FACULTY OF ENGINEERING LAB SHEET EOP3036 Fabrication and Packaging Technology TRIMESTER 2 2017-2018 FP2 Simulation of fabrication processes and modelling of photodiodes Notes: 1. Maximum of TWO students are allowed to form a group. 2. On-the-spot evaluation will be carried out during or at the end of the experiments. Questions regarding Silicon planar photodiodes, device processing and fabrication, as well as device simulation will be asked during the evaluation. 3. Students are advised to read through this lab sheet before conducting this experiment. Your individual performance during on-the-spot evaluation, participation in the simulation work, teamwork effort, and learning attitude will count towards the lab marks, in addition to the lab report. 4. Please bring along a USB stick (thumb drive) for the purpose of data saving. 5. This laboratory experiment contains the Learning Outcome 2 of the subject [LO2 design the fabrication and packaging processes of optoelectronics materials and devices (cognitive creating level 6)].
Objective To study the effect of difference ion implantation conditions on the current-voltage characteristic of the photodiode. Experiments/Apparatus 1. SILVACO Software. 2. Computer. Introduction Figure 1: Cross section of a planar p + -n silicon photodiode. Figure 1 shows the cross section of a planar p + -n silicon photodiode. The p + area can be realised by ion-implantation of boron into a predefined area on the surface of a highly resistive n-type silicon wafer so that a p-n junction could be formed within the device. The geometry of the predefined area can be defined by depositing a layer of silicon dioxide (SiO 2 ) diffusion mask having windows etched in it using common photolithography technique. The depth of the junction is basically relied on the choice of the wavelengths of light to be detected. For example, if the device is designed mainly for detecting blue and ultraviolet wavelengths, a shallow junction is required so that the photogenerated carriers are able to diffuse to the junction, being swept out from the device and contribute to the photocurrent. The front contact is usually realised by litt-off the metal that is previously evaporated on a layer of pre-patterned photoresist. The back contact can be realised by multilayer metallisation process. Due to the gradient of the doping concentration of boron after the implantation process, holes from the p + region diffuse across the n-type silicon region while electrons from the n-type region diffuse into the p + region so that the charge neutrality could be maintained at the interface between these regions. Thus, a depletion region is formed at the interface between the p + region and n-type silicon regions with an electric field across 2
it which is commonly known as the built-in electric field. This depletion width increases with the applied reverse bias and causes the capacitance of the device drops. When light (photon) is absorbed in the depletion width, an electron-hole pair is generated if the energy of the light is greater than the bandgap energy (~ 1.1 ev) of silicon. The photogenerated hole and electron are being swept out from the depletion region into the p + and n-type regions, respectively in the reverse bias mode. This eventually yields the photocurrent that flows in the external circuitry. On the other hand, photogenerated holes and electrons have the possibility of recombine together before they are being swept out from the depletion region and hence do not contribute to photocurrent. The parameter used to quantify the sensitivity a photodiode in absorbing light is given by the responsivity, R [A/W] which is defined as the ratio of the photogenerated current over the incident optical power as below [4] R I P ph opt q hv μm 1.24, [A/W] (1) where is the quantum efficiency (percentage of the capability of converting light into current) of the photodiode, q = 1.6 10-19 C, h is the Plank constant, v is the frequency of light, I ph is the photocurrent and P opt is the incident optical power. For example, silicon based photodetectors operating at wavelengths in the range of 900 nm have quantum efficiency value of 80 % and give a responsivity value of 0.58 A/W. Equation (1) also shows that the responsibility of the photodetector with the same quantum efficiency will drop when incident photons with shorter wavelength is shone on the photodetectors. When light is shone on the photodiode, electron-hole pairs can be generated in or outside the depletion region (space-charge region that forms between the p + -n boundary). The generation rate of these carries is decreasing exponentially as a function of the depth of the photodiode. If the depth of the junction is poorly designed and formed deep into the surface of the p + region, there is only a small fraction of light that can arrive at the junction. In this situation, almost all of the photogenerated carriers within the surface of the p + region to the p + -n interface have recombined before they are able to diffuse to the depletion region. This yield in low photocurrent level because only those electron-hole pairs that are generated, by the small fraction of light, in or at a distance within a diffusion length away from the depletion region are able to contribute to photocurrent. As a result, the quantum efficiency is less than unity which degrades the responsivity of the photodiode as well as its performance. The relationship between the current that flows through the photodiode with the applied reveries bias is given by the diode equation and it is written as below q Vrb Vbi nk BT I I 0e 1, (2) 3
