**2.4 CNT-based microbolometer pixels**

Networks of SWCNTs are considered as potential replacements for VOx and amorphous silicon in uncooled microbolometer-based infrared focal plane arrays

#### **Figure 6.**

*Schematic of superimposed thermal network for calculating temperature map of CNT-based bolometer absorber [14].*

**Figure 7.**

*Temperature map of bolometer pixel when net absorbed power Hnet is 1 nW, and CNT thermal resistance is (a) 5 × 108 K/W and (b) 1 × 109 K/W. The pixel is assumed to be tightly packed with CNTs [16].*

**79**

**Figure 9.**

*oriented MWCNT growth [16].*

*Nanostructure Technology for EO/IR Detector Applications*

are in the same thickness range as the SWCNT films.

0.07%/K, in contrast to 0.17%/K for the SWCNT films.

that of unsuspended SWCNT films at comparable IR power.

improving light absorption and reducing thermal transfer.

electron microscopy (SEM) image of representative MWCNT films in unsuspended and suspended forms, respectively [17]. Unlike their SWCNT counterparts, the MWCNT films leave substantial portions of the substrate uncovered. In addition, some minor recess deformation is visible on the suspended MWCNT films, which

In **Figure 9(b)**, a transmission electron microscopy (TEM) image of an individual representative MWCNT is shown. This MWCNT has a large hollow center ~10 nm in diameter and contains approximately 40–50 shells. **Figure 9(c)** presents the dense "forest-like" growth of oriented MWCNTs with good length/diameter uniformity [16]. Here the CNT growth is easily distinguishable from the substrate. All the MWCNT films studied show semiconductive resistance-temperature (R-T) behavior characterized by the representative curve depicted in **Figure 10** [17]. It is seen that the increase in resistivity of the MWCNT films with decreasing temperature is much less than that of the SWCNT films, which is not unexpected considering the much smaller bandgap of MWCNTs. This reduced temperature dependence implies smaller absolute TCR values for the MWCNTs. For example, the absolute TCR value at room temperature for the MWCNT films is about

Two notable differences are discernable between the MWCNT and SWCNT films: the former have significantly higher *∆R*/*R*0 and much shorter response times compared to the latter. The *∆R*/*R*0 for MWCNT samples is typically in the range of a few percent, which is above an order of magnitude higher than that of SWCNT films suspended in an aqueous solution and two orders of magnitude higher than

**Figure 11** compares the photoresponse *R*/*R*0 of MWCNT films in unsuspended and suspended cases, where *R*0 is the sample resistance before IR radiation was turned on and the change in the resistance *ΔR* caused by IR radiation is given as *R*–*R*0. Physical suspension of the MWCNT films (**Figure 11(b)**) and SWCNT films (**Figure 11(d)**) leads to further improvement in *R*/*R*0 compared to that of their unsuspended counterparts in **Figure 11(a)** and **(c)**, respectively [2]. Considering the lower absolute TCR values of MWCNTs, the much enhanced photoresponse of these MWCNT films can be attributed to the naturally suspended inner CNT shells, which may provide an ideal configuration to enhance the bolometric effect by

Carbon nanotubes as absorbing materials in microbolometers show much promise for providing greater TCR values, and thus improved IR detection and imaging performance. Nevertheless, additional work is needed in the development, testing, and optimization of microbolometer arrays that can offer superior performance to current well-established Si- and VOx-based technologies. The other nanostructure

*(a) SEM images of unsuspended (left) and suspended (right) MWCNT films. (b) TEM image of a representative MWCNT, for which the shell number is estimated to be in the range of 40–50 [17]. (c) Dense* 

*DOI: http://dx.doi.org/10.5772/intechopen.85741*

#### **Figure 8.**

*CNT bolometer test fixture to evaluate CNT film quality [14].*

(IRFPAs). These SWCNT-based microbolometer pixels can be fabricated on top of the CMOS readout circuit unit cell. Several potential benefits of SWCNTs over existing state-of-the-art films include low thermal mass, high absorption coefficients in the IR, and TCRs greater than 4%/K.

Another benefit of utilizing SWCNTs involves their compatibility with CMOS wafer processes, which can enable cost-effective manufacturing for uncooled IRFPAs. SWCNTs suspended in either aqueous solutions or other solvents may be applied uniformly to a silicon wafer using standard wafer fabrication equipment. Once this process is optimized, the back end of line (BEOL) standard CMOS process can be modified to continue processing SWCNT microbolometers.

The adaption of SWCNTs to CMOS process technologies has demonstrated the potential for very small pixel sizes with sufficient yields [13]. In addition, there is evidence that unlike devices using VOx and amorphous Si, SWCNT-based microbolometers are not limited in performance due to the 1/*f* noise floor [14].

Optimization of the growth of SWCNTs and MWCNTs has been performed. Grown CNTs have been shown to be easily separable from the growth substrate. Good length/diameter uniformity has also been demonstrated. **Figure 8** shows the fixture used to evaluate the CNT films for bolometric applications [14]. This fixture was employed for evaluation of the optoelectronic characteristics of the CNT samples.

