CHAPTER 2

DESIGN OF TEST STRUCTURES FOR ALGAAS/GAAS HBT INTERCONNECT CHARACTERIZATION

Introduction

The creation of test structures for characterization and modeling processes and interconnect is an active endeavor in IC technology [Schr90][Ghan82][Ghee85]. Typically, a regular set of test circuits are included as a process or control monitor in the kerf. These structures are designed by process designers and are probed during and after the run to collect key process statistics. In the case of a mature process, these measurements should show a high consistency from run to run. The same structures are used with special structures designed by device and circuit modeling engineers to model the process for circuit designers. Again, in the case of a mature process any variation is attributed to the design and simulation tools, which are then calibrated accordingly, and another set is run for verification. Typically, these special test structures are designed using simple scaled geometries that ignore the effect of real on-chip topologies and circuit environments. In addition, these test structures have limited suitability for predicting circuit behavior in the middle of a reticle field. Therefore, delay and interconnect modeling based on these isolated test structures is not accurate enough to extract maximum performance out of a circuit. Effects such as non-planarity of the multilayer interconnect with dominating 3-D fringing fields, or self and mutual heating with resulting device slow down are common in such technologies. These effects will not generally be seen in small planar test structures. Modeling verification and performance prediction must be done by measuring structures at the relevant dimensions with the actual materials and processing history to be implemented [Edel95][Deut94]. This becomes very important for high performance chips which try to squeeze maximum performance out of a process. Thus, test structures must approximate a typical chip environment and provide the desired data with a high level of accuracy.

This is especially important for a technology involving new semiconductor materials and new circuit concepts [Zucc80]. The continuously evolving nature of the AlGaAs/GaAs HBT process technology became apparent when the first batch of chips developed by the FRISC group came back from the foundry and the measured results were found to be significantly different from the simulation results based on the earlier models [Nah93]. This indicated the need of an additional degree of safety margin in subsequent design efforts to compensate for the unexpected variations in the process itself. Non-availability of any characterization structures on that run made it very difficult to correlate the measured results with simulation.

An opportunity arrived, in the fall of 1993, in the form of the High-Speed Circuit Design (HSCD) project to make demonstration test circuits and drive the calibration and augmentation of the CAD tools to reflect the test results. A strong emphasis was placed on the suitability of the technology for high-speed circuit design. Hence, key issues were the multilayer interconnect characteristics over short and long distances with varying topology and geometry, and device switching performance under transient, loaded, unloaded, low current density, and high-current density conditions. A test chip was designed to carry structures for this purpose.

A reticle containing the test chip and a few other demonstration and experimental circuits was submitted to the foundry in the Fall of 1994 and processed wafers were received by year end. The testing and correlation process continued for another year after receiving the first wafers. This chapter gives a brief overview of the strategy employed to evaluate key circuit parameters and the design of the test structures. The testing scheme and the measurement results are described in the next chapter.

HBT Technology

The HBT devices are formed in organo-metallic vapor phase epitaxy (OMVPE) grown layers on semi-insulating GaAs substrates. Figure 2.1 shows a cross-section of the HBT device [Asbe91] used in this effort. Many HBTs are made using an AlGaAs/GaAs heterostructure due to a close match in their lattice constants [Lee93]. These devices have a smaller base spreading resistance compared to a homojunction BJT. Another advantage is that the emitter-base capacitance can be made very small by a relatively low doping of the emitter. The baseline device has a ft of 50 GHz at a collector current of 2.0 mA.

Figure 2.1: Cross-section of a HBT (ft = 50 GHz, Ic = 2 mA, Vbeo = 1.4 V).

Interconnect Process

The interconnect process is non-planar employing three levels of gold conductors, Si3N4 and polyimide interlevel dielectrics, and thin film - NiCr and WSiNx - resistors. The interconnect cross-section is shown in Figure 2.2. The nominal dielectric constants of polyimide and silicon nitride are 2.9 and 6.8 respectively. Silicon nitride lies on top of the first level of metal and is used to make capacitors for analog components of the circuits or for providing bypass capacitors. The gold interconnect and nitride layer on top of the devices also act as good heat spreaders. The overall fabrication technology is discussed in great detail in [Ali91].

