Characterization of ALD Ta₂O₅, Al₂O₃, and Ta₂O₅/Al₂O₃ Nanolaminate as Metal-Insulator-Metal Capacitor Dielectric for GaAs HBT Technology

Abstract

In order to reduce die size and increase capacity, the metal-insulator-metal (MIM) capacitor area in circuit designs fabricated using GaAs hetero-junction bipolar transistor (HBT) technology has to be reduced. This can be achieved by using a capacitor dielectric material that has a high dielectric constant or high capacitance density. In GaAs HBT technology, the MIM capacitor dielectric film is typically also required to have a high breakdown voltage (>20 V) and low leakage current. Furthermore, the deposited dielectric film has to be compatible with GaAs processing and has to be deposited at a temperature of < 300 o C, due to the limited process thermal budget in the HBT technology. As a result of these requirements, there are only a few materials available for MIM capacitor dielectric application in GaAs HBT technology. In this study, 60 nm tantalum pentoxide (Ta 2 O 5 ), aluminum oxide (Al 2 O 3 ), and Ta 2 O 5 /Al 2 O 3 nanolaminate deposited using atomic layer deposition (ALD) have been characterized and evaluated as MIM capacitor dielectric for GaAs HBT technology. The results show that the capacitor with 60 nm of ALD Ta 2 O 5 has the highest capacitance density (3.84x10 -15 F/μm 2 ), followed by the capacitor with 60 nm Ta 2 O 5 /Al 2 O 3 nanolaminate, and the capacitor with 60 nm Al 2 O 3 capacitor dielectric film with a capacitance density of 2.40x10 -15 F/µm 2 and 1.37x10 -15 F/μm 2 , respectively. The calculated dielectric constant of ALD Ta 2 O 5 and Ta 2 O 5 /Al 2 O 3 nanolaminate is 26.1 and 16.3, respectively, while that of ALD Al 2 O 3 is 9.3, as shown in Figure 1. Figure 2 shows the I-V characteristics of MIM capacitors with 4055 μm 2 area and with 60 nm of ALD Ta 2 O 5 , Ta 2 O 5 /Al 2 O 3 nanolaminate, and Al 2 O 3 capacitor dielectric. As can be seen, the capacitor with Al 2 O 3 capacitor dielectric has significantly lower leakage and higher breakdown voltage (46 V), while the capacitor with Ta 2 O 5 /Al 2 O 3 and Ta 2 O 5 capacitor dielectric has higher leakage current and lower breakdown voltage of 32.7 V and 22.4 V, respectively. Figure 3 shows the C-V characteristics of capacitors with the 3 ALD capacitor dielectric films. The results show that the capacitance of MIM capacitor using these three films increased marginally by 2.2% to 4.3%, when the temperature was increased from 25 o C to 125 o C. No significant change in capacitance density of these ALD capacitor dielectric films was observed, when the applied voltage was varied from -5 V to +5 V. Figure 1