Microwave Integrated Circuits

UNIT - I

Microwave Integrated CircuitsMicrochip for Microwave

frequencies.It can incorporate innumerable

components of different types, (passive and active) into a small chip to form a complete microwave subsystem.

Size, weight and cost are reduced much.

Types of Microwave circuits

Discrete circuit:

Packaged diodes/transistors mounted in coaxial and waveguide

assemblies.

Devices can usually be removed from the assembly and replaced.

Hybrid MIC:

Diodes/transistors, resonators, capacitors, circulators etc., are

fabricated separately on most appropriate material and then mounted

into the microstrip circuit and connected with bond wires

MMIC :

Diodes/transistors, resistors, capacitors, microstrip etc., are

fabricated simultaneously, including their interconnections, in

semiconductor chip

HMICHybrid MICs have only one layer of

metallization for conductors and transmission lines and discrete components like resistors, capacitors, diodes and transistors, etc. are bonded to the substrate.

Alumina, quartz and Teflon fiber are commonly used substrates.

Transmission line conductors for hybrid MICs are typically copper or gold.

Hybrid Microwave Integrated Circuit (HMIC)

Photograph of one of the 25,344 hybrid integrated T/R modules used in Raytheon’s Ground Based Radar system. This X-band module contains phase shifters, amplifiers, switches, couplers, a ferrite circulator, and associated control and bias circuitry.

Microstrip Circuit elements commonly used in HMIC

The components that can be fabricated as part of the microstrip transmission line are:

Matching stubs and transformers Directional couplersCombiners and dividersResonatorsFiltersInductors and capacitorsThin film resistors

Components Added After Micro strip Fabrication

The MIC Components that are fabricated separately and added to the micro strip circuits are:

Bond wireChip resistorChip capacitorsDielectric resonatorsCirculatorsDiodes and transistors

Microstrip coupler

Coupled line filter

Hybrid coupler

Branch line coupler

Typical spiral inductor and interdigitated capacitor

Loop inductor

High impedance transmission line inductor

Figure: Microstrip elements used in HMIC

Bond wires

Chip capacitor and resistor

Dielectric resonator

MMICThe substrate of an MMIC must be a

semiconductor material to accommodate the fabrication of active devices and devices consisting, several layers of metal, dielectric and resistive films.

Potentially, the MMIC can be made at low cost because the manual labour in the fabrication of hybrid MICs is eliminated and that a single wafer can contain a large number of circuits, all of which can be processed and fabricated simultaneously.

Monolithic Microwave Integrated Circuit (MMIC)

Photograph of a monolithic integrated X-band power amplifier. This circuit uses eight heterojunction bipolar transistors with power dividers/combiners at the input and output to produce 5 watts.

Courtesy : Internet

Advantages and Disadvantages of HMIC

Advantages:1-Each component can be designed for optimal

performance: Each transistor can be made of the best material. Other devices can be made of the most

appropriate material. The lowest loss microwave components can be

made by choosing the optimal micro strip substrate.

2- It has high power capability since the high power generating elements can be optimally heat-sinked.

3- Standard diodes and transistors can be used and made to perform different functions by using different circuit design.

4- Special-purpose devices for each function are not required.

5- Trimming adjustments are possible.6- The most economical approach when small

quantities, up to several hundred, of the circuits are required.

Disadvantages:1-Wire bonds cause reliability problems. Each circuit

element that is not part of the microstrip assembly must be attached to the microstrip by a wire bond.

2-The number of devices that can be included is limited by the economics of mounting the devices onto the circuit and attaching them by a wire bonds. The circuit is usually limited to a few dozen compartments.

Advantages and Disadvantages of MMICs

Advantages:1- Minimal mismatches and minimal signal delay.2- There are no wire bond reliability problems.3- Up to thousands of devices can be fabricated at one

time into a single MMIC.4- It is the least expensive approach when large quantities

are to be fabricated.

Disadvantages:1- Performance compromised, since the optimal materials

cannot be used for each circuit element.2- Power capability is lower because good heat transfer

materials cannot be used3- Trimming adjustments are difficult or impossible.4- Unfavorable device-to-chip area ratio in the

semiconductor material. 5- Tooling is prohibitively expensive for small quantities of

MMIC.