where q = 1.6 10-19 C, k B = 1.38 10-34 J/m, V bi is the built-in voltage, V rb is the applied reverse bias and T is the temperature in Kelvin. n is the ideality factor of the photodiode which n = 1 for an ideal photodiode and n > 1 for a non-deal photodiode. The parameter I 0 is the saturation current of the photodiode which consists of the minority currents of the electrons and holes that are diffusing across the p + -n junction and it is written as below 2 2 Dnn Dpn i i I 0 qa, (3) Ln N a Lp N d where A is the cross section of the junction of the photodiode, n i is the intrinsic concentration, D n and D p are the electron and hole diffusion constants, and L n and L p are the electron and hole diffusion lengths, respectively. In this lab work, the diode equation is represented by the continuity equations for electron and hole and they are numerically solved in the Silvaco software so that the current-voltage characteristic of the photodiode can be determined. Device Fabrication Processes The device that is shown in figure 1 can be realised by a series of fabrication processes which includes photolithography, etching, thin film deposition, ion implantation and metallisation. The actual sequence of the fabrication steps that is used to realise the device shown in figure 1 is described below. Fabrication procedures: 1. Deposit a layer of SiO 2 to serve as the hard-mask for the ion implantation of boron. 2. Transfer the patterns (opening area for ion implantation) from the optical mask to the SiO 2 layer by means of photolithography process. 3. Etch away the areas of SiO 2 which are not covered by the photoresist so that the areas for ion implantation are exposed. 4. The dopants (boron) are implanted into the n-type silicon wafer by ion implantation process. 5. The dopants are further drive-in into the silicon wafer using the thermal diffusion process. 6. Remove the photoresist. 7. Transfer the top metal contact patterns from the optical mask to the silicon wafer by means of photolithography process. 8. Perform metallisation process so that aluminium contacts are formed at the predefined areas on the surface of the silicon wafer. 9. Perform litt-off process to remove the unwanted metals and then remove the photoresist. 10. Perform another metallisation process to deposit metal on the rear surface of the wafer. 4
Simulation Procedures The fabrication procedures that had been outlined in the previous section can be simulated using the ATHENA module in the Silvaco software. The ATHENA is a simulation module that is made to model or simulate difference real fabrication processes. As shown in figure 1, the p + region of the silicon photodiode can be realised via the ion-implantation technique in which the dopant (Boron) is implanted into the substrate by aiming a high energy ion beam that consists of the dopants on the prepatterned substrate. The dopants hit and penetrate into the wafer, travelling a distance before they eventually come to rest due to the nuclear and electronic stopping processes that deaccelerate the dopants. The simulation of the ion-implantation process can be implemented via the ATHENA module in the Silvaco software. The default model used in the ATHENA to describe the ion-implantation process is called SIMS verified Dual Pearson (SVDP) model [1]. In this model, the concentration of the implanted dopants in terms of the longitudinal direction is described by a linear combination of two Pearson functions [1] and the simulated dopant profile had been experimentally verified. Usually, the implanted dopants are located about a few hundred nanometers deep into the surface and the thermal drive-in process is required following the implantation process if a deep junction is required. The Fermi diffusion model based on the Fick s law is employed in the Silvaco to describe the thermal drive-in of the implanted dopants into the n-type silicon substrate [1]. The mechanism involve in the dopant diffusion depends on the concept of Pair Diffusion in which the diffusion of the dopants is assisted by the presence of the point defects such as self-interstitial and lattice vacancy surrounding the dopant, and they are moving together as a pair into the silicon wafer. How deep the dopant diffuses into the substrate is controlled by the drive-in time which is basically determined by the required depth of the junction. The temperature required in the drive-in process is used to provide the dopant with enough thermal energy so that they can diffuse into the substrate at a faster rate. Section ONE This section requires the students to perform the following steps to simulate the fabrication processes of photodiodes by using ATHENA (Please take note that the Click action here is referred to Right Click instead of Left Click of the mouse). A. To start the ATHENA under DECKBUILD 1. Open a Terminal window and type deckbuild an or click on the TCAD icon on the desktop. B. Generate the input file 1. Click on File. 2. Select Save (Save menu will then popup). 3. Specify the file name as mysipd.in. 5
