Heterogeneous Integration for High Performance Electronic and Photonic Devices

In this paper, two representative heterogeneous integrated applications with great performance based on epitaxial layer transfer process are reported. The SiC integrated Si PIN limiter has shown power handling capability of 150W (continues Wave) and 370W (long pulse) at 5.4GHz, which shows outstanding performance than conventional GaAs limiter. While the SiC integrated InP PIN photodetector has shown a high performance of 3-dB bandwidth over 67GHz, and 42% increasing of the optical responsibility over 0.51 A/W compared to the traditional PD. The results verified great potential of transistor level heterogeneous integration for high performance electronic and photonic devices.


Introduction
As Moore's law is running out of steam, heterogenous integration technologies such as multichip modules, ball grid array, flip chip, 2.5D/3D and other advanced integration technologies, have received great interest for the improvement of performance and reduction in power consumption [1][2].Chiplet-based approach can integrate multiple chips with separate designs and different manufacturing process.However, there are still many challenges for promoting the uppermost performance of each material and device, owing to the limitation of integration density and interconnection distance from individual chips [3].Transistor level integration, which is based entirely on micro/nano processing technology, allows designers to use the best transistor in places where it's needed most.Different materials, different functions and different devices can be combined tightly in one single chip.The distance between neighboring transistors and devices can be greatly reduced, leading to negligible transmission losses and other parasitic effects.In this paper, epitaxial layer transfer process for transistor level integration was first introduced, then two representative applications with great performance were demonstrated.

Experimental Details
A sequential epitaxial layer transfer process is shown in Figure 1.First the electronic or photonic devices is fabricated through the standard wafer front side process at the beginning of the heterogeneous integration.Then the device wafer is temporarily bonded to a carrier wafer face to face by using adhesive materials.And the original substrate is selectively removed by mechanical grinding and chemical etching methods, without further damage to the epitaxial functional layers through etch stop layer.After the entire removal of original substrate and etch stop layer, thin intermediate layer is fabricated on the backside of functional layer.Then the released epitaxial layer is transferred to SiC substrate by wafer bonding, and then the carrier wafer is debonded.The electrical connection between the transferred devices and passive circuits is fabricated through microstrip line, in which case less parasitic effects will introduced.Finally, the SiC substrate is thinned with the normal semiconductor process.
Figure 1：Schematic representation of the epitaxial layer transfer process for heterogeneous integration.

Si PIN limiters on SiC substrate for high power monolithic limiter
RF limiter is a very important microwave control device, which is widely used in transmit/receive modules in a variety of wireless communication systems [4].It is mainly used to protect the very sensitive components such as low noise amplifier from damage caused by large external, reflection from the antenna or other signals.With the application of GaN based high power devices, the power handling capability of traditional limiters is facing a bottleneck.Besides, it is difficult to meet the fast growing demand in the increasing peak power, with trade-offs between power handling, insertion loss and miniaturization [5].An innovative integrated monolithic limiter based on transistor level heterogenous integration was studied.A three layer Si PIN diode less than 10μm are vertically integrated to high thermal conductivity semi-insulating SiC substrate through layer transfer process as shown in Figure 2. Figure 3 shows the S-parameters and VSWRs of the integrated monolithic limiter.For the frequency from 5.0 to 6.0GHz, the insertion loss is less than 1.0dB with 1.4 of input VSWR and 1.3 of output VSWR.Power handling results are also measured.The integrated PIN limiter has shown power handling capability of 150W (continues Wave) and 370W (long pulse) at 5.4GHz, with a total flat leakage less than 23dBm, which shows outstanding performance than conventional GaAs limiter.Note that, owing to the lack of higher power input source and limited power handling capability of the connectors and cables, higher power handling capability of the integrated limiter may be proceeded.Besides, the relative higher flat leakage can be reduced from optimization of the size and resistance of PIN diode.Figure 3. Small-signal S-parameter measurements of the integrated limiter over a frequency range of 5.0 GHz to 6.0 GHz.In addition, the infrared temperature measurement of the limiter is also shown in Figure 4, at a substrate temperature of 70°C and an input power of 100W (continue wave).It can be seen that the color of those PIN diodes at the same stage is almost the same, which indicates that the limiter and integration process has good consistency.

InP photodetectors on SiC substrate for wideband photo-detector
PIN photodetectors (PDs) are widely used in optical communication, intelligent drive, phased arrays and other systems, and more efforts have been focused on optimizing PD performance with higher optical responsibility, high-saturation current, as well as wide bandwidth [6].However, the three aspects may affect and interact with each other.In this case, a top-illuminated InP/InGaAs PD structure based on transistor level heterogenous integration is also studied.A multilayer InP PIN diode less than 5μm is vertically integrated to high thermal conductivity SiC substrate through layer transfer process as shown in Figure 5. Optical image of InP Photo diode on SiC substrate.Frequency response under 3V reverse bias measured by vector network analyzer is plotted in Figure 6.The integrated 1550nm PD with a 14μm optical window diameter possesses a high performance of 3-dB bandwidth over 69GHz.Besides, the corresponding responsibility value is also measured through the tapered fiber.The integrated PD exhibits a responsivity as high as 0.51A/W at the wavelength of 1550nm, which is in good agreement with the theoretical calculations.Figure 7 shows the responsivity of the heterogeneous integrated PD as well as the traditional PD, it is obvious that the responsibility has increased by 42% compared to 0.36A/W for traditional PD.

Conclusions
In this paper, two novel vertical devices based on epitaxial layer transfer heterogenous integration have been introduced.The results verified great potential of heterogenous integration for enhancing device and system performance for post-Moore era.

Figure 2 .
Figure 2. (a) The cross-sectional diagram of the heterogeneous integrated monolithic limiter, (b) Scanning electron microscopy of Si PIN diode on SiC substrate.Figure3shows the S-parameters and VSWRs of the integrated monolithic limiter.For the frequency from 5.0 to 6.0GHz, the insertion loss is less than 1.0dB with 1.4 of input VSWR and 1.3 of output VSWR.Power handling results are also measured.The integrated PIN limiter has shown power handling capability of 150W (continues Wave) and 370W (long pulse) at 5.4GHz, with a total flat leakage less than 23dBm, which shows outstanding performance than conventional GaAs limiter.Note that, owing to the lack of higher power input source and limited power handling capability of the connectors and cables, higher power handling capability of the integrated limiter may be proceeded.Besides, the relative higher flat leakage can be reduced from optimization of the size and resistance of PIN diode.

Figure 4 .
Figure 4.the infrared temperature measurement of the limiter.

Figure 5 .
Figure 5. (a) The cross-sectional diagram of the heterogeneous integrated photo detector, (b)Optical image of InP Photo diode on SiC substrate.Frequency response under 3V reverse bias measured by vector network analyzer is plotted in Figure6.The integrated 1550nm PD with a 14μm optical window diameter possesses a high performance of 3-dB bandwidth over 69GHz.Besides, the corresponding responsibility value is also measured through the tapered fiber.The integrated PD exhibits a responsivity as high as 0.51A/W at the wavelength of 1550nm, which is in good agreement with the theoretical calculations.Figure7shows the responsivity of the heterogeneous integrated PD as well as the traditional PD, it is obvious that the responsibility has increased by 42% compared to 0.36A/W for traditional PD.

Figure 6 .
Figure 6.(a) Frequency response of the integrated PD at the reverse bias of 3 V, (b) Responsivity of the heterogeneous integrated PIN PD.

Figure 7 .
Figure 7. Responsivity of the heterogeneous integrated PD as well as the traditional PD.