AU2021102313A4 - A silicon on insulator based optical gas sensing device and its fabrication process thereof - Google Patents
A silicon on insulator based optical gas sensing device and its fabrication process thereof Download PDFInfo
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- 239000012212 insulator Substances 0.000 title claims abstract description 25
- 238000004519 manufacturing process Methods 0.000 title abstract description 10
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 claims abstract description 26
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 claims abstract description 26
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Abstract
The present disclosure relates to a silicon on insulator based optical
gas sensing device and its fabrication process thereof. The process
facilitates modelling, analysis and development of SOI optical sensor at
mid-infrared wavelength of 3.39 pm. The gas sensor is designed on SOI
substrate. The structure of optical gas sensor consists of SOI channel
waveguides with grating coupler at either side of the waveguide. In the
analysis the refractive index value of 1 to 1.5 is considered. The analysis
reports a waveguide sensitivity in the range from 0.85 to 0.92. The power
absorption of SOI optical gas sensor is found to be almost five times
greater than that of air clad SOI optical waveguide. This type of
subwavelength integrated optical sensors is used for the detection of
percentage of carbon dioxide/methane present in the air.
23
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L-
Description
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The present disclosure relates to a silicon on insulator based optical gas sensing device and its fabrication process thereof. In more details, the process facilitates in fabricating silicon on insulator based optical integrated sensing device.
Compared to electrochemical and metal-oxide semiconductor-based sensors, the optical gas sensors on SOI substrate are proven to be more beneficial for many applications due to their fast response, selectivity and minimal drift. In recent research on biosensors and gas sensors, silicon photonics has been extensively exploited in the visible and near-infrared spectral ranges.
In recent literature, silicon photonic integrated components at mid infrared wavelength region for gas sensors are reported. These sensors are developed on SOI substrate using grating couplers, strip and slot waveguides, slotted and un-slotted photonic crystal waveguides, photonic crystal microcavities. The silicon based photonic integrated components are proven to be more suitable for quantitative measurement of carbon dioxide (C02), methane concentrations present in air. SOI platform can be employed for photonic devices, such as gas sensors, in the mid infrared wavelength range of 3 pm to 4.5pm. It is demonstrated that a silicon waveguide of 400 nm height and 1600 nm width requires a buried oxide buffer of 3.5pm so that the leakage loss is below 0.01 dB/cm and reported refractive index-based sensitivity in the range of 0.4 to 0.85. Many other silicon photonic integrated components such as evanescent field waveguides, subwavelength grating waveguides, a metal-assisted silicon slot waveguide, grating-assisted strip and slot waveguides, Bragg grating air-slot optical waveguide, silicon nitride (Si3N4) horizontal long period grating, Aluminium doped Si-waveguide, arrayed waveguide grating on SOI micro ring resonator are integrated on a silicon on insulator substrate for refractive index and absorbance based gas sensing applications.
In one solution, an electronic device and manufacturing method thereof is disclosed. In the present invention, the etching hole 21 is formed in the polysilicon film 14 which is a hollow wall member. Hydrofluoric acid is injected from the etching hole 21 to dissolve the silicon oxide film 13 to form a cavity 22. The detection unit 12 of the sensor is exposed in the cavity 22. Next, an Al film 16 is deposited in the etching hole 21 and on the upper surface of the substrate by sputtering, and then a portion of the Al film 16 located on the polysilicon film 14 is removed by etching back to perform etching. Only the metal plug 16a made of Al sealing the hole is left. Since the sputtering process is performed under a pressure of 5 Pa or less, the pressure in the cavity 22 can be maintained at a low pressure.
In another solution, an infrared gas detector is disclosed. <P>PROBLEM TO BE SOLVED: To provide an infrared gas detector capable of reducing or removing gap dispersion of a Fabry-Perot filter. <P>SOLUTION: In this infrared gas detector 100, equipped with the Fabry-Perot filter 130 for making infrared rays selectively transmitted by wavelength, a detection part 120 for receiving the infrared rays transmitted through the Fabry-Perot filter 130, and outputting a detection signal corresponding to an infrared absorption quantity; and a can package part 140, having a window part 145 transmitting the infrared rays for storing the detection part 120 and the Fabry-Perot filter 130 in an internal space 144, at least one ventilation part 146 for circulating gas in between the internal space 144 and the can outside is provided in the can package part 140. In another solution, an optical sensor is disclosed. An optical sensor (10) comprises an optical cavity defined by a dielectric body and responsive to one or more physical environmental conditions, and a waveguide (70) having a terminal end spaced apart from the optical cavity such that light is optically coupled from the terminal end of the waveguide (70) to the optical cavity. The waveguide (70) is arranged such that, in use, it is maintained at a first temperature that would not damage the optical coupling to the optical cavity when the dielectric body is maintained at a second temperature sufficient to damage the optical coupling to the optical cavity.