4. Press the Save button. This file will then be stored in /home/user (user here refers to the user name, i.e. tcad3, that you used to log on to this computer). C. Defining initial grid 2. Select Mesh Define (Mesh Define Menu will then popup). 3. Click on Location field and enter a value of 0.0. 4. Click on Spacing field and enter a value of 0.5. 5. Click Insert button and these commands will appear in the scrolling list of the Mesh Define Menu. 6. Repeat step 3 to 5 to insert another new X location and spacing with the values of 20.0 and 0.5, respectively. 7. Click on the Y direction button, follow the steps from 3 to 5 to insert the following Y locations and spacing. Location Spacing 0.0 0.02 0.2 0.02 1.0 0.1 10.0 4.0 8. Add comment of Mesh Definition at the Comment line. 9. Preview the grid by selecting the View button (View Grid window will then popup). 10. Click on the Write button to write the mesh information into the input file. D. Initialise the substrate region 2. Select Mesh Initialize (Mesh Initialize Menu will then popup). 3. Select and enter the following parameters accordingly. Silicon Orientation 100 Impurity Phosphorus Concentration 1 10 14 cm -3 Dimensionality Auto Grid scaling factor 1.0 4. Add comment of Initialise Silicon substrate at the Comment line. 5. Click on the Write button to write the initialisation information into the input file. E. Deposition of oxide mask 2. Click on Process and then Deposit. 3. Select the Deposit (Deposit menu will then popup). 4. Select and enter the following parameters accordingly. 6
Type Conformal Oxide Thickness 0.5 Grid Specification (total no. of grid layer) 1 5. Add comment of Deposition of oxide mask at the Comment line. 6. Click on the Write button to write the oxide layer into the input file. F. Open the ion-implantation window by etching 2. Click on Process and then Etch. 3. Select the Etch (Etch menu will then popup). 4. Select and enter the following parameters accordingly. Etch Method Geometrical Geometrical type Right Oxide Etch Location 5.0 5. Add comment of Etching for implantation window at the Comment line. 6. Click on the Write button to write the etch command into the input file. G. Ion-implantation of dopant 2. Click on Process and then select the Implant (Implant menu will then popup). 3. Select and enter the following parameters accordingly. Impurity Boron Dose 1 10 14 cm -3 Energy 35 kev Model Dual Pearson Tilt angle 0 Rotation angle 0 type Crystalline Damage Do not click any of them (Defect free) 4. Add comment of Ion-implantation for Boron at the Comment line. 5. Click on the Write button to write the implantation command into the input file. H. Thermal drive-in the dopant 2. Click on Process and then select the Diffuse (Diffuse menu will then popup). 7
3. Select and enter the following parameters accordingly. Time 10 minutes Temperature 900 C Temperature mode Constant Ambient Nitrogen Gas Pressure 1 atm. 4. Add comment of Thermal drive-in dopant at the Comment line. 5. Click on the Write button to write the thermal drive-in command into the input file. I. Etch away the oxide mask after the ion-implantation and thermal drive-in processes. 2. Click on Process and then Etch. 3. Select the Etch (Etch menu will then popup). 4. Select and enter the following parameters accordingly. Etch Method Geometrical Geometrical type Left Oxide Etch Location 5.0 5. Add comment of Etch off oxide mask at the Comment line. 6. Click on the Write button to write the etch command into the input file. J. Reflect the current structure to get a complete structure The structure that has been made until this stage actually represents only half of the whole structure. We can make use of the structure command in the ATHENA to make a mirror image of the current structure to yield the full structure. This can be done by applying the following steps. 2. Click on Structure and then Mirror (Mirror menu will then popup). 3. Select and enter the following parameters accordingly. Mirror Right 4. Click on the Write button to write this command into the input file. 5. Add comment of Reflect the current structure after the # symbol which is located just right above the struct statement that is written just now in the TextEdit Window of the DECKBUILD window. 8
K. Realise top contact metal This subsection deposits a thin layer of aluminium as the contact material: 2. Click on Process and then Deposit (Deposit menu will then popup). 3. Select and enter the following parameters accordingly. Type Conformal Aluminium Thickness 0.2 Grid Specification (total no. of grid layer) 2 4. Add comment of Deposition of Al contact layer at the Comment line. 5. Click on the Write button to write the aluminium layer into the input file. This subsection etches away the unwanted thin layer of aluminium to form the contact at the pre-defined region: 6. Open the Commands menu. 7. Click on Process and then Etch. 8. Select the Etch (Etch menu will then popup). 9. Select and enter the following parameters accordingly. Etch Method Geometrical Geometrical type Left Aluminium Etch Location 7.0 10. Add comment of Etch away unwanted Al layer at the Comment line. 11. Click on the Write button to write the etch command into the input file. 12. Repeat step 6 to 11 for the following values. Etch Method Geometrical Geometrical type Right Aluminium Etch Location 10.0 L. Define the electrodes for ATLAS This subsection defines the top contact as anode. 2. Click on Structure and then Electrode (Electrode menu will then popup). 3. Select and enter the following parameters accordingly. 9
