SENSITIVE NH3OH AND HCL GAS SENSORS USING SELF

SENSITIVE NH3OH AND HCL GAS SENSORS USING SELF-ALIGNED AND SELFWELDED MULTI-WALLED CARBON NANOTUBES Yan Xie EECS Department Case Western Reserve University Cleveland, OH 44106

Massood Tabib-Azar EECS Department Case Western Reserve University Cleveland, OH 44106 Abstract— Self-aligned and self-welded multi-walled carbon nanotubes were grown for the first time using a metal-catalyzed chemical vapor deposition technique between 18 µm-high silicon posts with 2-6 µm gaps with excellent ohmic electrical contacts and mechanical bonding strengths in excess of 0.1 µN/CNT. The electrical conductivity of 5-10 MWCNTs spanning the gap between adjacent silicon posts changed drastically upon exposure to ammonia and hydrochloric gases at room temperature. In devices reported here the electrical contact between both sides of the MWCNTs and the silicon posts are intimately formed during the growth process. Thus, the gas sensitivity of these selfwelded MWCNT devices are less affected by the contact barrier changes. Moreover, since the MWCNTs are selfaligned and self-welded during the growth, the only post processing steps that are needed are dicing and wirebonding to a chip carrier (package), making these devices inherently more reliable and cost effective.

technique achieves strong self-welding by using carboncovered posts to enable the CNTs’ to grow into the opposing posts and firmly attach themselves by incorporating carbon atoms from the surface of the post into the CNT structure very similar to the directed and selfwelded silicon nanowire growth. II. Experimental Procedures The starting silicon-on-insulator (SOI) posts were defined using photolithography and deep-reactive ion etching as shown in figure 1. These posts were designed with line widths ranging from 1 to 10 um. The wafers were subjected to an in-situ rf cleaning before the metal deposition. The wafers were patterned with photoresist and the aluminum was wet etched. After cleaning the resist with acetone and methanol, the wafers were patterned again with photoresist for the DRIE process. The SOI wafers were then DRIE etched at an outside facility [1]. The silicon epitaxial layer was 18±2 µm thick.

I. Introduction Carbon nanotubes (CNT) are being studied for their interesting electrical properties for applications in electronics. Single-walled and multi-walled carbon nanotubes with metallic and semiconducting properties are considered in many of these applications. For applications in electronic devices, semiconducting single-walled CNTs are desirable but multi-walled CNTs are also being considered [1]. In CNT field-effect transistors and devices, many different techniques involving in-situ assembly as well as post processing are developed to situate CNTs between drain and source contacts. In some cases, an electric field was used to direct the growth of the CNT between two contacts. Here for the first time we report a growth technique that results in self-welded multi-walled CNTs grown between silicon posts separated by 2 µm up to 6 µm [1-13]. To achieve self-welding, we functionalized the surface of one of the contacts such that when the CNTs grew into that surface, they formed a very strong bond. Our

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(b) Figure 1 a) The silicon structure with contact pads. b) SEM of the microfabricated silicon structure.

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(c) Figure 1 c) Fabrication steps used to deposit thin layers of iron and amorphous carbon on adjacent posts to enable ironcatalyzed growth of CNTs. The CNTs bridge the gap and weld themselves to the post covered with a layer of thin carbon.

(b) Figure 2 b) SEM of self-assembled CNTs grown between silicon posts in the gap region.

The above devices were pre-processed to deposit a thin layer of iron on a set of posts and a thin layer of amorphous carbon on the corresponding and opposing posts as schematically shown in figure 1. The iron layer (100 Å) was deposited off-axis in a thermal evaporator and both CVD amorphous carbon and sputtered carbon layers (50 Å) were deposited off-axis. Figure 2a shows the deposited material near the top of the silicon post. The material was confined to a narrow band near the top of the silicon post by off-axis deposition. Subsequently, these devices were loaded into a low-pressure chemical vapor deposition (LPCVD) chamber and were thermally heated in an argon gas flow to 750-800C in 10-4 Torr. The heating cycle typically took around 5 minutes.

(a) Figure 2 a) SEM image of a silicon post showing only the top region covered by the evaporated material.