Figure 2.2: Interconnect structure of a baseline GaAs process.

Design of the Test Chip

Early attempts to characterize GaAs technology targeted MESFET [Zucc80] [Brow93], and low-integration HBT Gate arrays [Ying93]. This was the first attempt to characterize the HBT technology for high-density and high-speed full custom digital applications. A quarter of the reticle space (2 cm x 2 cm) was allocated for this purpose. Therefore, the selection of appropriate test structures and the test scheme to implement in a limited area required a lot of iterations from concept to implementation.

A typical high-density chip contains gates with varied loads, high fan-out nets, short local interconnects, long global interconnects, clock interconnection networks, and dense circuit macros such as register files and datapaths. Maximizing performance requires accurate information about device and interconnect behaviors under different circuit conditions. Low complexity large regular test structures were designed to make global performance estimates and small high-density structures were designed to simulate local environment. The structure description is broken up into two main sections: passive and active structures, and is described in the following sections. Figure 2.3 shows the design flow adhered to in the design process. The layout of the test chip is shown in Figure 2.4 and the layout of the full reticle is given in Figure 2.5.

Figure 2.3: HBT test chip design flow.

Figure 2.4: Layout of the test chip illustrating different test structures.

Figure 2.5: Layout of the test reticle with a size of 2 cm x 2 cm.

Passive Structures

The passive structures consisted, mostly, of capacitive and resistive structures to determine, electrically, parameters such as line width, sheet resistivity, thickness, and dielectric constant of metal and/or insulator layers in the process and to calibrate extraction tools against these results. These structures are described in this section.

Capacitors

There were three types of capacitors on the test chip: MIM/parallel, finger, and crossover. The main difference between these capacitors was the direction of the electric field between the capacitor plates. All the capacitors were designed to be measured by 1-port s-parameter measurements limiting the value of capacitors to a few pico farads. These capacitors are probed with a coplanar GSG probe with a 150 µm pad pitch [Casc83]. All the capacitors are described below.

Metal-Insulator-Metal (MIM) Capacitors

The MIM capacitors were obtained by sandwiching the silicon nitride layer between the first and second level metal layers. The schematic and layout of a MIM capacitor is given in Figure 2.6. Y and X dimensions were varied to obtain two such capacitors. Their dimensions and extracted capacitance, using QuickCap, are given in Table 2-1. The capacitance of these structures comprises of a large parallel plate component and a small fringing field component as shown in Figure 2.7.

Figure 2.6: Side and top view of a MIM capacitor.

Figure 2.7: Model of a MIM capacitor with parallel plate and fringe components.

Table 2-1: Summary of MIM capacitors.

Structure

Name
Type
Structure

Area [µm x µm]
X/Y

[µm]/[µm]
Extracted Capacitance

[pF]
cap4pf
M1/M2
301 x 416
100/60
2.08
cap8pf
M1/M2
382 x 416
200/160
8.32

Parallel Plate Capacitors

Parallel plate capacitors were made by overlapping any two of the available three metal layers. Schematic and layout of a parallel plate capacitor between the M1 and M2 layers is shown in Figure 2.8. The only difference between a MIM capacitor and an M1/M2 parallel plate capacitors is the removal of polyimide layer in the former to obtain a high capacitance value. Again, X and Y were varied to get capacitors of different values and of different layer combinations - M1/M2, M1/M3, M2/M3.

Figure 2.8: Side and top view of a M1/M2 parallel plate capacitor.

A circular dot would have been the best type of structure as it has the minimum periphery for a given area, but boston geometries were not allowed in the layout. The overlap capacitors included in the test set with the dimensions and extracted capacitances, using QuickCap, are given in Table 2-2. Figure 2.9 illustrates a model of an M1/M2 capacitor.

Table 2-2: Summary of parallel plate capacitors.

Structure

Name
Type
Structure Area

[µm x µm]
Y/X

[µm]/[µm]
Extracted Capacitance

[pF]
capm1m21
M1/M2
450 x 382.5
450/160
1.09
capm1m22
M2/M3
450 x 542
450/320
2.18
capm1m24
M2/M3
450 x 982.2
450/760
5.18

Figure 2.9: Model of an M1/M2 parallel plate capacitor.