Name anode Electrode Type Specified Position X Position 8.5 Y Position 0.0 4. Click on the Write button to write this command into the input file. 5. Add comment of Top contact (Anode) after the # symbol which is located just right above the electrode statement that is written just now in the TextEdit Window of the DECKBUILD window. This subsection defines the rear contact as cathode. 6. Select and enter the following parameters accordingly. Name Electrode Type cathode Backside 7. Click on the Write button to write this command into the input file. 8. Add comment of Rear contact (Cathode) after the # symbol which is located just right above the electrode statement that is written just now in the TextEdit Window of the DECKBUILD window. M. Save the structure into a file 1. Place the mouse over the TextEdit area within the DECKBUILD window and left click at the line below the electrode statement of the rear contact. 2. Type the below statements to save the processed structure into a file. # Save processed structure structure outf=sipd_01.str N. View the completed structure 1. Click on the Tool, select Plot and then Plot Structure. 2. Press OK (TONYPLOT window will then popup). 3. Click File on TONYPLOT window and then select Load Structure. 4. Select SiPD_01.str structure file. 5. Press Load and Dismiss (The structure file is then plotted in the TONYPLOT window). O. Extract device structure parameters This subsection extracts some important structure parameters after the whole device structure is obtained. The extract statement which is available in the VWF Tool [3] will be used in this section to extract the structure parameters. 2. Click on the Extract (Extraction menu will then popup). 3. Select Junction Depth (Junction Depth menu will then popup). 10
4. Select and enter the following parameters accordingly. Name Xj Silicon Occurrence 1 Junction Occurrence 1 Result Data File Xj01.final Extract Location X 19 5. Click on the Write button to write this command into the input file. 6. Add comment of Extract junction depth after the # symbol which is located just right above the extract statement that is written just now in the TextEdit Window of the DECKBUILD window. Device Simulation Procedures This section simulates the electrical characteristics of the silicon planar photodiode that was fabricated in the previous part (Device Fabrication Processes). The influences of the carrier recombination, described by Shockley-Read-Hall (SRH) and Auger recombination, on the carrier transport are included in the simulation. These processes will cause some of the carriers recombine before they arrive at the contacts and contribute to electrical current. The concentration dependence of mobility of the carriers and the velocity saturation effect are also included in the simulation to simulate the transport behaviour of the carriers within the device [2]. This simulation will obtain the current-voltage (I-V) characteristic of the photodiode in response to an incident light that is having a wavelength value of 623 nm and intensity value of 5 W/cm 2. The simulation result is then obtained by performing the Newton method to solve all the models that are used to describe the transport of the carriers in the device. a. Type the below statements in the input file (displayed in the text edit window of DECKBUILD) that have been created previously (mysipd.in) to carry out the simulation. # Device simulation part go atlas # Set contact material to be opague material material=aluminium imag.index=1000 # Model Specification material material=silicon taup0=2.e-6 taun0=2.e-6 11
# Define monochromatic beam beam num=1 x.origin=20.0 y.origin=-1.0 angle=90.0 wavelength=0.623 # model used models srh auger conmob fldmob impact selb # method newton trap solve init # save the simulation output log outf=iv_01.log master solve vcathode=0.0 vstep=0.5 vfinal=5.0 name=cathode b1=5 save outf=sipd_02.str # plot the final structure tonyplot SiPD_02.str quit b. Click File on the DECKBUILD window and then Save to save these commands into the mysipd.in input file. c. Press the Run button on the DECKBUILD window to perform the simulation. Procedures for Obtaining, Plotting and Saving Simulated Results This section describe the procedures to obtain, plot and save the simulated results so that you can bring those data back for further analysis and report writing. Perform the below procedures to obtain, plot and save the simulated results. This subsection is to save the net doping of the device structure into a pdf file. 1. Click on Plot at the TONYPLOT window to display the structure. 2. Select Display (2D Mesh menu will then popup). 3. Click on the Edges (2 nd icon), Region (3 rd icon), Contour (4 th icon), Junction (7 th icon) and Electrode (8 th icon). 