Proceedings of the 2011 6th IEEE International Conference on Nano/Micro Engineered and Molecular Systems February 20-23, 2011, Kaohsiung, Taiwan
Infrared Enhanced Absorption of TiNx Nano Films Yongjun Zheng, Xiaomei Yu*, Mingquan Yuan, Kan Yu National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Institute of Microelectronics, Peking University Beijing 100871, China
[email protected] Abstract: In the design of infrared detectors, nano films are more and more frequently used to improve infrared absorption and mechanical sensitivity. In this paper, a Titanium nitride (TiNx) film was studied theoretically and experimentally as an infrared enhanced absorption layer. The TiNx films at two different thicknesses and in three different content ratios were deposited and infrared absorption experiments in the range of 8~14ȝm were done. Experiment results show that the infrared absorbance is enhanced through depositing 5nm thick TiNx nano films on SiNx layer and the infrared absorbance of the films can be increased as the flow ratio of N2/Ar get increased.
II.
Infrared radiation on the surface of films can be described by formula (1).
r at 1
To decrease r and t is an effective method for increasing infrared radiation absorption. There is a high reflective metal on the surface of infrared absorption layer for thermal infrared detector with optical readout, as shown in Figure 1, so the transmittance can be ignored and the reflectivity is the only issue to be solved.
INTRODUCTION
Infrared detector has a wide range of military applications such as night vision, navigation, aviation, remote sensing and target tracking. Nowdays more and more attention has been focused on the civilian market such as environmental monitor medical diagnostics and heat detection [1] . Infrared detectors include photoelectric detectors and uncooled infrared detectors. Because expensive and bulky refrigeration equipments are needed in photoelectric detectors [2] , the present study has focused on uncooled infrared detectors. And in uncooled infrared detectors, microcantilever infrared detector owns its irreplaceable advantages and has good application prospect for its simple device structure, compatible fabrication process with microelectronic technology and very low cost. Microcantilever infrared detectors detect the infrared radiation through the bending of cantilever due to bimaterial cantilever composed of two materials with the great difference of thermal expansion coefficients, and optical method is proved to be a simple and efficient way to readout the image results [3]. In the design of infrared detectors, infrared absorption and mechanical sensitivity are two most important factors that affect the detectors performance, so thin films is brought in to improve them as infrared radiation absorber.
Optical readout Metal layer Infrared absorption layer
Infrared radiation
Figure 1. Bimaterial structure with optical readout When Infrared radiate from material 1 to material 2, the reflectivity can be calculated by formula (2).
r =
To meet the need of miniaturization and high sensitivity of infrared detectors, infrared absorbing nano films is required. Nano-infrared absorption films can not only improve the sensitivity of the device, but also can effectively improve the efficiency of infrared absorption which is the key factor to ameliorate the performance infrared detector. Some metals or metal alloys nano films will solve this problem effectively. Some simulations were done by M.F. Toy et al. to get an optimal thickness of TiNx [4]. In this paper, three samples were simulated: only TiNx, TiNx on 0.2ȝm thick SiNx and TiNx on 0.5ȝm thick SiNx, and optimal thickness of TiNx films were obtained correspondingly: 6nm, 4nm and 2nm.
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(1)
Where r is the reflectivity, a is the absorption, and t is the transmittance.
Keywords:Nano film, Infrared, Absorption, TiNx
I.
THEORY OF NANO FILMS INFRARED ABSORPTION
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R 1 -R 2 R 1+R 2
(2)
Where R1 is the sheet resistance of material 1 and R2 is the sheet resistance of material 2. If zero reflectivity is to be achieved, the sheet resistance of material 2 should be designed equal to the resistance of material 1. With more and more infrared detectors are fabricated using bulk silicon processing technology to reduce the infrared loss caused by silicon substrate, infrared radiate directly from vacuum to the infrared absorption material. The impedance of free space is 377ȍ/Ƒ, so as the sheet resistance of infrared absorption material is more close to 377ȍ/Ƒ, the reflectivity of infrared is lower. The sheet resistance of material can be calculated by the following formula (3).