However, there are various gas sensors available for sensing carbon dioxide and methane gas, but these gas sensors are less sensitive. In the view of the forgoing discussion, it is clearly portrayed that there is a need to have a silicon on insulator based optical gas sensing device and its fabrication process thereof SUMMARY OF THE INVENTION
The present disclosure seeks to provide a silicon on insulator based optical gas sensing device and a process for fabricating silicon on insulator based optical gas sensing device.
In an embodiment, a silicon on insulator based optical gas sensing device is disclosed. The device includes a silicon core fabricated on top of buried oxide (BOX) layer which is mounted on a silicon on insulator (SOI) substrate, wherein two sides and top of the silicon core are exposed to air.
The device further includes a pair of optical fibers placed on top of silicon core for illuminating and collecting light. The device further includes a silicon waveguide surrounded by air and gas cladding acts as a refractive index (RI) based sensor for sensing gas.
The device further includes a pair of linear grating couplers for mode confinement between optical fiber and silicon waveguide at either ends of the silicon waveguide. The device further includes a pair of linear tapers fabricated at either side of the SOI straight waveguides separated by gap for adjusting lighting.
In an embodiment, the waveguide is of channel type with a rectangular cross section having 220 nm height and 1.2 pm width.
In an embodiment, thickness of the BOX layer is 2pm, length of input waveguide is 1mm and length of output waveguide is 1mm, thickness of silicon top layer is 220nm and height of waveguide is 220nm.
In an embodiment, the optical gas sensors cladding region contains mixture of air and gas, wherein one of the is gas selected from carbon dioxide or methane gas.
In another embodiment, a process for fabricating silicon on insulator based optical gas sensing device is disclosed. The process includes cleaning silicon on insulator (SOI) wafers through RCA clean technique to remove organic contaminants and metal contaminants.
The process further includes dicing the SOI wafer using precision diamond dicing blades. The process further includes developing pattern on the dices of SOI wafer using a direct E-beam lithography process.
The process further includes performing silicon dry etching using an inductively coupled plasma reactive ion etching (ICPRIE) tool to etch silicon at top layer of the SOI wafer.
In an embodiment, two levels of RCA cleaning process is used for SOI wafers, wherein RCA-1 is used to remove any organic contaminants by cleaning the SOI wafer using 5:1:1 mixture of DI water: H202 : NH40H at 75degree Celsius for a duration of 10 minutes, wherein RCA-2 is used to remove the metal contaminants by cleaning the SOI wafer using 6:1:1 mixture of DI water: H202 : HCI at 75degree Celsius for a duration of 10 minutes.
In an embodiment, the SOI dices is of 1cmx1cm from SOI wafer are obtained.
In an embodiment, a direct E-beam lithography process comprises:
performing spin coating of Ma-N 2400, negative photoresist at 3000 rpm to obtain a thickness of nearly 3pm; wherein the photoresist is developed using MF26A developer; and executing direct LASER writing of complete structure on SOI wafer using a Raith E-beam Lithography.
In an embodiment, ICPRIE consists of a bottom electrode over which the sample is placed and a top electrode which provides inductive coupling.
In an embodiment, an anisotropic etch of silicon is performed using CHF3 gas, wherein etch rate is 200nm/min and etch time is approximately minute and 06 seconds.
An object of the present disclosure is to develop SOI optical sensor at mid-infrared wavelength of 3.39 pm.
Another object of the present disclosure is to determine refractive index-based waveguide sensitivity.
Another object of the present disclosure is to perform testing of the fabricated SOI optical sensor for different concentrations of carbon dioxide and methane gas sensing applications.
Yet another object of the present invention is to deliver an expeditious and cost-effective process for fabricating silicon on insulator based optical gas sensing device.