Finger Capacitors

Finger, or interdigital, capacitors were made by interdigitating signal and ground electrodes in a plane using only one metal layer. These capacitors emphasize the line to line capacitance as the field in these capacitors is mainly in the horizontal direction. This kind of capacitor is present in the circuit between a base and emitter contacts in bipolar transistors [Asbe87] or between a differential wire pair. The layout and schematic of a typical structure is shown in Figure 2.10 and the photomicrograph of a fabricated capacitor is shown in Figure 2.11. Two finger capacitors were put on the chip in M1 layer with the dimensions shown in Table 2-3. These dimensions are typical of the M1 wiring on F-RISC/G chips. The spacing between two adjacent fingers was kept at 20 µm to make interfinger coupling negligible. Since the metal layers are not very thick the number of fingers was increased to twenty to amplify the mutual capacitance. Both silicon nitride and polyimide are present as insulators between these metal1 fingers. An approximate model of the finger capacitor is given in Figure 2.12.

Figure 2.10: Side and top view of a finger capacitor.

Figure 2.11: Photomicrograph of an M1 finger capacitor.

Table 2-3: Summary of finger capacitors.

Structure

Name
Type
Structure Area

[µm x µm]
Length(L)/Width (w)/Spacing(s)

[µm]/[µm]/[µm]
Finger Spacing (P)

[µm]
Extracted

Capacitance

[pF]
finm1str
M1
416 x 747.6399/2/3
22
0.70
finm2str
M1
416 x 757.5398/2/4
20
0.63

Figure 2.12: Model of an M1 finger capacitor.

3-D/Crossover Capacitors

Cross-over capacitors were obtained by orthogonally crossing conductors in two metal layers at regular intervals. Such a structure maximizes the field lines between two conductors and simulates the worst case wiring capacitance when all the wiring tracks are occupied. The schematic and photomicrograph of a crossover capacitor is shown in Figure 2.13 and Figure 2.14. The characteristics of these capacitors, provided in Table 2-4, are based on standard routing pitches used currently in F-RISC/G chips. Use of the same routing pitches was important for later correlation with the measured results, and the direct insertion of the result into chip routing tools. A simple model of an M1/M2 crossover capacitor is also shown in Figure 2.14.

Figure 2.13: Side and top view of an M1/M2 crossover capacitor.

Figure 2.14: Photomicrograph and a simple model of an M1/M2 crossover capacitor.

Table 2-4: Summary of crossover capacitors.

Structure

Name
Type
Number of

Crossovers

/ Area [µm x µm]
Extracted Capacitance [pF]
Comments
xovrm1m2str1
M1/M2
1000/

453.6x429.1
1.271
M1 pitch = 4 µm (1 track)

M2 pitch = 3 µm (1 track)

xovrm1m2str2
M1/M2
2730/

1519.2x483.1
1.886
M1 pitch = 22 µm (3 tracks)

M2 pitch = 21 µm (3 tracks)

xovrm1m3str1
M1/M3
4200/

699.9x430.1
1.102
M1 pitch = 6 µm (1 track)

M3 pitch = 8µm (1 track)

xovrm2m3str1
M2/M3
2100

460.6x430.1
1.076
M2 pitch = 6 µm (1 track)

M3 pitch = 8 µm (1 track)


Resistors

The resistor structures were designed to investigate the effect of line width, corners, and processing steps on the sheet resistivity of resistive and metal layers. A non-planar process produces variation in interconnect sheet resistance. Although this variation is minuscule, long signals routed on top of devices in a dense register file can increase the line resistances affecting the current levels adversely. Each resistor structure consists of five separate resistors as shown in Figure 2.15. The resistors are measured by the four-point method and are probed by a multi-channel (SSPGSSGPSS) cascade probe with a 150 µm pad pitch [Casc91]. The six signal pads on the probe can be used to measure voltage across five resistors (R1, R2, R3, R4, and R5). This arrangement provides additional accuracy by measuring the resistance between any two points in the whole structure.

Figure 2.15: Layout of a resistor test structure.

