Thin-film RF components for smartphones
The flatter, the better
One of the primary challenges facing developers of smartphones is to design phones with the slim form factor demanded by users. As a result OEMs are requiring ever thinner modules and components in order to produce the slim devices. TDK is using its advanced thin-film process technology to manufacture miniature components that feature improved performance and are flatter than ever.
Thin-film process technology was originally developed and refined by TDK for the manufacture of read/write heads for hard disk drives. These extremely reliable heads consist of a miniature coil that is formed by an ultra-thin conductive layer of copper applied to a ferrite material, thus forming a highly sensitive and extremely flat coil for reading and writing the hard disk.
Now, TDK is applying the leading-edge technology to the development and manufacture of RF components, especially for smartphones and compact mobile devices. This technology enables the development of a new line of compact, low-profile components with outstanding performance. Due to the precision achievable using the thin-film process, components can be manufactured with an extremely tight tolerance.
At present, a wide range of TDK components for RF applications such as directional couplers, filters, baluns, and capacitors for RF power circuits are manufactured using a low temperature co-fired ceramics (LTCC) process. The current LTCC process, however, has reached its limits as far as the further miniaturization of discrete components is concerned. Components produced with thin-film technology that meet the exacting demands for the thickness and size reduction can replace the standard LTCC discrete components.
|Figure 1: Cross-section comparison of LTCC products and thin-film components|
It can be seen from the cross sections of conductor parts in Figure 1 that the edges of conductor sections manufactured using thin-film technology are much more exact and well defined, and that their surfaces are much smoother than those of LTCC components. This means that less variance and more favorable electrical properties at higher frequencies can be expected than with the LTCC process. Figure 2 shows the electrical properties of a coupler. RF couplers manufactured with thin-film processes exhibit lower insertion losses and superior performance in the 2-GHz band.
|Figure 2: Insertion loss of couplers made with LTCC and thin-film technologies|
Innovative thin-film RF components
TDK has introduced ultra-low-profile thin-film couplers with an insertion height of just 0.25 mm with thin-film process technologies. In addition to the superior performance of the TDK TFSC directional coupler (Figure 2), the new thin-film component – at just 0.65 × 0.50 × 0.25 mm³ – is significantly smaller and thinner than the previous products that were made using the conventional LTCC process (1.0 × 0.5 × 0.35 mm³).
Based on thin-film technology, TDK developed a miniature combined component for the filtering and protection of smartphone designs. The TDK TCE1210 series comprises the first thin-film common-mode filter to offer both high-speed common-mode noise suppression and ESD protection in a single component. To do so, it integrates a micro-gap ESD protection element into a common-mode filter. The milestone in miniaturization was realized using TDK’s leading-edge thin-film patterning technology combined with compact, high-precision coil pattern and terminal formation processes. Thanks to its much smaller footprint, the new filter supports the high-density mounting of electronic components and contributes to significant space-saving in electronic devices. The TCE1210 series is designed for use in the input/output portions of DisplayPort and USB terminals of smartphones.
Thin-film micro-wiring technology enables further slimming
The new TDK TFSB and TFSD series of thin-film band pass filters and diplexers offer a footprint of 1.0 × 0.5 mm² and an insertion height of just 0.3 mm. The filter is essentially a series of resonators, each consisting of an optimally tuned capacitor formed by the upper and lower conductors separated by a dielectric film and an inductor by the conductor patterns. In this way, the TFSB and TFSD series achieve the same performance of the much larger LTCC components. The filters are designed for the 2.4-GHz and 5-GHz bands, making them suitable for Bluetooth and WLAN applications in smartphones and other mobile phones. The TFSZ and TFSL series of thin-film baluns and low pass filters measure in at 0.65 × 0.50 × 0.25 mm³ and represent a further advance in miniaturization to meet the demands of smartphone manufacturers.
The external dimensions of the Z-match capacitor for impedance matching, which was developed for high-frequency modules, are the same as earlier products that were developed with a multilayer technique at 0.4 × 0.2 × 0.2 mm³. The result is a new line of compact, low-profile components with outstanding performance. Thanks to TDK’s thin-film process achieved by building up thin-film conductive layers on thin layers of ceramic dielectric material, Z-match capacitors achieve an extremely tight tolerance and, furthermore, the new components offer an excellent high self-resonant frequency (SRF) of 6.8 GHz (2.2 pF).
The thin-film capacitors feature lower ESL and low ESR with a tolerance at ±0.05 pF, along with compactness, high Q, and the narrow tolerance sought by designers of RF modules. With these features, the new series exhibits superior RF characteristics in impedance matching circuits. In addition, by transforming the terminal structure by placing the terminals on the base plate and by using a high-precision cutting process, higher dimensional accuracy than that of previous products can now be achieved with stable mounting on modules that require high-density mounting. Figure 3 shows the TDK portfolio of thin-film RF components for smartphones.
|Figure 3: The portfolio of thin-film RF components from TDK|
With their low profiles and compact dimensions, thin-film components are also ideal for integration in modules, for example, in TDK’s SESUB (semiconductor embedded in substrate) technology. See the article, SESUB modules for smartphones to learn how highly complex semiconductor chips with a thickness of approx. 50 µm can be actually embedded in the substrate to create advanced miniaturized SIP (system in package) modules.