To further clarify advantages and features of the present disclosure, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings. It is appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting of its scope. The invention will be described and explained with additional specificity and detail with the accompanying drawings. BRIEF DESCRIPTION OF FIGURES
These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Figure 1 illustrates a block diagram of a silicon on insulator based optical gas sensing device in accordance with an embodiment of the present disclosure; Figure 2 illustrates a flow chart of a process for fabricating silicon on insulator based optical gas sensing device in accordance with an embodiment of the present disclosure; Figure 3 illustrates a structure of integrated SOI waveguide for gas sensing applications in accordance with an embodiment of the present disclosure;
Figures 4A and 4B illustrate mathematical modelling of a SOI channel waveguide with air clad and SOI channel waveguide with (Air
+ Gas) cladding in accordance with an embodiment of the present disclosure; Figure 5 illustrates a modal power confinement in silicon core in accordance with an embodiment of the present disclosure; Figure 6 illustrates power absorption in waveguide for cladding RI of air and cladding (Air + Gas) in accordance with an embodiment of the present disclosure; Figure 7 illustrates variation of real value of effective RI as a function of wavelength in accordance with an embodiment of the present disclosure; Figure 8 illustrate variation of real value of effective RI as a function of wavelength in accordance with an embodiment of the present disclosure; Figure 9 illustrates waveguide sensitivity in accordance with an embodiment of the present disclosure; Figure 10 illustrates SEM images of SOI gas sensor after Direct E beam lithography process in accordance with an embodiment of the present disclosure; Figure 11 illustrates SEM images of SOI gas sensor after ICP RIE process in accordance with an embodiment of the present disclosure; and Figure 12 illustrates AFM height measurements (Silicon waveguide) of fabricated device in accordance with an embodiment of the present disclosure.
Further, skilled artisans will appreciate that elements in the drawings are illustrated for simplicity and may not have been necessarily been drawn to scale. For example, the flow charts illustrate the method in terms of the most prominent steps involved to help to improve understanding of aspects of the present disclosure. Furthermore, in terms of the construction of the device, one or more components of the device may have been represented in the drawings by conventional symbols, and the drawings may show only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the drawings with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated system, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.
Reference throughout this specification to "an aspect", "another aspect" or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, appearances of the phrase "in an embodiment", "in another embodiment" and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process or method that comprises a list of steps does not include only those steps but may include other steps not expressly listed or inherent to such process or method. Similarly, one or more devices or sub-systems or elements or structures or components proceeded by "comprises...a" does not, without more constraints, preclude the existence of other devices or other sub-systems or other elements or other structures or other components or additional devices or additional sub-systems or additional elements or additional structures or additional components.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The system, methods, and examples provided herein are illustrative only and not intended to be limiting.
Embodiments of the present disclosure will be described below in detail with reference to the accompanying drawings.
Referring to Figure 1, a block diagram of a silicon on insulator based optical gas sensing device is illustrated in accordance with an embodiment of the present disclosure. The device 100 includes a silicon core 102 fabricated on top of buried oxide (BOX) layer 104 which is mounted on a silicon on insulator (SOI) substrate 106. Two sides and top of the silicon core 102 are exposed to air.
In an embodiment, a pair of optical fibers 108 is placed on top of silicon core 102 for illuminating and collecting light. The optical fibers 108 are placed to the ends of the silicon core 102.
In an embodiment, a silicon waveguide 110 is surrounded by air and gas cladding acts as a refractive index (RI) based sensor for sensing gas. The silicon waveguide 110 is constructed in a semi-circular shape. In an embodiment, the waveguide 110 is of channel type with a rectangular cross section having 220 nm height and 1.2 pm width.
In an embodiment, a pair of linear grating couplers 112 are used for mode confinement between optical fiber and silicon waveguide 110 at either ends of the silicon waveguide 110.
In an embodiment, a pair of linear tapers 114 is fabricated at either side of the SOI straight waveguides 110 separated by gap for adjusting lighting. The linear tapers 114 are adjacent to the linear grating couplers 112. In an embodiment, thickness of the BOX layer 104 is 2pm, length of input waveguide is 1mm and length of output waveguide is 1mm, thickness of silicon top layer is 220nm and height of waveguide is 220nm.