R,
U
E21 exp(ik2s) E22 exp(ik2s) a2[E31 exp(ik3s) E32 exp(ik3s)] (7)
(3)
d
When x=d+s,
Where RƑ is the sheet resistance, ȡ is the resistivity and d is the thickness. The optimal sheet resistance of infrared absorption material is 377ȍ/Ƒ, then the optimal thickness of absorption layer can be calculated when the resistivity of the material is known.
E31 exp[ik3(d s)] E32 exp[ik3(d s)] E41 exp[ik4 (d s)] (8) E31 exp[ik3(d s)] E32 exp[ik3(d s)] a3E41 exp[ik4 (d s)] (9) Where k is the transmitting coefficient, and the expression of k1, k2, k3, k4 are as follows: k1=2ʌ/Ȝ, k2=2ʌ(Ș+iț)/Ȝ, k3=2ʌn/Ȝ, k4=2ʌ/Ȝ. And a1=k2/k1, a2=k3/k2, a3=k4/k3.
-7
The resistivity of Ti is 4.2×10 ȍ/m, and the optimal thickness is calculated to be 1.11nm, which is too thin to deposite and measure. The resistivity of TiNx will be bigger than that of Ti because of the mix of nitrogen, so the optimal thickness of TiNx must be thicker than 1.11nm. We can calculate the resistivity of TiNx after the sheet resistance and the thickness are measured. III.
The reflectivity R and transmittance T can be calculated.
In Figure. 2, infrared radiates from left side through metal and dielectric flims. For the metal film, s is thickness, Ș is index of refraction, ț is extinction coefficient, ı is conductivity, İ is dielectric constant, and ȝ is permeability. For the dielectric film, d is thickness and n is index of refraction.
E12 E11
E22
AL
AR
4
E31
x=s
T
E41 E41* / E11 E11*
(11)
1 sin 2 k3 d ) 2 n
f 1 n ( 2 1) 2 sin 2 k3 d ( f 2) 2 cos 2 k3 d n
(12)
2
And we can get AR by the same method.
4f f 1 n 2 ( 2 1) 2 sin 2 k3 d ( f 2) 2 cos 2 k3 d n
(13)
Where f = 4ʌț2s/Ȝ and the physical meaning of f is the ratio of sheet resistance of vacuum and metal film.
E32
E21
x=0
3 Dielectric
(10)
4 f (cos 2 k3 d
For this structure, material 1 and 4 is vacuum, material 2 is metal film and material 3 is dielectric film. Two conditions will be considered. One condition is the infrared radiating from the left side and the other one is the infrared radiating from the right side.
2 Metal
E12 E12* / E11 E11*
So the absorption A can be calculated by formula (1). We call absorption as AL when infrared radiates from the left side and AR when infrared radiates from the right side.
CALCULATION OF INFRARED ABSROPTION FILMS
1
R
The range of far infrared varies from 8ȝm to 12ȝm, so we make the wave length Ȝ equal to 10ȝm and the index of refraction of dielectric n equal to 2.5. For different thicknesses of dielectric, we can get the relationship between A and f.
E41
In the Figure 3, we can see that AR is higher than AL in the same f and d. So in the experiment, we will make the infrared radiates from left side through dielectric and metal.
x=d+s
We can get f when the absorption is on the peak, and base on the physical meaning of f and the formula (3), the optimal thickness of metal films can be calculated.
Figure 2. System of infrared radiation For the first condition, infrared radiates from left side through metal film and dielectric film. We can get boundary equations through the theory of electromagnetic wave [5].
For titanium as the metal films, the optimal thickness can be calculated. When d=0.2ȝm, s=2.45nm; when d=0.4ȝm, s=3.7nm; when d=0.6ȝm, s=4.3nm. As is concerned above, the optimal thickness of TiNx will be thicker than that of titanium.
When x=0,
E11 E12
E21 E22
(4)
E11 E12
a1 ( E21 E22 )
(5)
Figure 3(a) is the simulation result of absorption when infrared radiates from left side, and Figure 3(b) is the simulation result of absorption when infrared radiates from right side.
