Vanadium oxide thin films for bolometric applications deposited by ...

Vanadium oxide thin films for bolometric applications deposited by reactive pulsed dc sputtering N. Fieldhouse, S. M. Pursel, R. Carey, and M. W. Horn Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, Pennsylvania 16802

S. S. N. Bharadwajaa兲 Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802

共Received 2 October 2008; accepted 23 March 2009; published 30 June 2009兲 Vanadium oxide 共VOx兲 thin films were deposited by reactive pulse dc magnetron sputtering process using a pure vanadium metal target. The structural, microstructure, and electrical properties were correlated as a function of processing parameters such as substrate temperature, Ar:O partial pressures ratios, and pulsed dc power to fabricate these films. The VOx films deposited at various substrate temperatures between 30 and 300 ° C using a range of oxygen to argon partial pressure ratios exhibited huge variation in their microstructure even though most of them are amorphous to x-ray diffraction technique. In addition, the electrical properties such as temperature coefficient of resistance 共TCR兲, resistivity, and noise levels were influenced by film microstructure. The TCRs of the VOx films were in the range of −1.1% to − 2.4% K−1 having resistivity values of 0.1– 100 ⍀ cm. In particular, films grown at lower substrate temperatures with higher oxygen partial pressures have shown finer columnar grain structure and exhibited larger TCR and resistivity. © 2009 American Vacuum Society. 关DOI: 10.1116/1.3119675兴

I. INTRODUCTION Night vision via infrared detection is an important field of technology for both civilian and defense applications. There are two physical principles used to acquire IR images: photonic and thermal detection.1 The former involves absorption of IR photons, which excites the charge carriers in the material and thus produces a change in the output voltage and/or current. The later involves detection of temperature by means of change in the material’s resistance as it absorbs the IR radiation.1 Even though both these methods are useful, thermal based night vision offers several advantages in terms of wide spectral response and uncooled IR detection capability. Recent development in fabrication, efficiency, and low manufacturing cost has resulted in the use of uncooled microbolometers in many microdevices for the civilian and defense applications. Much progress has been made in the past decade in night vision technology with the development of several different uncooled bolometer materials.2 Microbolometers consist of an array of thermally sensitive pixels that change resistance as an infrared radiation is focused onto the array. This change in resistance is measured with respect to the initial offset followed by the signal processing. Vanadium oxide thin films are commonly used in microbolometers due to their low resistivity 共0.025– 10 ⍀ cm兲 and high temperature coefficient of resistance 共TCR兲.3 Reactive sputtering,4 pulsed laser ablation,5 ion beam assisted deposition,6 and chemical vapor deposition7 are a few deposition methods that have been used to grow VOx films. Most of these earlier studies suggest that electrical and thermal properties are mainly controlled by a兲

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value of x in VOx thin films. Vanadium oxide exists in multiple phases 共both crystalline and Magneli phases兲8,9 due to its multivalent nature. As a result, it is difficult to precisely control the oxidation state and electrical properties of the VOx film. This study focuses on vanadium oxide thin films grown using pulsed dc sputtering. There are several factors when sputtering that can allow one to manipulate the film composition and structure when grown. The amount and type of gas present during deposition in addition to the substrate temperature are two of the more important parameters worth investigating. For instance, a more reactive gas present in the chamber is likely to affect the composition of the film produced. While heating the substrate may supply the film with enough energy to allow for a more ordered structure. Sputtering yields high deposition rates and has a comparatively simple setup, giving it several advantages over the other methods of deposition Several types of electrical modes can be used when sputtering 共dc, rf, and pulsed dc兲. dc sputtering is appropriate when a conductive target is sputtered in an oxygen-free atmosphere shown in Fig. 1共a兲. If dc power is used on a metallic target 共such as vanadium兲 in an oxygen rich atmosphere, an insulating layer of VOx will form on the target effectively reducing the sputtering rate and uniformity of film. To prevent this from happening, a rf or pulsed dc source is used; examples can be seen in Figs. 1共b兲 and 1共c兲. Continuously changing the polarity of the target prevents the insulating oxide layer from forming. One of the challenges of an oxide layer forming on a target surface is the unavoidable surface arching during

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©2009 American Vacuum Society

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Fieldhouse et al.: Vanadium oxide thin films for bolometric applications

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FIG. 1. Three modes of target voltage operation.

deposition. To understand in simple terms, the electrical charge that can buildup on the target under high voltages with a surface layer can be given as Q = VC,

共1兲

where Q is the charge, V the voltage, and C the capacitance due to thin oxide layer formation. The “capacitor” described uses the conductive target and the ionic plasma as the charge carriers while the insulating metal oxide formed on the surface of the target acts as the dielectric. One can further define the capacitance as C = 共␧␧0兲共A/d兲,

共2兲

where A is the area, d is the thickness, and ␧ is the dielectric constant of the surface layer of the metal oxide layer and ␧0 is the vacuum permittivity 共8.854⫻ 10−12 F / m兲. In addition, expressing the voltage in terms of field 共E兲 across the surface layer thickness 共d兲 as V = Ed,

共3兲

the effective buildup charge over time 共t兲 by a current density J 共assuming J is constant兲 is given by Q = JAt.