In an embodiment, the optical gas sensors cladding region contains mixture of air and gas, wherein one of the is gas selected from carbon dioxide or methane gas.
In an embodiment, the power absorption of SOI optical gas sensor is found to be almost five times greater than that of air clad SOI optical waveguide 110. This type of subwavelength integrated optical sensors can be used for the detection of percentage of carbon dioxide/methane present in the air. Figure 2 illustrates a flow chart of a process for fabricating silicon on insulator based optical gas sensing device in accordance with an embodiment of the present disclosure. The process is used for modeling, development, and sensitivity analysis of SOI optical channel waveguide 110 for gas sensing applications in the spectral range of 3pm to 4.5pm is presented.
At step 202, the process 200 includes cleaning silicon on insulator (SOI) wafers through RCA clean technique to remove organic contaminants and metal contaminants. In an embodiment, two levels of RCA cleaning process is used for SOI wafers, wherein RCA-1 is used to remove any organic contaminants by cleaning the SOI wafer using 5:1:1 mixture of DI water: H202 : NH40H at 75degree Celsius for a duration of minutes, wherein RCA-2 is used to remove the metal contaminants by cleaning the SOI wafer using 6:1:1 mixture of DI water: H2O2 : HCI at degree Celsius for a duration of 10 minutes.
At step 204, the process 200 includes dicing the SOI wafer using precision diamond dicing blades. In an embodiment, the SOI dices is of 1cm x1cm from SOI wafer are obtained.
At step 206, the process 200 includes developing pattern on the dices of SOI wafer using a direct E-beam lithography process.
At step 208, the process 200 includes performing silicon dry etching using an inductively coupled plasma reactive ion etching (ICPRIE) tool to etch silicon at top layer of the SOI wafer. In an embodiment, ICPRIE consists of a bottom electrode over which the sample is placed and a top electrode which provides inductive coupling.
In an embodiment, a direct E-beam lithography process includes performing spin coating of Ma-N 2400, negative photoresist at 3000 rpm to obtain a thickness of nearly 3pm. The photoresist is developed using MF26A developer. Thereafter, executing direct LASER writing of complete structure on SOI wafer using a Raith E-beam Lithography.
In an embodiment, an anisotropic etch of silicon is performed using CHF3 gas, wherein etch rate is 200nm/min and etch time is approximately minute and 06 seconds.
Figure 3 illustrates a structure of integrated SOI waveguide for gas sensing applications in accordance with an embodiment of the present disclosure. Figure 3 shows the structure of integrated optical structure on silicon on insulator (SOI) substrate 106. The structure consists of optical fiber for light illumination and collection, grating couplers 112 along with linear grating for mode confinement between optical fiber and silicon waveguide 110 at either ends of the silicon waveguide 110. The gas chamber is placed on top of the silicon waveguide 110 for sensing applications. The proposed SOI based integrated optical structure shown in Figure 3 is designed to operate at mid infrared wavelength of 3.39 pm.
Figures 4A and 4B illustrate mathematical modelling of a SOI channel waveguide with air clad and SOI channel waveguide with (Air
+ Gas) cladding in accordance with an embodiment of the present disclosure. Mathematical modelling of SOI channel waveguide 110 at mid infrared wavelength of 3.39 pm is discussed.
The wavelength of operation is chosen to be 3.39 pm. At this wavelength, carbon dioxide (C02) and methane possesses a significant optical absorption. The dimensions of the subwavelength SOI waveguide 110 for air clad and air + Gas mixture clad are depicted in Figures 4A and 4B respectively. The waveguide 110 is of channel type with a rectangular cross section. The dimensions of 220 nm height and 1.2 pm width are chosen for TE10 dominant mode confinement in the silicon channel waveguide 110. The core 102 is made of silicon that lies on top of buried oxide layer 104 (BOX). The two sides of the core 102 and its top are exposed to air, as shown in Figure 3. The thickness of the BOX layer 104 is 2pm. Table.1 shows the refractive index (RI) and absorption loss of optical materials silicon and BOX.
Table 1: Optical properties of silicon and BOX at operating wavelength 3.39 pm.
Material Refractive Index Absorption [dB/cm] Silicon (Si) 3.434 0 BOX (Si0 2 ) 1.401 0
Equation (1) gives the confinement factor of the waveguide 110.