When x=s,
E21 exp(ik2s) E22 exp(ik2s) E31 exp(ik3s) E32 exp(ik3s) (6)
426
d=0.2ȝm d=0.4ȝm d=0.6ȝm
AL
f
a
AR
f Figure 4. The crystal structure of titanium nitride b
V.
Figure 3. Absorption with different dielectric thickness IV.
THE STRUCTURE OF TITANIUM NITRIDE
EXPERIMENTS OF TINX INFRARED ENHANCED ABSORPTION
In the experiment, very thin nano films can not be deposited and measured because of the limitation of the experimental conditions, so TiNx with the thickness of 5nm and 10nm were deposited on 200nm thick SiNx. Figure 5 is a comparison of infrared absorption spectrum between 10nm thick TiNx films on 200nm thick SiNx and only 200nm thick SiNx on silicon substrate. It can be seen that the absorption decreased when 10nm thickness TiNx was deposited. If TiNx is thicker than the optimal thickness, the reflectance of infrared will play the main role, so the infrared absorption was less than that of only SiNx. Figure 6 is a comparison of infrared absorption spectrum between 5nm thick TiNx films on 200nm thick SiNx and only 200nm thick SiNx on silicon substrate is shown. It can be seen the absorption increased when 5nm thickness TiNx was deposited. The infrared reflectance decreased as the thickness of TiNx is closer to the optimal thickness.
TiNx is a transition metal nitride which has a structure combined with covalent bond, ionic bond and metal bond. When the content of nitrogen is low, there is more metal bond in the structure and the metal property is obvious. On the contrary, there is more covalent bond in the structure and the covalent property is obvious. As shown in Figure.4, the ideal TiNx is face-centered cubic structure more like NaCl [6]. Physical Vapor Deposition (PVD) is a common method to grow TiNx. The specific method is sputtering titanium in the nitrogen atmosphere, and there will be different structure in different proportion of titanium and nitrogen. The roughness of TiNx surface increased as the content of nitrogen increase. And the roughness of surface can decrease the reflectivity of infrared, so increasing the content of nitrogen in TiNx is an effective method to improve the infrared absorption.
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reactive gas. In this experiment, TiNx deposited in three different flow ratios of N2/Ar 1:1, 1:2 and 1:9 were tested. As shown in Figure 7, we can see infrared absorption increased as the flow ratio of N2/Ar increase. When the content of nitrogen increased, the surface of TiNx films became rougher. And this will decrease the infrared reflectance, relatively, then the infrared absorption gets increased. Because of the infrared loss caused by the substrate, infrared absorption is less than the calculated value in this experiment. After deposition and absorption measurement, we have tried to measure the sheet resistance of the TiNx nano films. But the films are too thin that the probe of resistance meter pierced it, and no correct results were obtained. In the next experiment, a thinner film will be deposited for the convenience of measuring the infrared absorption and a thicker film will be deposited to get the sheet resistance measured. Also other metal and metal alloy films will be considered.
Figure 5. Infrared absorption of TiNx (10nm)
VI.
SUMMARY
In this paper, we present the theory, calculation and experiment for the infrared absorption of TiNx nano films. Through the analysis and calculation, the thickness of the TiNx films is optimized. Also in the experiment, we proved that the infrared absorption is improved when the thickness of TiNx nano films is closer to the optimal thickness. Through depositing TiNx at different conditions, we also found that infrared absorption of the films will increase as the flow ratio of N2/Ar increase. ACKNOWLEDGEMENT This work was supported by Natural Science Foundation of China (founded No. 90923028 and 60911130236), Hi-tech Research and Development Program of China (founded No. 2009AA04Z316) and National Basic Research Program of China (founded no. 2009CB320305)
Figure 6. Infrared absorption of TiNx (5nm)
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Figure 7. Infrared absorption of TiNx with different proportions of nitrogen and titanium
[5] L.N.Hadley, D.M.Dennison, Reflection and Transmission Interference Filter, Journal of the optical society of America: Volume 37, Number 6,
Different ingredients and characters of TiNx can be obtained using different proportions of nitrogen and titanium, so different infrared absorption results can be obtained. In the deposition, Argon is the sputtering gas and nitrogen is the
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