共4兲

Thus the electric field strength across the target would be E = 共J/␧␧0兲t.

共5兲

Therefore, pulsing the target between two opposite polarities for short time periods 共microsecond order兲 enables a self-sustaining dc plasma by avoiding oxide layer formation on the target surface.10 To prevent target poisoning, alternative power modes, such as radio frequency 共rf兲 or pulsed dc source, are useful. A continuous changing of the polarity on the target can prevent the insulating oxide layer formation on its surface. While both rf and pulsed dc sources would suffice, pulsed dc yields higher deposition rates than rf mode since the target is negative for longer amounts of time. Therefore, this work tests the viability of pulsed dc sputtering as a method used to fabricate VOx thin films and to characterize the electrical properties of the resultant films was investigated. II. EXPERIMENTAL PROCEDURE The VOx thin films were deposited via pulsed dc magnetron sputtering in an oxygen/argon 共both 99.9% pure兲 atmoJ. Vac. Sci. Technol. A, Vol. 27, No. 4, Jul/Aug 2009

FIG. 2. Schematic of deposition chamber.

sphere under vacuum 共Kurt J. Lesker; model: CMS 18兲. A pulsed dc power source 共Advanced Energy; model: MDX Sparc—LE 20兲 was used to control the current 共 ⬇ 0.85– 0.91 A兲 and voltage 共 ⬇ 350 V兲 supplied to the vanadium target 共99.9% Kurt J. Lesker兲. The pulse operated at a frequency of 20 kHz and had a width of 5 ␮s. All films were grown on 6 in. 关001兴 Si wafers coated with a SiO2 layer approximately 1500 Å thick. The target was pulsed at 20 kHz and a power of 300 W. All films were grown in a load-locked chamber on a rotating stage to ensure reproducible and uniform films. Figure 2 shows the schematic of the deposition system that was used to deposit VOx thin films in a pulse dc mode. Wafers were mounted onto a steel plate in an upside-down configuration during the deposition and the target was located at the bottom of the chamber facing the substrate. The target to substrate throw distance was 25.4 cm and the substrate temperature was controlled using a set of quartz infrared lamps in conjunction with an automated temperature controller. All the VOx films were grown with different oxygen partial pressures 共5%, 7.5%, 10%, 15%, and 30%兲 to vary the oxygen content in the resultant films. The wafer substrates were kept at a constant temperature and were rotated during the deposition. The ratio between Ar:O was fixed at a value by adjusting the flow rates of these gases during the deposition. The substrate temperature during deposition was found to be around 40 ° C without heating it. Other sets of films were also grown with different substrate temperatures as noted in Table I. The purpose of varying the processing conditions is to control the resultant VOx thin film properties

TABLE I. List of deposition variables. Deposition temperature 共°C兲 40 200 300

%O2 5 5 5

7.5 7.5 7.5

10 10 10

15 15 15

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TABLE II. AFM results of films deposited at 40, 200, and 300 ° C. Deposition temperature 共°C兲

%O2

Grain size 共nm兲

Roughness 共nm兲

40

5 10 15

50 50 40

0.76 0.395 1.165

200

5 10 15

30 40 25

1.61 2.11 0.98

300

5 10 15

30 40 30

2.32 1.06 0.78

such as resistivity, TCR, and stability via controlling chemical composition, structure, and microstructure features. A profilometer 共TENCOR 500兲 was used to measure thickness on the films. Cross sectional microstructures of the films were examined using a field emission scanning electron microscopy 共FESEM兲共model: LEO 1530兲. X-ray diffraction studies 共Scintag X2 theta-theta powder diffractometer兲 were carried out between 10° 艋 2␪ 艋 60° to determine crystallinity of the as deposited VOx samples and the results indicated all the films were x-ray amorphous. Atomic force microscopy studies were carried out 共Digital Instruments Nanoscope IIIa scanning probe microscope兲 to analyze the surface morphology of the films grown at different processing conditions. Nickel strip electrodes 共⬃5 mm in length兲 were deposited through a stencil mask using a dc magnetron sputtering onto the surface of the films for electrical measurements. Currentvoltage 共I-V兲 characterization was done in a two probe configuration over a range of 20– 120 ° C in ambient conditions using an automated picoampere meter 共HP model: 4140B兲 and a heated stage. The TCRs of the resultant films were estimated using the following equation: %TCR =