In addition, preliminary TCR measurements have been carried out with CNT materials, examples of which are shown in **Figure 9**. **Figure 9(a)** shows a scanning *Nanorods and Nanocomposites*

**Figure 7.**

*(a) 5 × 108*

**Figure 8.**

 *K/W and (b) 1 × 109*

(IRFPAs). These SWCNT-based microbolometer pixels can be fabricated on top of the CMOS readout circuit unit cell. Several potential benefits of SWCNTs over existing state-of-the-art films include low thermal mass, high absorption coeffi-

*Temperature map of bolometer pixel when net absorbed power Hnet is 1 nW, and CNT thermal resistance is* 

 *K/W. The pixel is assumed to be tightly packed with CNTs [16].*

Another benefit of utilizing SWCNTs involves their compatibility with CMOS wafer processes, which can enable cost-effective manufacturing for uncooled IRFPAs. SWCNTs suspended in either aqueous solutions or other solvents may be applied uniformly to a silicon wafer using standard wafer fabrication equipment. Once this process is optimized, the back end of line (BEOL) standard CMOS process can be modified to continue processing SWCNT microbolometers.

The adaption of SWCNTs to CMOS process technologies has demonstrated the potential for very small pixel sizes with sufficient yields [13]. In addition, there is evidence that unlike devices using VOx and amorphous Si, SWCNT-based microbo-

Optimization of the growth of SWCNTs and MWCNTs has been performed. Grown CNTs have been shown to be easily separable from the growth substrate. Good length/diameter uniformity has also been demonstrated. **Figure 8** shows the fixture used to evaluate the CNT films for bolometric applications [14]. This fixture was employed for evaluation of the optoelectronic characteristics of the CNT

In addition, preliminary TCR measurements have been carried out with CNT materials, examples of which are shown in **Figure 9**. **Figure 9(a)** shows a scanning

lometers are not limited in performance due to the 1/*f* noise floor [14].

cients in the IR, and TCRs greater than 4%/K.

*CNT bolometer test fixture to evaluate CNT film quality [14].*

**78**

samples.

electron microscopy (SEM) image of representative MWCNT films in unsuspended and suspended forms, respectively [17]. Unlike their SWCNT counterparts, the MWCNT films leave substantial portions of the substrate uncovered. In addition, some minor recess deformation is visible on the suspended MWCNT films, which are in the same thickness range as the SWCNT films.

In **Figure 9(b)**, a transmission electron microscopy (TEM) image of an individual representative MWCNT is shown. This MWCNT has a large hollow center ~10 nm in diameter and contains approximately 40–50 shells. **Figure 9(c)** presents the dense "forest-like" growth of oriented MWCNTs with good length/diameter uniformity [16]. Here the CNT growth is easily distinguishable from the substrate.

All the MWCNT films studied show semiconductive resistance-temperature (R-T) behavior characterized by the representative curve depicted in **Figure 10** [17]. It is seen that the increase in resistivity of the MWCNT films with decreasing temperature is much less than that of the SWCNT films, which is not unexpected considering the much smaller bandgap of MWCNTs. This reduced temperature dependence implies smaller absolute TCR values for the MWCNTs. For example, the absolute TCR value at room temperature for the MWCNT films is about 0.07%/K, in contrast to 0.17%/K for the SWCNT films.

Two notable differences are discernable between the MWCNT and SWCNT films: the former have significantly higher *∆R*/*R*0 and much shorter response times compared to the latter. The *∆R*/*R*0 for MWCNT samples is typically in the range of a few percent, which is above an order of magnitude higher than that of SWCNT films suspended in an aqueous solution and two orders of magnitude higher than that of unsuspended SWCNT films at comparable IR power.

**Figure 11** compares the photoresponse *R*/*R*0 of MWCNT films in unsuspended and suspended cases, where *R*0 is the sample resistance before IR radiation was turned on and the change in the resistance *ΔR* caused by IR radiation is given as *R*–*R*0. Physical suspension of the MWCNT films (**Figure 11(b)**) and SWCNT films (**Figure 11(d)**) leads to further improvement in *R*/*R*0 compared to that of their unsuspended counterparts in **Figure 11(a)** and **(c)**, respectively [2]. Considering the lower absolute TCR values of MWCNTs, the much enhanced photoresponse of these MWCNT films can be attributed to the naturally suspended inner CNT shells, which may provide an ideal configuration to enhance the bolometric effect by improving light absorption and reducing thermal transfer.