2 - ff|E(x,y)WG dxdy 2 ff|E(X,Y)TotalI dxdy
In equation (1), |E(x,y)wG 2 represents the power confinement in the core 102 part of SOI waveguide 110 and |E(X,Y)Totall 2 represents the power confinement in the complete optical waveguide 110 including cladding region. In the waveguide 110 geometry shown in Figure 4, the cladding represents the sensing region of gas sensor. The proposed waveguide 110 structure acts as a RI based sensor. The waveguide 110 sensitivity can be determined using equation (2).
S-= aneff (2) anc
Where, anc, represents the change in RI of cladding and aneff represents the change in effective RI of the propagating mode in silicon core 102.
Figure 5 illustrates a modal power confinement in silicon core in accordance with an embodiment of the present disclosure. The modal analysis of SOI waveguide 110 structure shown in Figure 3 is carried out using finite difference Eigen mode solver. The modes are determined at A=3.39pm.
The cladding region of SOI waveguide 110 shown in Figure 4, acts as a sensing region. In optical gas sensors cladding region contains the mixture of air and gas. In particular, for optical gas sensing applications gas can be either carbon dioxide or methane. Therefore, in simulation the refractive RI of cladding region is varied between 1 to 1.45. Figure 5 shows the modal power density in silicon core 102 of the waveguide 110. The analysis is carried out for power absorption for cladding of air with its RI as unity and RI of 1.45 for air and gas composition.
Figure 6 illustrates power absorption in waveguide for cladding RI of air and cladding (Air + Gas) in accordance with an embodiment of the present disclosure. Figure 6 shows power absorption in waveguide 110 for cladding RI of air (1) and 1.45 RI cladding (Air + Gas).
Figure 6 shows the power absorption is more for air and gas mixture in comparison with air clad. The power absorption for air and gas mixture is almost 5 times more than that of power absorption in air clad. The power density is almost 1 nW/M2 for air clad whereas it is increased to almost 5.5 nW/m2 for air and gas mixture cladding. It clearly indicates that the proposed waveguide 110 is feasible for absorbance-based gas sensors.
Figure 7 illustrates variation of real value of effective RI as a function of wavelength in accordance with an embodiment of the present disclosure. The imaginary RI of the effective index is found be almost zero up to wavelength range of 3.737 pm and it gradually increases from 3.737 pm to 4.5 pm wavelength range as shown in Figure 7.
Figure 8 illustrate variation of real value of effective RI as a function of wavelength in accordance with an embodiment of the present disclosure. The variation of real value of effective RI as a function of wavelength is shown in Figure 8. It shows that real value of effective RI of complete waveguide 110 structure gradually reduces with increase in wavelength. The imaginary RI of the effective index is found be almost zero up to wavelength range of 3.737 pm and it gradually increases from
3.737 pm to 4.5 pm wavelength range as shown in Figure 7. The effective index of the waveguide 110 is found to be 1.70 at operating wavelength 3.39 pm. At this wavelength the imaginary value of effective RI is zero and there will be no optical losses in the waveguide 110.
In optical gas sensors, the presence of carbon dioxide or methane or any gas molecules in air will gradually alter the RI of gas - air mixture which will modify the atmospheric refractive index. The RI of different percentage by volume of carbon dioxide- air mixture is obtained by Lorentz-Lorenz mixing rule. In the simulation the RI of gas - air mixture is varied from 1 to 1.5 for different composition and volumes of air and gas mixtures. Table 2 indicates the variation of effective index for different values cladding RI.
Table.2: The Variation of effective RI for different values of cladding RI at A=3.39pm
Cladding 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45 1.5 RI
Effective 1.7 1.714 1.725 1.736 1.748 1.761 1.774 1.787 1.802 1.817 1.833 Index
Figure 9 illustrates waveguide sensitivity in accordance with an embodiment of the present disclosure. The sensitivity of the waveguide 110 is determined and it is plotted in Figure 8 using equation (2). Figure 9 shows that the waveguide 110 sensitivity gradually increases from 0.85 to 0.92 for variations in gradual variation in RI of waveguide 110 cladding from 1 to 1.5. The reported sensitivity 0.85 to 0.92 is comparable with silicon nanowire optical rectangular waveguide sensor.
