Firstly, we make the test HfO2 capacitors in order to obtain the optimum condition of the process temperature for the SOHOS memory device in this study. The detail of all process parameters (also called split table) will be describes later.
3.2.1 Fabrication of Test HfO2 Capacitor
Figure 3-1 (a)-(b) schematically depicts the cross-section and the process flow of the proposed test HfO2 capacitors. The fabrication process of the capacitors were carried out on 4-inch p-type (100)-oriented silicon substrate wafers. The resistivity of the wafer is about 15-25 Ω cm. First of all, the wafers were cleaned down by standard RCA cleaning. Before growing the tunnel oxide film, the all wafers were dipped into diluted HF solution to remove the chemical oxide which grown during the standard RCA cleaning.
Subsequently, the cleaned wafers were steeped into H2O2 solution at room
temperature immediately. The immersion process let wafers grow about 10 Å chemical oxide in 20 min [60]. Then the chemical oxide carried out the nitridation process through LPCVD (Low Pressure Chemical Vapor Deposition) furnace in low-pressure (180 mTorr) NH3 ambient at 750 for 15 min. Next, the nitrided ℃ chemical oxide was placed in atmospheric O2 ambient at 900℃ for 70 sec. Now, the whole tunnel oxide process is completed, and the initial chemical oxide already became the oxynitride film. Nevertheless, the tunnel oxide of another control sample is only grown in atmospheric furnace at 900℃. Then 45~50 Å HfO2 film was
deposited by Dual E-gun evaporate deposition system, followed by post-deposition annealing (PDA) in the nitrogen ambient using a RTA (Rapid Temperature Annealing) system. The split table of PDA temperature is listed in Table 3.1. Afterward, about 150 Å blocking oxide was deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition). In order to improve the quality of the blocking oxide film, the samples were carried out RTA densification in O2 ambient at 600℃ for 30 sec.
To form the top electrode, about 5000 Å aluminum (Al) metal film was deposited by sputtering system. Then the Al film was patterned and sintered to define the capacitors. Finally, a 5000 Å Al film was also deposited on the backside of the wafers as the lower electrode of the capacitors. Before the lower electrode film deposition, the backside native oxide must be stripped by using BOE (Buffered Oxide Etchant) to reduce the contact resistance.
In the same process condition, a nitrogen distribution profile across the 35Å
oxynitride tunnel oxide layer was revealed by secondary ion mass spectrometry (SIMS) in Fig. 3-2. Apparent high nitrogen concentration with a peak located at the tunnel oxide top surface is observed. Such high nitrogen concentration is more helpful in resisting the boron penetration phenomenon of the gate dielectric from the P+-poly-silicon gate. Moreover, the lower nitrogen concentration at the interface lying in between tunnel oxide and Silicon substrate also improves the reliability of the devices.
3.2.2 Fabrication of HfO2 Trapping Layer Memory Device
Figure 3-3 (a)-(b) schematically depicts the cross-section and the process flow of the SOHOS Flash memory cell with HfO2 trapping layer. The experimental process of the memory device were carried out on 4-inch p-type (100)-oriented silicon substrate wafers. The resistivity of the wafer is about 15-25 Ω cm. First of all, the wafers were cleaned down by standard RCA cleaning. Before growing the tunnel oxide film, the all wafers were dipped into diluted HF solution to remove the chemical oxide which grown during the standard RCA cleaning.
In the same way, the characteristics of HfO2 memory devices should be compared between the control and experimental samples. Therefore, the tunnel oxide of control sample grew in atmospheric furnace at 900 ℃ . Subsequently, the
experimental devices were steeped into H2O2 solution at room temperature immediately. The immersion process let wafers grow about 10 Å chemical oxide in 20 min. Then the chemical oxide carried out the nitridation process through LPCVD
(Low Pressure Chemical Vapor Deposition) furnace in low-pressure (180 mTorr) NH3
ambient at 750 for 15 min. Next, the nitrided chemical oxide was placed in ℃ atmospheric O2 ambient at 900℃ for 70 sec. Now, the whole tunnel oxide process is completed, and the initial chemical oxide already become the oxynitride film. Then 45~50 Å HfO2 film was deposited by Dual E-gun evaporate deposition system, followed by post-deposition annealing (PDA) in the nitrogen ambient using a RTA (Rapid Temperature Annealing) system. Afterward, about 150 Å blocking oxide was deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition). In order to improve the quality of the blocking oxide film, the samples were carried out RTA densification in O2 ambient at 600 for 30 sec. Then, the poly℃ -silicon gate 3000 Å is formed by LPCVD at 900 ℃ for 30 min. After poly-silicon/blocking oxide/HfO2/tunnel oxide stack formation, gate pattern were defined by lithography and etched back. Subsequently, the Source/Drain was formed in POCl3 ambient at 900℃ for 30 min and then carried out Drive-in process in nitrogen ambient at 900℃
for 30 min. Next, the passivation layer 5000 Å followed removing the PSG (Phosphorous Silicon Glass) on the gate, source and drain region. Then we define the contact hole by lithography and BOE etching back.
To form the top metal pad, about 5000 Å aluminum (Al) metal film was deposited by sputtering system. Then the Al film was patterned and sintered to define the metal pad. Finally, a 5000 Å Al film was also deposited on the backside of the wafers as the substrate contact of the device. Before the substrate contact film deposition, the backside native oxide must be stripped by using BOE (Buffered Oxide
Etchant) to reduce the contact resistance.
Figure 3-4 schematically describes the construction of measurement system.
The electrical characteristics of SOHOS memory device and test capacitors are measured by HP4284 Precision LCR Meter and HP4156C Precision Semiconductor Parameter Analyzer.