Carbon nanotubes as absorbing materials in microbolometers show much promise for providing greater TCR values, and thus improved IR detection and imaging performance. Nevertheless, additional work is needed in the development, testing, and optimization of microbolometer arrays that can offer superior performance to current well-established Si- and VOx-based technologies. The other nanostructure

#### **Figure 9.**

*(a) SEM images of unsuspended (left) and suspended (right) MWCNT films. (b) TEM image of a representative MWCNT, for which the shell number is estimated to be in the range of 40–50 [17]. (c) Dense oriented MWCNT growth [16].*

**Figure 10.** *Resistance vs. temperature curves for SWCNT and MWCNT films [17].*

#### **Figure 11.**

*Photoresponse of unsuspended and suspended CNT films. (a) Unsuspended MWCNT film (f = 10 Hz in IR, P = 3.0 mW/mm<sup>2</sup> ); (b) suspended MWCNT film (f = 10 Hz in IR, P = 3.0 mW/mm2 ); (c) unsuspended SWCNT film (f = 1/30 Hz in IR, P = 3.5 mW/mm<sup>2</sup> ); (d) suspended SWCNT film (f = 2 Hz in IR, P = 3.5 mW/mm<sup>2</sup> ) [2].*

**81**

*Nanostructure Technology for EO/IR Detector Applications*

**3.1 Overview of nanostructured AR coating technology**

properties, particularly with air as the ambient medium.

the coating surface be specular.

*n*λ/4 ≈ √

technology we shall now discuss, though comprising an applied rather than a core component, has demonstrated the capability to extend and enhance the performance of a wide range of electro-optical and infrared systems and devices.

**3. Nanostructured antireflection coatings for optical sensor applications**

Reducing optical reflections from surfaces, which is important to many optical applications including optical lenses, windows, photovoltaic devices, and photodetectors, has commonly been achieved through coating, or texturing, the surfaces of interest. Nanostructures that minimize reflection loss have been investigated for the development of high performance antireflection (AR) coatings. Such nanostructured AR coatings having optimal index profiles can demonstrate broadband AR

Glass, a dielectric material widely used in a variety of optical applications including lenses, windows, and as a cover or encapsulation for semiconductor optoelectronic devices, is basically transparent to wavelengths longer than 400 nm. However, due to Fresnel reflection losses glass reflects about 4% of the incident light from its front surface, or ~8% from both surfaces. This undesired reflection in certain applications can degrade the efficiency of the underlying device (e.g., sensor or photovoltaic cell), reduce its signal-to-noise ratio (e.g., in the case of a photodetector), and cause glare (e.g., from electronic displays). For such applications, it is important not only to reduce reflectance but also to improve the transmittance through the surface. This requires that the coating material be nonabsorbent, and

AR coatings have conventionally been composed of single layers having optical thicknesses equal to one quarter of the wavelength (λ/4) of interest. Ideally, such a single-layer λ/4 AR coating should have a refractive index *nλ*/4 given by [18]:

\_\_\_\_\_\_\_\_\_\_\_\_

However, due to the unavailability of materials having the precise desired refractive index, the performance of λ/4 AR coatings often deviates from the optimum, which is especially the case for low-index substrates such as glass. For example, an ideal single-layer λ/4 AR coating on a glass surface in an air ambient would require a material with a refractive index of (1.46)1/2 ≈ 1.21. Due to the unavailability of optical materials with very low (*n* < 1.4) refractive indexes, near-perfect graded-index

Recently, however, a new class of optical thin film materials consisting of tunable nanostructures has enabled the realization of very low refractive index materials [9, 19]. Using these nanostructured materials, AR coatings can greatly minimize reflection losses and enhance the sensitivity and performance of detection and imaging systems. The multilayer AR coatings feature step-graded refractive indexes which decrease in discrete steps from that of the substrate (e.g., ~1.5 for SiO2 or ~3.5 for Si) to a value of close to that of air (e.g., 1.18). These AR coatings with specular surfaces comprising multiple discrete layers of non-absorbing materials exploit thin film interference effects to reduce reflectance and maximize transmittance. Such discrete multilayer AR coatings, which have been shown outperform continuously-

AR coatings for glass substrates have not been practically achievable.

graded AR coatings, offer very effective antireflection performance [20].

*nsubstrate* <sup>×</sup> *nair* (10)

*DOI: http://dx.doi.org/10.5772/intechopen.85741*

*Nanorods and Nanocomposites*

**Figure 10.**

*Resistance vs. temperature curves for SWCNT and MWCNT films [17].*

*Photoresponse of unsuspended and suspended CNT films. (a) Unsuspended MWCNT film (f = 10 Hz in* 

*); (b) suspended MWCNT film (f = 10 Hz in IR, P = 3.0 mW/mm2*

*); (d) suspended SWCNT film (f = 2 Hz in IR,* 

*); (c) unsuspended* 

**80**

**Figure 11.**

*IR, P = 3.0 mW/mm<sup>2</sup>*

*P = 3.5 mW/mm<sup>2</sup>*

*SWCNT film (f = 1/30 Hz in IR, P = 3.5 mW/mm<sup>2</sup>*

*) [2].*

technology we shall now discuss, though comprising an applied rather than a core component, has demonstrated the capability to extend and enhance the performance of a wide range of electro-optical and infrared systems and devices.
