RF‐to‐IF mixer, IF amplifiers, quadrature IF‐to‐baseband mixers, PLL, and frequency tripler. The input/output (I/O) pads are peripheral, with 60 on the transmitter and 53 on the receiver chips.
The emergence of wireless SoCs or single‐chip radios called for compatible antenna solutions, which provided an excellent opportunity for researchers of prepared minds to seriously explore the feasibility of integrating an antenna in a chip package using packaging materials and processes in the late 1990s, leading to the development of antenna‐in‐package (AiP) technology [5]. This chapter recounts how AiP technology has been developed to its current state. Section 1.2 describes the idea of AiP with respect to the ideas of antenna on chip (AoC), antenna in module (AiM), antenna on board (AoB), and active integrated antenna (AIA). Section 1.3 reviews the early attempts to explore the idea of AiP. Section 1.4 reflects on the milestones in the development of the idea of AiP into a mainstream antenna and packaging technology. Finally, Section 1.5 gives concluding remarks.
1.2 The Idea
The idea of AiP was triggered by the demand for innovative antenna solutions to wireless SoCs [6]. It features using packaging technology to implement an antenna (or antennas) with a radio or radar die (or dies) in a chip package. It emphasizes only the addition of the unique function of radiation to the package. In this sense, it is different from the concept of system‐in‐package (SiP).
The idea of AoC sounds attractive [7]. It attempts to integrate an antenna (or antennas) with other circuits on a die directly using semiconductor technology. It is obviously a subset of the concept of SoC. Then why do we specifically differentiate it from SoC? The reason is to highlight the unique property of radiation, which is not necessarily being improved like digital circuits as the technology scales down. It is clear that AoC is more suitable for terahertz applications for cost and performance reasons.
The idea of AiM was proposed for multichip 60‐GHz radios [8]. It uses micro‐assembly technology to mount a few monolithic microwave integrated circuits (MMICs) and a small flat antenna in a hermetically sealed package. A window for the propagation of electromagnetic waves is formed above the antenna at the lid of the package. The window is also hermetically sealed.
The idea of AoB is similar to the idea of AiP. However, it relies on printed circuit board (PCB) technology to make an antenna (or antennas) on one surface of a board and to solder a packaged chip (or chips) on the other surface of the board. A few techniques, such as probe feeding or aperture coupling, are available to interconnect the packaged chip with the antenna. Of course, the antenna, the packaged chip, and the necessary feed networks can be contained on the same surface of the board. Recently, the idea of AoB has received considerable attention for millimeter‐wave (mmWave) fifth‐generation (5G) base stations [9].
A typical AIA consists of active devices such as Gunn diodes or transistors that form an active circuit and a planar antenna. The idea of AIA was proposed to eliminate the lossy and bulky interconnect between the active device and radiating element [10]. Later, the idea of AIA was employed for quasi‐optical power combining. The output power from an array of many solid‐state devices was combined in free space to overcome the power limitations of individual solid‐state devices at mmWave frequencies.
Although the origin of the above ideas can be traced back to the invention of microstrip antennas in the early 1970s [11], it should be noted that they extended the concept of microstrip antennas to different levels of integration.
1.3 Exploring the Idea
In this section, the early attempts to explore the idea of AiP are reviewed. It should be mentioned that researchers in university labs devoted their efforts regarding Bluetooth radios to 2.4 GHz or other RF applications, while researchers in company labs focused on 60‐GHz radios and other mmWave applications. At 2.4 GHz, a key challenge was how to miniaturize the antenna size, while at 60 GHz, it was how to minimize the interconnect loss between the die and antenna.
1.3.1 Bluetooth Radio and Other RF Applications
In 1998, Zhang started to work in the Division of Circuits and Systems at the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore. The division soon initiated a strategic research project entitled “Software radio on a chip.” Zhang was tasked to develop an antenna technology for the project. Inspired by the structural similarity shared by a microstrip antenna and a microchip, shown in Figure 1.3, and foreseeing the outcome of an interesting antenna solution, Zhang immediately started to investigate the antenna performance of the microchip. First, Zhang did antenna experiments with used microchips, as shown in Figure 1.4a. Then, Zhang tried PCB mock‐ups, as shown in Figure 1.4b. Encouraged by good antenna results from the microchips and PCB mock‐ups, the research team led by Zhang realized more sophisticated designs, as shown in Figure 1.4c,d, with low‐temperature co‐fired ceramic (LTCC) technologies in 2004 [12]. It is interesting to note that differential microstrip patch and meander antennas were designed to suit high‐level integration of wireless SoCs. They were integrated on the top surfaces of two ball grid array (BGA) packages. Both packages had cavities to house wireless SoC dies. The interconnects from the die to the antenna were cascaded bond wires, traces, and vias. The interconnected die was encapsulated with epoxy.
In 2000, Song et al. at University of Birmingham presented an integrated antenna package [13]. An electrically small feed antenna was designed on a semiconductor substrate, which also supported the RF front‐end circuits. The parasitic radiator placed above the feed antenna also acted as a top cover, sealing the entire package. Later, Song et al. presented another integrated antenna package [14]. A small antenna was embedded within the chip encapsulating material. A parasitic radiator was placed in close proximity to the embedded antenna, where it enhanced the poor gain and bandwidth of the packaged antenna.
In 2001, package engineers started to tackle the same problem. Lim et al. at the Georgia Institute of Technology managed to integrate RF passives, a patch antenna, and chips at the package level to enhance the overall performance of and to add more functionalities to an SoP paradigm [15]. Mathews et al. disclosed a package with an integral shield and antenna for a complete Bluetooth radio design [16].