The structure of Integrated SOI waveguide 110 shown in Figure 9 is fabricated on SOI substrate 106. The fabricated SOI integrated optical component consists of grating couplers 112, linear taper 114 at either side of the SOI straight waveguides separated by gap. Table.3 shows the dimensions of the optical waveguide 110 for fabrication process.
Table.3: Dimensions of the optical waveguide.
Parameter Dimensions Length of input waveguide 1 mm Length of Output waveguide 1 mm BOX thickness 2 pm Silicon top layer thickness 220 nm Height of waveguide 220 nm
The thickness of the top silicon layer is 220 nm and that of BOX layer 104 is 2 pm. The fabrication of complete optical structure shown in Figure 3 involves the RCA cleaning, SOI wafer dicing, direct E-beam lithography and silicon dry etching respectively. Silicon RCA cleaning process is done before processing the SOI wafer. The two levels of RCA cleaning process is used for SOI wafers. RCA-1 is used to remove any organic contaminants. In this process, the SOI wafer cleaning is done by using 5:1:1 mixture of DI water: H202 : NH40H at 75degree Celsius for a duration of 10 mins. RCA-2 is used to remove the metal contaminants. In this process, the SOI wafer cleaning is done by using 6:1:1 mixture of DI water: H202 : HCI at 75degree Celsius for a duration of 10 mins. After RCA cleaning process wafer dicing is done using precision diamond dicing blades. SOI dices of, 1cmx1cm from SOI wafer are obtained for further process of the fabrication.
Figure 10 illustrates SEM images of SOI gas sensor after Direct E beam lithography process in accordance with an embodiment of the present disclosure. Figure 10 shows SEM images of SOI gas sensor after Direct E-beam lithography process (a) Input Grating end for launching light from optical fiber (b) Interface between Grating 112 and Taper 114 (c) SOI straight waveguide 110 with gap for sensing (d) Interface between taper and grating at the output end.
The pattern development on the dices of SOI wafer is carried out using direct E-beam lithography process. This process involves the following steps:
• Spin coating of Ma-N 2400, negative photoresist at carried out at 3000 rpm to obtain a thickness of nearly 3pm.
• Develop the photoresist using MF26A developer.
• Direct LASER writing of the complete structure on SOI wafer using Raith E-beam Lithography.
The SEM images of lithography patterning of the complete optical sensor having gratings, gratings and taper 114 interface, waveguides with gap are shown in Figure 10.
Figure 11 illustrates SEM images of SOI gas sensor after ICP RIE process in accordance with an embodiment of the present disclosure. Figure 11 shows SEM images of SOI gas sensor after ICP RIE process (a) Input Grating end for launching light from optical fiber (b) Interface between Grating 112 and Taper 114 (c) SOI straight waveguide 110 with gap for sensing (d) Interface between taper 114 and grating 112 at the output end.
After direct E-bem lithography process, silicon dry etch is then performed using an inductively coupled plasma reactive ion etching
(ICPRIE) tool to etch the Silicon at the top layer of SOI wafer. In ICPRIE process the patterned silicon wafer is loaded into the tool and the recipe is initiated. The system consists of a bottom electrode over which the sample is placed and a top electrode which provides the inductive coupling. An anisotropic etch of Silicon is done with the help of CHF3 gas. The etch rate is 200nm/min and thus the etch time is approximately minute and 06 seconds.
Figure 12 illustrates AFM height measurements (Silicon waveguide) of fabricated device in accordance with an embodiment of the present disclosure. The atomic force microscopy (AFM) characterization is dine to measure the thickness of fabricated SOI channel waveguide 110. The measurement of AFM characterization is shown in Figure 12. It indicates that the height of the fabricated waveguide is 220 nm.
The modelling, analysis and development of SOI optical sensor at mid infrared wavelength of 3.39 pm for optical gas sensor applications is presented. The SOI channel waveguide of 220 nm height and 1200 nm width has dominant mode confinement with almost zero optical loss and effective index of 1.7 to 1.833. The refractive index of 1 to 1.5 is considered for cladding region in the simulation. In the analysis the refractive index-based waveguide sensitivity is determined and is found to be in the range of 0.85 to 0.92. The reported sensitivity 0.85 to 0.92 is comparable with the work reported in R.R. Singh et al., silicon nanowire optical rectangular waveguide sensor. The power absorption for air and gas clad silicon waveguide is found to be 5 times greater than that of air clad silicon waveguide. The detailed fabrication steps for the SOI based optical integrated sensor along with SEM and AFM characterization results is reported. Further testing of the fabricated SOI optical sensor needs to be carried out in the future work for different concentrations of carbon dioxide and methane gas sensing applications. This type of subwavelength integrated optical sensors can be used for the quantitative detection of percentage of carbon dioxide/methane present in the air.
