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^oC, 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 (Ta₂O₅), aluminum oxide (Al₂O₃), and Ta₂O₅/Al₂O₃ 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 Ta₂O₅ has the highest capacitance density (3.84x10^-15 F/μm²), followed by the capacitor with 60 nm Ta₂O₅/Al₂O₃ nanolaminate, and the capacitor with 60 nm Al₂O₃ capacitor dielectric film with a capacitance density of 2.40x10^-15 F/µm² and 1.37x10^-15 F/μm², respectively. The calculated dielectric constant of ALD Ta₂O₅ and Ta₂O₅/Al₂O₃ nanolaminate is 26.1 and 16.3, respectively, while that of ALD Al₂O₃ is 9.3, as shown in Figure 1.

Figure 2 shows the I-V characteristics of MIM capacitors with 4055 μm² area and with 60 nm of ALD Ta₂O₅, Ta₂O₅/Al₂O₃ nanolaminate, and Al₂O₃ capacitor dielectric. As can be seen, the capacitor with Al₂O₃ capacitor dielectric has significantly lower leakage and higher breakdown voltage (46 V), while the capacitor with Ta₂O₅/Al₂O₃ and Ta₂O₅ 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^oC to 125^oC. 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.