⳵共ln R兲 ⳵共ln ␳兲 ⫻ 100 = ⫻ 100, ⳵T ⳵T

共6兲

where R is the resistance and ␳ is the resistivity. III. RESULTS AND DISCUSSION The surface morphology studies by atomic force microscopy 共AFM兲 on several of the films indicated an insignificant change in surface roughness values or average grain size as a function of either deposition temperature or partial pressure of oxygen; which can be seen in Table II. The maximum surface roughness value of the film was ⬃2.5 nm and the average grain size value was ⬃30– 35 nm, respectively. Profilometry results indicated that all the films were approximately 100 nm thick for a deposition time of 900 s 共rate of deposition is ⬃0.11 nm/ s兲. In addition, cross sectional scanning electron microscopic images were used to estimate the thickness as well as microstructure of the films, as shown in Fig. 3. With a scale as a reference, the thickness of the films was measured through an image analysis proJVST A - Vacuum, Surfaces, and Films

FIG. 3. SEM images of the VOx films grown with oxygen partial pressures and temperatures. 共a兲 5% 40 ° C; 共b兲 5% 200 ° C; 共c兲 5% 300 ° C; 共d兲 15% 40 ° C; 共e兲 15% 200 ° C; 共f兲 15% 300 ° C.

gram. Proper precautions were taken into account to minimize the error values in estimating thickness values using appropriate angle correction factors. Simultaneous comparison of cross sectional images of the films processed at different growth conditions indicated the growth rates of ⬃0.1 nm/ s and were found consistent with the profilometery thickness analyses. Furthermore, the cross sectional SEM images showed a columnar structure in all the grown films. Even though the films possess distinct columnar structure, x-ray analysis determined that these films was amorphous. However, selective area transmission electron microscopy 共TEM兲 studies indicated some of the films possess micro/ nanocrystallites and further studies are still in progress to determine the distribution of the crystallites across the films. A high TCR is an important factor when designing microbolometers because it directly describes larger resistivity changes with respect to temperature changes. Additionally the resistivity is an equally important factor since it determines the necessary amount of power required to operate the device. Sensors that have lower resistivities will require less power and enable the fabrication of efficient microbolometers. Thus, pulse dc sputtering would be a viable and alternative method to fabricate VOx based microbolometer. However, issues such as noise and stablity of film resistance analysis are still needed to optimize the processing conditions for VOx thin films for microbolometer applications. Currently, these studies are in progress. Current versus voltage characteristics of the films were found to be linear 共or Ohmic兲 between 20 and 130 ° C. The electrical measurements were performed using a pair of surface electrodes. All of the sample sets showed an increase in resistivity and TCR as the amount of oxygen supplied during deposition was increased. The resistivities of the films ranged from 0.01 to 100 ⍀ cm while the TCRs varied between −0.4 to − 2.3% K−1.

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FIG. 5. Effect of the partial pressure of oxygen on resistivity at 25 ° C 共a兲 and TCR in the VOx thin films 共b兲.

conduction mechanisms of these various films with their structure using Raman spectroscopy and TEM analysis. FIG. 4. Resistivity measurements as a function of temperature for films deposited at 共a兲 40 ° C, 共b兲 200 ° C, 共c兲 and 300 ° C respectively.

Interestingly, there is a general trend that as the deposition temperature was increased, the resistivity of the films made under a fixed oxygen to argon pressure ratio decreased. The 200 and 300 ° C films in Fig. 4 had lower room temperature resistivity than the 40 ° C samples. These TCR and electrical resistivity values are comparable to VOx deposited using ion beam sputtering and currently incorporated into commercial microbolometer arrays. Figure 5 plots the room temperature resistivity and TCR of VOx thin films prepared at various substrate temperatures as a function of oxygen partial pressure. Notice that both the TCR and resistivity increase as the percent oxygen in the sputtering ambient increases. At a fixed oxygen content, there is significant difference in both resistivity and TCR for films grown at different substrate temperatures. Trends indicate that if a film with a high TCR is desired, one can increase the amount of oxygen present during the deposition. If, however, films with a high TCR and relatively low resistivity are important, then depositing the film at higher temperatures may be necessary to achieve both of these simultaneously. We are currently correlating the J. Vac. Sci. Technol. A, Vol. 27, No. 4, Jul/Aug 2009

IV. SUMMARY Vanadium oxide films deposited via pulsed dc sputtering showed electrical properties comparable to films manufactured from traditional ion beam systems. The TCRs of the VOx films were found to be in the range of −1.1% to −2.4% K−1 having resistivity values of 0.1– 100 ⍀ cm. The resistivity and TCR could be controlled by adjusting the argon to oxygen partial pressure ratio as well as the substrate temperature conditions during deposition. High temperature depositions 共200 and 300 ° C兲 can be used to produce lower resistivity values than those of room temperature samples, while maintaining a comparable TCR values. ACKNOWLEDGMENTS The authors acknowledge Sean Pursel for assisting with the FESEM analysis and Dr. Orlando Cabarcos helping to acquire the AFM results. This research was sponsored by the U.S. Army Research Office and U.S. Army Research Laboratory under Cooperative Agreement No. W911NF-0-2-0026. 1

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