The metal resistor structures determine the effect on the sheet resistivity of metal layers due to different types of contacts, crossovers, and vias in a high-density layout. One example is shown in Figure 2.16. All the resistor structures are summarized in Table 2-5.

Table 2-5: Summary of resistor structures.

Structure

Number
Type
R1

[]
R2

[]
R3

[]
R4

[]
R5

[]
Max Current

[mA]
1
NiCr
7125
21525
7125215257125 2
2
NiCr
3675
21525
3650214621300 2
3
WSiN
36750
215250
3650021462513000 1.2
4
M3
36
36
363636
24
5
M1/M2
1500
1500
333.3333.3333.3
6

Figure 2.16: Layout of a metal structure.

Active Structures

Response of a known passive network to a known excitation can be exactly determined by network analysis or by experimentally applying the stimulus to a sample. The uncertainty in this approach is the variation in the passive network properties and in replication of the excitation waveform encountered in real circuits. Even a fast ramp on the order of 25 ps -30 ps requires very expensive equipment. Again, measurements with network analyzers and time domain reflectometry techniques require de-embedding the effect of the probes and pads and contact parasitics. Some of these, such as contact resistances, are dependent on the force applied and can leave uncertainty in the measurements. On the other hand, in-circuit test schemes can be easily used to measure the network response to a large signal, and calibrate parasitic extraction tools. One common scheme is the use of ring oscillators made of a chain of gates. A ring oscillator can produce and apply a multi-GHz stable stimulus very easily to produce direct time domain results. Ring oscillators can also, simultaneously, provide a low frequency synchronization signal to trigger an external scope to capture a high-speed signal. Ring oscillators have been used in many testing situations [Dutt95][Lane87] before.

Active structures use a ring oscillator as the basic structure on the test chip and modify it to isolate the desired variable. At first it was thought to be good enough to put a few low-level wiring oscillators comprised of several interconnected stages and test their oscillation period for gate-delay measurement [Lane87]. This idea can also be used to monitor the interconnect instead of the gate [Dutt95]. Here, the ring oscillators were used to evaluate, simultaneously, both the devices and the 3-D interconnect effects with help from passive structures. The ring oscillators were loaded with the typical interconnect loads encountered in digital designs.

The basic idea is shown in Figure 2.17. An odd number of inverters, or a chain of buffers with an odd number of inversions, can be connected in a circular chain to make an oscillator. This oscillator will oscillate at a frequency given by [2. 1]

where N is the number of oscillator stages and di is the delay through the ith stage. Hence, if the delay through each stage is equal to d, the resulting oscillation frequency is simply given by [2. 2]

Figure 2.17: Schematic of a ring oscillator with a chain of odd-number of inverters.

Therefore if the delay through each stage is kept the same by careful design and layout, the maximum frequency of oscillation depends on d only which varies from dunloaded to dloaded, where dloaded is given by [Bako90] [2. 3]

Here dloaded, dunloaded, and dinterconnect signify the loaded gate, unloaded gate, and the interconnect delays respectively. dunloaded is the intrinsic gate delay and is calculated by measuring an unloaded ring oscillator. Once dunloaded is known, equation 2.3 can be used to obtain dinterconnect as dloaded is calculated by measuring the loaded ring oscillators. Depending on the interconnect, as shown in Figure 2.18, this delay can be due to lumped capacitive load, distributed RC load, or distributed RLC load.

A current mode logic (CML) gate acts like a current source to charge a lumped capacitive load. The delay in charging a capacitor C through voltage swing V by a current I is given by [2. 4]

If V and I are known, the above equation determines capacitive load at the gate output. The load at the output is also extracted by QuickCap. Any discrepancy among measured and extracted results leads to a change in the process technology file for QuickCap. The same process technology file is used to verify capacitive structures described earlier. Therefore, this triple check method provides a process technology file with a high accuracy. The device characteristics are measured with a separate structure to verify device models. Thus, device performance with varying load can be estimated simultaneously employing ring oscillators with unloaded and loaded gates. The design parameters of the oscillators designed for these measurements are given in Table 2-6. The unloaded and loaded ring oscillators are described in the following sections.

(a) (b)

(c) (d)

Figure 2.18: Ring oscillators with (a) unloaded (b) capacitive (c) distributed RC, and (d) distributed RLC Load.

