GaN device is one of the candidates for high power RF applications because of a wide bandgap energy. Otherwise, GaN has a heat problem. So, the packaging for heat problems should be with GaN device. The aluminum substrate can be a solution for the heat dissipation. Our Laboratory members have researched many circuits with integrated passive device(IPD) on the aluminum substrate. Our research purpose is device for high power application, so IPD capacitor should increase their reliability because of the high voltage.
Breakdown voltage from original IPD method is relatively high, but not stable. One reason is that 1st copper metal of IPD capacitor is not flat due to the thermal transformation. So, nickel addition for barrier metal gives more flat metal surface rather than before. Breakdown voltage of nickel material based MIM IPD capacitor was increases.
Other approaches for high breakdown voltage is MIM capacitor on silicon substrate. Proposed Si MIM Capacitor is smaller than single layer on ceramic(SLC) capacitor with high breakdown voltage. It can be used for DC blocking capacitor.
Another issue for capacitor is tolerance. IPD capacitor has a miss-alignment due to its MIM structure. However, proposed inter-digital capacitor consumes just 1 layer for the capacitor. It is easy to make for the small capacitance. It was made with IPD fabrication process. So inter-digital capacitor has same advantages of aluminum substrate. Library data for 108 units was made. Total size of capacitor depends on capacitance, so small capacitance capacitor has a decided advantages than SLC capacitor on business.
2.4GHz RF GaN Power Amplifier was designed on the aluminum substrate with modified nickel IPD capacitor fabrication. Passive device was integrated on the aluminum substrate and GaN active chip was used. DC model was based on DC pulse model and fitting method. EEHEMT model and Materka model were selected for large signal model. Small signal model was offered by datasheet. Stability and s-parameters were simulated and CLC matching network was used with IPD process.
Anodized aluminum was used for substrate and NiCr and Ti/Cu/Ni metal were used for metal layer. SiNx layer was made from PECVD and benzo-cyclo-butene (BCB) layer was used as an passivation layer. Die--attach process was used electrical thermal conductive epoxy.
DC I-V curve data form fabricated 2.4GHz PA were obtained. After replacing an integrated capacitor to Si MIM capacitor, proper DC I-V curve data was obtained. Some thermal degradation was observed but Si MIM capacitor can be used as a DC blocking capacitor of miniaturized RF circuit.
In this thesis, reliability for passive capacitor was successfully improved. First, breakdown voltage of modified MIM capacitors with nickel was improved. Second, breakdown voltage of Si MIM capacitor was also improved and can be one of the candidates for the DC blocking capacitor.
본 논문에서는 RF High Power로의 응용을 위한 집적회로에 사용될 신뢰성 향상을 위한 커패시터를 구현함으로써 고출력 응용으로의 신뢰성 향상이 가능하도록 하였다. 이를 바탕으로 RF GaN Power Amplifier를 Anodized Aluminum Substrate를 이용하여 Passive 소자들을 집적하여 구현하고, 제안된 Capacitor들을 응용할 수 있도록 하였다.
연구실에서 이제까지 사용된 Copper Ending의 MIM Capacitor의 경우, 높은 온도를 필요로 하는 공정에는 온도에 의한 변형이 일어나므로 Breakdown의 저하가 일어나게 될 우려가 있다. 이에 Breakdown Voltage가 낮아지는데, Breakdwon Voltage를 증진시키기 위해 편평한 1st Metal을 만들기 위해서 기존의 Copper Metal 위에 Nickel을 Barrier Metal로 사용하고, 산화 방지를 위한 Gold Ending 처리가 필요하다. 제안한 Nickel 물질의 추가를 통해 IPD MIM Capacitor의 Breakdown Voltage를 향상시켰다.
Aluminum 기판에서의 집적 외에도, 외부 Passive 소자의 응용과 상용 SLC 보다 작은 Capacitor 공정을 이용하기 위한 SMD-type Si MIM Capacitor를 제안하였고, 이것을 구현하여 작은 사이즈이면서 동시에 높은 Breakdown Voltage를 갖는 Passive 소자를 만들었다.
또한, 기존의 MIM Cap.에서 구조적으로 발생할 수밖에 없는 Tolerance의 개선을 위해 Matching단에 사용할 수 있는 작은 Capacitance 값을 갖는 Cap.을 위해 Inter-digital Capacitor를 제안하였다. 이를 IPD 공정과 같은 공정을 이용하여 Library Data를 잡고, 측정하였다. 작은 Size는 물론 Tolerance를 개선한 Capacitor를 제안하여 Reliability를 증가시켰다.
이러한 세 가지 Capacitor를 응용하여 고전력 RF 증폭기를 위한 기판에 집적하여 구현하였다. GaN Chip의 경우에는 Large Signal Model에 대해 세 가지 접근을 하여 IV Curve를 비슷하게 수정, 디자인 하였고, Nickel을 이용한 IPD MIM Cap.을 적용한 Cap을 포함한 IPD 기술로 Aluminum 기판에 2.4GHz RF HPA를 구현하였다. TGF 2023-01 Active Chip을 Wire Bonding 하여 동작시켰으며, 합당한 DC I-V Curve를 얻었다.
이 기판에서 DC Blocking Cap.으로 사용되고 있던, Nickel이 접목된 IPD MIM Cap. 대신 제안된 Si MIM Cap.을 적용하였고, 이것으로부터 얻은 DC I-V Curve 역시 합당한 결과를 얻어 제안된 Si MIM Cap.은 DC Blocking Capacitor로 동작이 가능하다는 것을 보였다.
