of SiC for high undercut etch rate 

Most importantly for the objectives of this project, the high surface areas 

necessary for a sensor application have been demonstrated. Also, as anticipated PSiC 

has proven to be a very robust material, an important consideration for the Aerospace 

Propulsion and Power Program. 

NASA/CR—2003-212198 11
Program for Year 2 of the proposal are: 

• Develop nanoporous SiC formation by metal-assisted electroless chemical 

etching in nonaqueous solution, and explore the achievable range of 

structures by this process. 

• Explore the formation of nanoporous SiC/metal composites by in situ

metal assisted etching and deposition. 

• Studies of lateral patterning to determine relationship between etching 

conditions and spatial resolution. 

• Create cantilevers by using dopant selective etching by undercutting and 

freeing cantilevers using the PEC method and/or dry etching method. 

Investigate resolution limits by using electron-beam defined lines for the 

porous SiC cantilevers. Delineate possible sensor fabrication process. 

NASA/CR—2003-212198 12
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18

Nanoporous Silicon Carbide for Nanoelectromechanical Systems Applications

T. Hossain, F. Khan, I. Adesida, P. Bohn, and T. Rittenhouse

Nanotechnology; Nanoporous; Silicon carbide; NEMS

Unclassified -Unlimited

Subject Category: 25 Distribution: Nonstandard

University of Illinois at Urbana-Champaign

Urbana, Illinois 61801

Project Manager, Michael Lienhard, Instrumentation and Controls Division, NASA Glenn Research Center, organization

code 5510, 216–433–8932.

A major goal of this project is to produce porous silicon carbide (PSiC) via an electroless process for eventual utilization in nanoscale sensing platforms. Results in the literature have shown a variety of porous morphologies in SiC

produced in anodic cells. Therefore, predictability and reproducibility of porous structures are initial concerns. This

work has concentrated on producing morphologies of known porosity, with particular attention paid toward producing

the extremely high surface areas required for a porous flow sensor. We have conducted a parametric study of electroless

etching conditions and characteristics of the resulting physical nanostructure and also investigated the relationship

between morphology and materials properties. Further, we have investigated bulk etching of SiC using both photoelectrochemical etching and inductively-coupled-plasma reactive ion etching techniques.