The drawings and the forgoing description give examples of embodiments. Those skilled in the art will appreciate that one or more of the described elements may well be combined into a single functional element. Alternatively, certain elements may be split into multiple functional elements. Elements from one embodiment may be added to another embodiment. For example, orders of processes described herein may be changed and are not limited to the manner described herein. Moreover, the actions of any flow diagram need not be implemented in the order shown; nor do all of the acts necessarily need to be performed. Also, those acts that are not dependent on other acts may be performed in parallel with the other acts. The scope of embodiments is by no means limited by these specific examples. Numerous variations, whether explicitly given in the specification or not, such as differences in structure, dimension, and use of material, are possible. The scope of embodiments is at least as broad as given by the following claims.
Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any component(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or component of any or all the claims.
Claims (10)
1. A silicon on insulator based optical gas sensing device, the device comprises:
a silicon core fabricated on top of buried oxide (BOX) layer which is mounted on a silicon on insulator (SIO) substrate, wherein two sides and top of said silicon core are exposed to air; a pair of optical fibers placed on top of silicon core for illuminating and collecting light; a silicon waveguide surrounded by air and gas cladding acts as a refractive index (RI) based sensor for sensing gas; a pair of linear grating couplers for mode confinement between optical fiber and silicon waveguide at either ends of the silicon waveguide; and a pair of linear tapers fabricated at either side of said SOI straight waveguides separated by gap for adjusting lighting.
2. The device as claimed in claim 1, wherein said waveguide is of channel type with a rectangular cross section having 220 nm height and 1.2 pm width.
3. The device as claimed in claim 1, wherein thickness of said BOX layer is 2pm, length of input waveguide is 1mm and length of output waveguide is 1mm, thickness of silicon top layer is 220nm and height of waveguide is 220nm.
4. The device as claimed in claim 1, wherein said optical gas sensors cladding region contains mixture of air and gas, wherein one of said is gas selected from carbon dioxide or methane gas.
5. A process for fabricating silicon on insulator optical gas sensing device, the process comprises: cleaning silicon on insulator (SOI) wafers through RCA clean technique to remove organic contaminants and metal contaminants; dicing said SOI waferusing precision diamond dicing blades; developing pattern on said dices of SOI wafer using a direct E beam lithography process; and performing silicon dry etching using an inductively coupled plasma reactive ion etching (ICPRIE) tool to etch silicon at top layer of said SOI wafer.
6. The process as claimed in claim 5, wherein two levels of RCA cleaning process is used for SOI wafers, wherein RCA-1 is used to remove any organic contaminants by cleaning the SOI wafer using 5:1:1 mixture of DI water: H2O2 : NH40H at 75degree Celsius for a duration of 10 minutes, wherein RCA-2 is used to remove the metal contaminants by cleaning the SOI wafer using 6:1:1 mixture of DI water: H2O2 : HCI at degree Celsius for a duration of 10 minutes.
7. The process as claimed in claim 5, wherein said SOI dices is of 1cm x1cm from SOI wafer are obtained.
8. The process as claimed in claim 5, wherein a direct E-beam lithography process comprises:
performing spin coating of Ma-N 2400, negative photoresist at 3000 rpm to obtain a thickness of nearly 3pm; wherein said photoresist is developed using MF26A developer; and executing direct LASER writing of complete structure on SOI wafer using a Raith E-beam Lithography.
9. The process as claimed in claim 5, wherein ICPRIE consists of a bottom electrode over which the sample is placed and a top electrode which provides inductive coupling.
10. The process as claimed in claim 5, wherein an anisotropic etch of silicon is performed using CHF3 gas, wherein etch rate is 200nm/min and etch time is approximately minute and 06 seconds.
Figure 3
Figure 10
Figure 11
Figure 12
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