Heterogeneous integration of micro- and nano-electromechanical systems (MEMS, NEMS) and photonic devices on top of integrated circuits (ICs) is an emerging technology that will lead to highly compact devices combining traditional ICs with advanced sensors and actuators. Different components, such as MEMS, photonic devices, and complementary metal oxide semiconductor (CMOS) circuits are fabricated on separate substrates and subsequently joined into one single substrate by wafer bonding [1,2]. This enables the integration of high-performance MEMS, NEMS, and photonic devices made of materials such as mono-crystalline silicon, germanium, III–V materials, piezoelectric materials, shape memory alloys (SMAs), carbon nanotubes (CNT), or nanowires directly on top of ICs, which is otherwise not possible.

We review heterogeneous integration approaches for various MEMS, NEMS, and photonics systems based on layer transfer by adhesive wafer bonding. The main advantages of adhesive wafer bonding are the low bonding temperatures (20 - 450°C), its insensitivity to wafer surface topology, the compatibility with standard CMOS wafers, and the ability to join practically any wafer materials [3]. Adhesive wafer bonding does not require special wafer surface treatments, such as planarization or excessive cleaning, and therefore, offers a simple and cost-effective transfer process. Layer transfer by adhesive wafer bonding consists of several steps. First, the donor wafer, containing the layer to be transferred, is aligned to the target wafer (e.g. CMOS wafer) and bonded using an intermediate polymer adhesive. Second, the bulk material (e.g. Si substrate) of the donor wafer is sacrificially etched while the transferred layer remains on the target wafer of top of the polymer layer. The transferred layer can subsequently be further processed using conventional process steps to create the desired devices. Compatible processes include structuring and etching of the transferred layer, material deposition such as chemical and physical vapour deposition (CVD, PVD), ion implantation, and annealing amongst others. These processes can be used to create electrical, mechanical, and photonic devices and electrical interconnections between the transferred layers and the underlying substrate (vias). Using adhesive bonding with an intermediate polymer further offers highly selective and controlled sacrificial release etching capabilities in O₂ plasma. This provides a simple approach for realizing free-standing MEMS and NEMS structures without wet etching and critical point drying (CPD), thus increasing manufacturing yield and reducing fabrication complexity. Lastly, the presented heterogeneous integration approach is compatible with pre-structuring of the target substrates, as well as of the transfer layer. Thus, it is possible to integrate not only layers of high-performance materials, but pre-processed devices and systems.

We illustrate the versatility of adhesive wafer bonding for heterogeneous integration with different application examples of MEMS and optical systems integrated onto various target substrates. Each example requires a slightly different heterogeneous integration approach. A suspended Si/SiGe quantum-well infrared microbolometer array was integrated on top of a CMOS substrate by adhesive transfer bonding of the Si/SiGe thermistor layer and subsequent patterning of the bolometer array [4]. Since the bolometer pixels needed to be suspended, Ni pillars were formed and the polymer adhesive was sacrificially removed to free-etch the bolometer pixels. The Ni pillars provide both electrical connection to the underlying CMOS circuits and mechanical support for the suspended bolometer pixels. For the integration of nano-electromechanical (NEM) switches for ultra-low power computing applications, TiW vias were used as electrical interconnects. In order to achieve short signal path lengths and thereby increase the performance and energy efficiency of the circuits, an ultra-thin adhesive bonding layer of just 200 nm was used. In optical systems that don’t require electrical interconnections, the polymer adhesive itself can be used as mechanical support. Finally, the capability for multiple layer transfer steps was demonstrated by fabricating hidden hinge micromirror arrays [5]. In this approach, a layer of mono-crystalline silicon was transferred onto the target wafer and the mirror hinges were subsequently patterned in this layer. Pillars extending from the hinge layer to the target substrate were formed, but the polymer adhesive was not removed at this stage. Instead, a second mono-crystalline silicon layer was transferred on top of the hinge layer using adhesive bonding. The mirrors are then formed in this second transferred layer. Pillars connecting the mirrors to the hinges were formed and finally both polymer adhesive layers were sacrificially removed in O₂-plasma, thus, creating free-standing micromirrors. In conclusion, the flexibility to precisely control the thickness of the adhesive polymer layers and selectively remove it makes adhesive wafer bonding a highly versatile approach for heterogeneous integration of a large variety of MEMS and photonic applications.