~ !i~i~ ~ III F E A T U R E I

p lessey has been involved in GaAs technology at its

Caswell site near Towcester since 1962 when work first started on basic material studies. In that time, over 28 years experience in GaAs has been gained and is demon- strated in Europe's premier GaAs IC manufacturing facility, the Plessey GaAs Foundry Service. This was established in 1985 and from the outset, emphasis has always been placed on process control and manu- facturability to ensure a high yield, reliable and reproducible process.

Figures 1 and 2 show the progress in complexity made over this period.

PROCESSES & C O M P O N E N T S The Plessey GaAs Foundry Service provides a MMIC process with two basic options:

Plessey GaAs Lives On -

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rhe Plessey GaAs Foundry Process is based on proven, reliable technology wi th the greatest emphasis placed on manufacturability, repro- ducibi l i ty and yield. Since the introduct ion of commercial processes in 1985, Plessey has main- tained a policy of enhancement and updating rather than replacement as new processes become available. For example, usable frequency limits have been pushed from 12GHz to 14GHz and now to 20GHz with 0.5pro technology and via holes. This policy will continue in the future. Advanced processes have been developed, for example, for the generation of many watts of mean power from a MMIC. A HEMT has also been demonstrated and this will form the core technology for the for thcoming Plessey F40 pro- tess which will operate at 40GHz.

F20 - employing 0.S~tm gate length MESFETs and through substrate via holes for use to 20GHz and beyond. F14 - employing 0.7~m gate length MESFETs for use to 14GHz and beyond and using implantation profiles optimised for par- ticular applications.

KEY FEATURES The key features of the Plessey F14 and F20 Foundry processes are:

• ion-implanted GaAs sub- strates for maximum unifor- mity and reproducibility • standard depletion mode, recessed gate MESFET tech- nology • 0.51am (F20) and 0.71~m (F14) gate definition by optical lithography for max- imum throughput • composite dielectric isola- tion providing two capaci- tance/unit area values • 2-1evel metallisation

Figure 1. Low Noise X-band Amplifier (1974).

scheme (no air-bridges) to maximise chip and process yields • RIE etched through GaAs vias for minimum ground- ing inductance and maxi-

mum flexibility in circuit topology • all gold bond pad surfaces with barrier metals for improved wire bond strength and reliability

• overall dielectric encapsu- lation for passivation and mechanical protection • 200~m final chip thickness for optimum performance and maximum wafer yield.

Figure 2. Phased Array Radar Transmit/Receive MMIC (1989).

Figure 3. Novel Spiral Inductor

TECHNOLOGY The Plessey GaAs Foundry Process is based on proven, reliable technology with the greatest emphasis placed on manufacturabil- ity, reproducibility and yield. The key features that set Plessey's process tech- nology apart from other similar processes are:

• the use of multi-layer dielectrics dispensing with the need for air-bridges, and • the use of 20011m sub- strates for all processes, including those with through substrate vias.

The use of 200~tm sub- strates greatly reduces the incidence of breakages and keeps wafer yields high, and without the need for air-bridges the overall pro- cess is simplified and chip yields are maximised.

Yet the real advantage of the multilevel dielectric process is to the circuit designer who can now truly design in 3-dimen- sions. With creative think- ing, components can be realised that just do not have counterparts in hybrid technology nor can they be manufactured in air-bridge IC processes. Figure 3 shows a novel pro- grammable spiral inductor incorporating indepen- dently addressed inter-turn switching MESFETs. Plessey's standard cell for a "stacked spiral" is also one such component and more will be released in the future with many others awaiting discovery.

PROCESS CONTROL The Plessey GaAs Foundry Process is fully document- ed through the use of pro- cess specifications, opera- tor instructions and test specifications. The system established for process monitoring and control is based on the acquisition of data both during and sub- sequent to manufacture. From this data information is derived relevant to the control of the process,

quality of the product, yield and performance of the equipment used.

Every wafer processed through the Foundry has process control monitor (PCM) cells distributed over it. These PCM cells provide test components and structures necessary for the control and release of wafers from the Foundry. The PCM cell provides data for the control and moni- toring of:

* GaAs material resistivity . device isolation • contact resistance • FET parameters (Figure 4) • resistor parameters • metall isation resistances • metall isation interconnect resistances • capacitance values (Fig. 5) • through GaAs via continuity

Strict quality control is employed with visual inspections carried out by both operators and QA staff within and after each major process stage and additionally at the com- pletion of the process as part of the wafer release conditions. Traceability of each slice and all related data is maintained using a batch card system which accompanies each batch of slices throughout the pro- duction cycle.

WAFER RELEASE In addition to the usual PCM release conditions, guaranteed GET S-parame- ters are available. This has been made possible by the extensive effort in charac- terisation that has been invested in supporting the Plessey GaAs Foundry Service Design Manual and its models.

An RF-on-wafer measur- able standard MESFET test cell is inserted and is included in the test specifi- cation for release of wafers. Test FETs are probed at sites evenly distributed across the wafer, and again a min- imum yield of 70% must satisfy the guaranteed lim- its on S-parameters.

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0 100 200 300 400 500

Figure 4. Transconductance control over 500 batch starts.

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11.5

10.5

9.5

8.5

7.5

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0 100 300 500 700

Figure 5. Capacitance control of lOpF silicon nitride capacitor - over 700 batch starts.

YHE FUTURE Since the introduction of commercial processes in 1985, Plessey has main- tained a policy of enhancement and updat- ing rather than replace- ment as new processes become available. For example, usable frequency limits have been pushed from 12GHz to 14GHz and now to 20GHz with 0.5Bm technology and via holes. Lately, selective implanta-

tion of optimum material profiles has added more functionality. Capabilities offered to customers are continually extended in an evolutionary way, com- patibility always being maintained with previous processes.

This policy will continue in the future. Advanced processes have been devel- oped for the generation of many watts of mean power from a MMIC by

using selective local heatsinking beneath active devices. Additionally, a High Electronic Mobility Transistor (HEMT) has been demonstrated and will form the core technol- ogy for Plessey's forthcom- ing F40 process (40GHz). However, both the HEMT and power MMIC process will be compatible with the existing F14 and F20 processes, thus enabling a smooth transition into manufacturing.

Plessey's research activi- ties in circuit design and CAD also play an essential role in providing engineer- ing support to Foundry Service customers. Over the years, techniques have been developed to realise a complete range of microwave circuit func- tions. Plessey has pio- neered work on electro- magnetic circuit simula- tors to analyse high pack- ing density, multifunction designs, a topic where Plessey leads the world.

The Plessey GaAs Foundry Service is now well established. It has achieved this with a clear focus on manufacturing disciplines in order to maintain the highest lev- els of yield, reliability and reproducibility. But the foundations of today's production processes are based on many years expe- rience in GaAs and have been supported by the most advanced and experi- enced R&D activity. It is the support of this R&D activity that will ensure the continuing success of the Foundry Service through the 1990s. •

Jim Turner, MBE and David Smith Plessey Research

Caswell Ltd, A Management Company within GEC-Marconi Ltd.,

Caswell, Towcester, Northants, NN12 8EO,, UK.