After the sample temperature reached 750 C, the acetylene gas (50 sccm) was introduced through an opening located above the chip and CNT growth was carried out with both acetylene and argon gases flowing at 50 sccm for 15 minutes. Subsequently, the acetylene was turned off and the sample was cooled down to room temperature in the argon gas flow (50 sccm). The CNTs were then examined by scanning electron microscope, atomic force microscope (Nano-Scope IV) and they were electrically tested by current versus voltage technique (Agilent 4155B). III. Experimental Results The CNTs grew horizontally and parallel to the substrate in the gap between the two silicon posts as shown in figure 2b. In this example there were around 10 CNTs bridging the gap and all seemed to be under tension and straight. It is also important to note that the CNTs are only present in the gap between the posts. This may be caused by the gas current patterns that create a volume slightly richer in feeder gas. It is also important to note that CNTs only grew between the posts and they are all suspended away from the substrate. Aspects of this behavior is reported in the past and explained by the gas flow patterns that exist around the raised posts. In our case, we believe that a combination of surface gas currents and van der Wall interaction are present. First we note that the CNTs in our case only grew inside the gap and not anywhere else. In our case, the presence of aluminum contact regions precludes the CNT growth over the contacts. In these regions, as reported by us previously, aluminum acts as a CNT growth poison under certain conditions suppressing the CNT growth. One possible mechanism for this growth suppression is that at 750 C, the aluminum is quite soft and starts dissolving the iron nanoparticles diminishing their catalytic ability. Second, we note that the CNTs also do not grow in other

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regions that are not covered with aluminum, for example on the walls of the raised SOI region away from the gap. This lack of growth, most probably is caused by the surface currents that generate strong boundary layer shear forces and currents that may “cool” down these regions below the CNT growth temperature. In between the gap regions, a stagnant volume may exist giving an opportunity for CNTs to grow. The surface current profiles, keep the CNT growth confined and directed and away from the bottom surface in these gap regions. The current versus voltage (I-V) characteristics of the CNTs measured by contacting the 150 µm pads is shown in figure 3. It is interesting to note that the I-V is very symmetric with a deflection region near the origin that is related to the band-gap of the CNT. The symmetry of the IV indicates that the two electrical contacts at the two ends of the CNT bridges are nearly identical. Thus, the growth side and welded side are electrically the same. The deflection region near the origin indicates that the CNTs are semiconducting. There were approximately 10 CNTs in this device resulting around 10 nA/CNT current conduction at 0.5. The resulting current densities at 0.5 V and assuming CNT diameter of around 10 nm, was around 6.3x103 A/cm2.

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Figure 4 The load-displacement characteristics of one of the CNTs shown in figure 2b measured using atomic force microscope (AFM). How these strong bonds come about between the CNTs and the amorphous carbon-covered silicon posts? To understand and examine such a “welding” mechanism, we studied the possibility of growing carbon nanotubes by simply using iron nanoparticles on amorphous carbon layers. Carbon coated samples with a thin layer of iron were loaded onto a different reactor that operated in atmospheric pressure at 1200 C in 50 sccm of argon flow. At lower temperatures, the CNT yield was not very high. Figure 5 shows the SEM of the CNTs that were grown directly from the amorphous carbon layer at 1200 C without any C2H2 gas. To the best of our knowledge, this is the first report of such a growth mechanism and process directly from a carbon layer in the absence any hydrocarbon gases. This experiment clearly shows that it is quite possible for the iron nanoparticle at the tip of the growing CNT to start incorporating carbon atoms from the amorphous carbon layer and grow into this layer forming a strong bond.

Figure 3 Current versus voltage characteristic of the CNTs shown in figure 2b. There were approximately 10 CNTs spanning the gap. To examine the strength of this bond, the AFM technique was used to image, locate, and then apply a vertical force on the CNT to measure the bond strength. We used AFM probes with a 0.02 N/m spring constant and ultra-sharp tips. The tip was then positioned at the mid-point between two posts over the CNT and 110 nm vertical steps were successively applied while measuring the force using the AFM system. The resultant load deformation plot is shown in figure 4. After AFM testing, the device was examined with SEM and it appeared that the CNTs were “stretched” rather than broken as also reported in other CNTs.

Figure 5 SEM of CNTs grown from an amorphous carbon layer by iron particles without any carbon-containing gases. The current through the CNTs was reduced upon exposure to ammonia and hydrochloric gases as can be seen in figures 5 and 6. The effect of ammonia was larger. The CNT device was inside a closed petry dish at room

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temperature and a small vial containing NH3(OH) or HCl were separately introduced inside the petry dish allowing the gas molecules to spread at the partial pressures of these substances.

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Acknowledgement: This work was supported by grants from NSF NER program and SRC. 1.0

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Figure 6 I-V of the CNT device before (1), during (2), and after (3) exposure to ammonia. The response time was faster than 2 s upon when the ammonia was first introduced. It took around 60 s for the current to become large again.

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Figure 7 I-V of the CNT device before (1) and after (2) exposure to HCl gas. The response time was faster than 10 s. It took much longer to recover. It is known that MWCNTs show p-type conductivity in air. An acidic agent such as HCl or a basic agent such as NH3(OH) may reduce the p-type conductivity by displacing adsorbed oxygen atoms at CNT surfaces.

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References 1.

Massood Tabib-Azar and Yan Xie, “Self-Welded MetalCatalyzed Carbon Nanotube Bridges.” Submitted to APL (2005).

2.

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