Based on the analysis of the characteristics of thin film circuits, this paper introduces several typical thin film circuits in T/R components. The analysis points out two new trends in the application of thin film technology in T/R components, and gives development. Suggest.
1 IntroductionThe use of thin film technology to fabricate thin film circuits is an important branch in the field of thin films. Main features of the thin film circuit: high manufacturing precision (low film line width and line spacing), small hole metallization, integrated passive components such as resistors, capacitors, inductors, air bridges, etc. A multilayer circuit is fabricated using a medium. The thin film multilayer circuit refers to a thin film process such as vacuum evaporation, sputtering, electroplating, and a pattern forming technique such as wet etching and dry etching (reactive ion etching, plasma etching, laser etching) on ​​a polished substrate. A multilayer interconnection structure in which a conductor (Cu or Au, etc.) wiring and an insulating dielectric film (PI or BCB or the like) overlap each other is formed on a material (ceramic, silicon, glass, or the like).
Thin-film multilayer circuit technology is very competitive in the microwave field due to its high interconnect density, high integration, high-power circuit manufacturing, and system-level functions of the entire package structure, especially on the airborne. In the spaceborne or aerospace field, its small size, light weight and high reliability are more prominent. It is a very promising microwave circuit module (low noise amplifier, filter, phase shifter, etc.), and even more demand The larger the T/R component substrate manufacturing technology.
Based on the analysis of the characteristics of the application of thin film circuits in T/R components, this paper introduces several typical application examples and gives development suggestions.
2. Analysis of the characteristics of thin film circuit technology applied in T/R componentsWith the development of radar technology, active phased array radar has become the mainstream, and its core is the T/R component, which usually contains thousands of T/R components per radar. T/R components are basically the same regardless of whether they are used at the same frequency or whether they are used in the same place. They are mainly composed of power amplifiers, driver amplifiers, T/R switches, phase shifters, limiters, and low noise. Amplifier, looper, logic control circuit, etc., its structural block diagram is shown in Figure 1 (1). These basic components, when the process is implemented, can be directly used on the circuit board, such as microstrip transmission lines, switches, couplers, filters, etc., some use external chip (such as power amplifier, drive, etc.), capacitors, loopers Wait to achieve. Therefore, from the use function and structure, the T/R component can actually be regarded as a microwave multi-chip module with transceiving function.
Due to the radar beam grating effect (the center distance of two adjacent radiating elements is less than half of the working wavelength) and the limitations of weight and cost, the miniaturization, integration and weight reduction of T/R components will be its development trend. In order to meet its performance requirements, it is inevitable to develop and produce T/R components by using multi-layer integration technologies such as low temperature co-fired ceramic LTCC, high temperature co-fired ceramic HTCC, thin film multilayer circuit technology and multilayer microwave printed circuit technology. A comparison of the multilayer techniques is shown in Table 1 (2~3).
As can be seen from the table, the thin film multilayer interconnection substrate has the following outstanding advantages:
(1) The wiring density is high, the volume can be small, and the weight is very light;
(2) High integration, it can embed passive components such as resistors, inductors, capacitors and active chips;
(3) Good high frequency characteristics, can be used in the field of microwave and millimeter wave;
(4) With high power density, high-density high-power multilayer substrates can be fabricated by using high-heat-conductivity metal, diamond, ceramic or aluminum carbonized silicon composite materials.
Thin film multilayer interconnect substrates have the following significant disadvantages compared to other types of substrates:
(1) The process adopts a string type, the yield is relatively low, and the manufacturing cost is high;
(2) The number of manufacturing layers is limited.
Thin film multilayer circuit technology has obvious advantages and disadvantages, so there are two options for the choice of manufacturing T/R components. First, the T/R module (4~5) can be directly fabricated on a ceramic substrate or a metal substrate by using a thin film technology, and the film has high precision, high integration, and high power performance, and the method is costly; Combining thin film technology with other multilayer circuit technologies (such as thick film technology, HTCC, LTCC, etc.) (6-8), manufacturing T/R components, making advantages and avoiding shortcomings, not only making the other substrates easy to realize multi-layer characteristics, thus overcoming the film The shortcomings of the technology itself are insufficient to produce layers, and the high precision and high performance of the thin film technology itself can be exerted.
3. Application examples of thin film technology in T/R components3.1 Thin film hybrid integrated T/R assembly on ceramic substrate
RCA Lab reported in 1985 that the T/R module (3) was fabricated on a high thermal conductivity ceramic BeO substrate using a thin film process. The dimensions were 7.0 cm & TImes; 9.0 cm & TImes; 1.6 cm, operating frequency 16.0 to 16.5 GHz, peak power 3.9~ 4.4 W, voltage tuning range 2.5~2.9, noise figure 5dB.
The MarTIn Marietta laboratory first reported in 1995 the use of thin-film technology to manufacture a W-band 8-unit T/R assembly with a frequency of up to 94 GHz (4), as shown in Figure 3. The dimensions of the assembly are 16.5 mm & TImes; 28.3 cm x 1.8 cm with a maximum gain of 47.8 dB. The main process is to first manufacture DC and control signal boards on a 0.5 mm thick molybdenum substrate using a copper conductor and polyimide film multilayer process, and then fabricate it on a 0.125 mm thick low loss Al2O3 ceramic plate. RF transmission line, and finally the RF part and chip, capacitor, etc. are assembled on the low frequency motherboard.
3.2 Thin film hybrid integrated T/R components on metal substrates
The production of T/R modules or multi-chip modules on metal substrates such as aluminum and molybdenum has also been reported in recent years (4-6). In 1995, Australia's O.Sevimli reported on a metal-based V-band (up to 110 GHz or more) thin-film multi-layer multi-chip module patent technology (5), the structure is shown in Figure 2. The technical process is as follows: firstly, the hole for mounting the chip is etched on the metal substrate, and then the chip is fixed in the hole by using a conductive adhesive, and the depth of the hole for mounting the chip is controlled so that the chip is in the same metal surface In-plane and precise positioning, the surface is coated with a dielectric material such as BCB suitable for use in the millimeter wave field, and finally the through hole is etched at the die pad to fabricate the thin film multilayer circuit. The outstanding advantage of this technology is that all chips or passive components (such as couplers, filters, etc.) can be assembled at the same time. The assembly does not use gold wire bonding or flip chip to solve the millimeter wave band gold wire bonding tape. The consistency control and parasitic problems come into being; at the same time, the power dissipation problem caused by flip chip can be solved, and the heat of the chip can be quickly dissipated through the metal base plate.
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