Materials used for MIC

Substrate materials sapphire, alumina, ferrite/garnet, silicon, RT/duroid,

quartz, GaAs, Inp, etc.,

Conductor materials copper, gold, silver, aluminum, etc.

Dielectric films SiO, SiO2,…etc

Resistive films Nichrome (cNiCr), tantalum (Ta)

Substrate Choice for HMIC1. The cost of the substrate must be justifiable for the

application

2. Is the technology to be thin- or thick film?

3- The choice of thickness and permittivity determines the

achievable impedance range and the usable frequency

range.

4- There should be low loss tangent for negligible

dielectric loss

5- The substrate surface finish should be good (~ 0.1

mm), with relative freedom from voids, to keep conductor

loss low and yet maintain good metal-film adhesion

6- There should be good mechanical strength and thermal

conductivity.

7- No deformation should be occur during processing of

circuit

8- A substrates with sufficient size are for the particular

application and complexity should be available.

Commonly used substrate materials1. Organic PCBs (Printed Circuit Boards) FR4

1) Low cost, rigid structure, and multi-layer capability.

2) Applications for operation frequency below a few GHz.

fop Loss

2. Plastic substrate

RT/Duroid

1) Low loss and good for RF applications.

2) Board has a wide selected range for permittivity. e.g. RT/Duroid 5870 with r =2.33, RT/Duroid 5880 with r =2.2, and RT/Duroid 6010 with r =10.2.

3) Board is soft leading to less precise dimensional control.

1) This is suitable for experimental circuits operating below a few GHz and array antennas operating up to and beyond 20 GHz.

3. Alumina1) Good for operation frequency up to 40 GHz.

2) Metallic patterns can be implemented on ceramic substrate using thin-film or thick-film technology.

3) Passive components of extremely small volume can be implemented because the ceramic substrate can be stacked in many tens of layers or more, e.g. low temperature co-fired ceramic (LTCC).

4) Good thermal conductivity.

5) Alumina purity below 85% should result in high conductor and dielectric losses and poor reproducibility.

4. Quartz

1) Production circuits for millimetric wave applications from tens of GHz up to perhaps 300 GHz, and suitable for use in finline and image line MIC structures.

2) Lower permittivity of property allows larger distributed circuit elements to be incorporated.

5. Sapphire

The most expensive substrate with following advantages:

1) Transparent feature is useful for accurately registering chip devices.

2) Fairly high permittivity (r =10.1~10.3), reproducible ( all pieces are essentially identical in dielectric properties), and thermal conductivity (about 30% higher than the best alumina).

3) Low power loss.

Disadvantages:

1) Relatively high cost.

2) Substrate area is limited (usually little more than 25 mm square).

3) Dielectric anisotropy poses some additional circuit design problems.

6. Beryllia (BeO) and Aluminium Nitride (AlN): Ceramic substrate. Excellent thermal conductivity – high power

applications. Dangerous to handle – Its dust is toxic and must

not be machined.

7. GaAs: Suitable for MMICs. Lownoise MESFET, Power

MESFET, Schottky diodes are fabricated on GaAs.

Conductor Materials

Properties:1. High conductivity2. High coefficient of thermal expansion3. Low resistance at RF/microwaves4. Good adhesion to the substrate5. Good etch ability and solder ability6. Easy to deposit or electroplate

Example:HMIC: Cr/Au, Pd/Au, Ta/AuMMIC: Cr/Au, Ti/Pd/Au, Ti/Pt/Au

Properties of Conductors :

Dielectric Films

1. Reproducibility 2. High breakdown voltage3. Low loss tangent 4. Ability to Process without developing

pinholes.

Capacitors Protective layers for active devices Insulating layers for passive circuits

Sio2 Vs GaAs

Properties of Dielectrics :

Resistive Films

1. Good stability2. Low Temperature Coefficient of

Resistance (TCR)3. Sheet resistivity (10-2000 /square)

TerminationsAttenuatorsBias Networks

Examples:Cr, NiCr, Ta, Cr-Sio, Ti

Properties of Resistive Films :

Properties of Various Manufacturing Technology