4. Press Apply and then Dismiss. 5. Usually, the net doping distribution of the structure will be displayed by default. If it is not, you can change the setting to display the net doping distribution by clicking on the Plot (Display (2D mesh) menu will then popup). Click Define and then chose Contour. Click on the Quantity on the popup menu and then select Net Doping. The purple line in the plot shows the boundary of the junction and the value of the junction depth is stored in the file, Xj01.final that was saved previously. 6. Click on Print, select Option and enter the following details to save the net doping distribution of the structure in to a postscript file. You can open the file and export/convert it to pdf format using GhostView programme. 12
Destination File name Printer Form File SiPD_02_str Postscript Letter (P) 7. Click Apply, Print and then Dismiss. This subsection is to obtain and save the simulated data into a file. 8. Click Tools on the TONYPLOT window. 9. Select Cutline (Outline menu will then popup). 10. Click on the most right icon to specify the cutline coordinates as below. Start X = 19 Y = 0 End X = 19 Y = 10 11. Press Enter key and then Confirm and Dismiss. 12. Click on File and select Export. 13. Export menu will then popup. Enter below details. Format Data File Basename Extension out Tonydata All data Tonydata dat 14. Use Microsoft Excel or any plotting software to plot the simulation result for the net doping concentration (data in column 2), acceptor concentration (data in column 5) and electric field in Y direction of the device (data in column 8) as a function of depth of the device (data in column 1). This subsection is to obtain and save the I-V characteristic of the device into a file. 15. Click File on TONYPLOT window. 16. Select Load Structure and click on the IV_01.log file. 17. Click Load and then Dismiss. 18. Place mouse over and click on the I-V graph area. 19. Click Plot on TONYPLOT window and select Display (XY Graph menu popup). 20. Change the X quantity to Cathode Voltage and Y quantity to Cathode Current. 21. Press Apply and Dismiss. 22. Click File and select Export. 23. Export menu will then popup. Enter below details. 13
Format Data File Basename Extension out Tonyplot User Data Display only IV_01 dat 24. Press Export, OK and then Dismiss. 25. Use Microsoft Excel or any plotting software to plot the simulated data for cathode voltage (at x-axis) and current (at y-axis). Section TWO Apply the procedures in Section ONE to study the effect of the difference ionimplantation energies and diffusion times on the electrical characteristics of the photodiode. 26. Copy all the result files into a new folder so that these files will not be overwritten by the new files from the new simulation setting. 27. Change the ion-implantation energy to 85 kev in the section G of the Device Fabrication Process. 28. Click File on the DECKBUILD window and select Save to save the changes. 29. Press the Run button on the DECKBUILD window to perform the simulation. 30. Repeat all the procedures, from step 1 until step 25, in Procedures for Obtaining, Plotting and Saving Simulated Results section to obtain, plot and save the simulated data into files. 31. By carry out the similar procedures as before (steps 26, 28-30) and set the ion implantation energy back to 35 kev, change the diffusion time to 45 minutes in the section H of the Device Fabrication Process to obtain another new set of data. Section THREE (Analysing and evaluating the electrical properties of photodiodes) Analyse and discuss how ion-implantation energy and diffusion time affect the follow photodiode characteristics: a) depth of the junction, b) profile of the acceptor concentration, c) net doping concentration, d) electric field, and e) current-voltage characteristics. Based on the results, evaluate and explain which fabrication condition yields the best performance of the photodiodes characteristics. 14
References 1. ATHENA User s Manual, ver. 5.10.R, SILVACO International, Jan. 11, 2006. 2. ATLAS User s Manual Device Simulation Software, ver. 5.10.R, SILVACO International, Dec. 8, 2005. 3. VWF Interaction Tool User s Manual, SILVACO International, July 18, 2005. 4. B. G. Streetman and S. K. Banerjee (2006). Solid State Electronic Devices. Prentice Hall Series on Solid State Physical Electronics, Nick Holonyak, Jr., Series Editor, Pearson Education Singapore. Guideline for Report Submission 1. The report should include an introduction of the device processing for the silicon planar photodiodes, simulation codes, simulation results, discussions, summary and references. 2. The report write-up and discussions cannot be duplicated but the TWO students who have conducted the experiment together can only share the simulation results. 3. No plagiarism is allowed. 4. The lab report could be type-written and submitted to the staff for Nanotechnology laboratory 2 (Nanolab 2). Please make sure you sign on the student list for your submission. 5. The lab report MUST be submitted within TEN days from the date you have conducted the lab session. 6. No late submission is allowed if not some marks will be taken away due to the late submission. 15