Table 2-6: Design parameters for ring oscillators.

Gate current level
I
Number of stages
N
Length of interconnect
L
Type of interconnect
M1/M2/M3
Type of load
parallel, finger, crossover, distributed RC

Differential Unloaded Ring Oscillators

The main design issues for unloaded oscillators were the area required and testing convenience. A six-channel 5 GHz ceramic probe from Cascade[Casc91] was available for testing. A 4-to-1 multiplexer was used to switch between four oscillators requiring three channels -two for multiplexer select signals and one for multiplexer output. The other three channels were used to monitor the on-chip power supply and the voltage swing in a special voltage-current monitor cell to calibrate the swing at 250 mV across all the oscillators.

The next problem was the number of stages in an oscillator. It is easier to measure a lower oscillation frequency achieved with more stages. The best choice turned out to be an 8-stage oscillator. A six-stage oscillator would have been close to the upper measurement frequency of the probe and a 7-stage oscillator would have disrupted the regular layout. The multiplexer too could have been included in the oscillating loop as shown in the Figure 2.19. This configuration doesn't lend itself to easy post-fabrication analysis as the constituent stages are not of the same type. Finally the configuration shown in Figure 2.20 was chosen to decouple the multiplexer from the ring oscillators.

The schematic shown in Figure 2.20 formed the basis of both unloaded and loaded oscillators. The layout of the unloaded oscillator structure is shown in Figure 2.21 with four 8-stage loops connected via a 4-input multiplexer to a 50 pad driver for scope outputs. These oscillators are made up of two types of devices - standard Q1, and a round emitter Q1. There were two oscillators of each type of device in the structure and their design parameters are given in Table 2-7.


Figure 2.19: An oscillator structure with a 4-to-1 mux in the oscillating loop.

Figure 2.20: Schematic of the unloaded 8-stage oscillator on the test chip.

Table 2-7: Summary of unloaded oscillators.

NoTypeLoad Type
Current

[mA]
Load Capacitance

[fF]
Oscillation Period [ps]
1,3Standard Q1Unloaded
1.2
14
354
2,4Round Q1Unloaded
2.0
25
266

Figure 2.21: Layout of the unloaded ring oscillator macro on the test chip.

Differential Ring Oscillators with Lumped Load

Three basic types of routing schemes were used in F-RISC/G HBT standard cell areas as enumerated in Table 2-8.

Table 2-8: Type of signal environments.

Route Type
Comments
Differential routing between standard cells wiring pitch = 6 µm
Differential routing inside standard cells wiring pitch = 4/5 µm
Single ended routing inside standard cells wiring pitch = 2/3 µm

Several ideas emerged regarding the design of test structures which would give an indication of capacitance of a wire under these routing conditions. These conditions can also be described as

  1. Line-to-line capacitance and its variation with spacing.
  2. single layer orthogonal crossovers and their occurrence (0%, 30%, 60%, 100%).
  3. orthogonal crossovers on more than one layer (0%, 30%, 60%, 100%).
  4. solid planes below or above signal layer

A strategy was developed to provide as many structures as possible and still be able to measure all the situations described above.

Since there are four oscillator spots available in any structure, as described in the section on unloaded oscillators, with all of them connected to a single multiplexer it was easier to calibrate the structures if all of them contained the same kind of oscillators. Finally, a test structure was designed with four oscillators of the same load type and varying signal length from 530 µm to 1908 µm. In total there was space for seven oscillator structures on the chip including the unloaded oscillators. The line lengths were similar in each group and only the loading type was varied - overlap, crossover, finger. One underlying assumption was that the interconnect capacitance would behave as a lumped load instead of a distributed one. Therefore, the maximum signal length was kept below 2 mm to ascertain a lumped load. The photomicrograph of one such structure is shown in Figure 2.22.

Table 2-9 lists all the different types of ring oscillators along with their load type, load capacitance, and their oscillation period - determined from PSPICE simulations - grouped according to the structure type. The structure type in the first column also cross-references the layout by the encircled numbers in Figure 2.23 which shows the combined layout of all the ring oscillator structures.

Figure 2.22: Photomicrograph of a test structure with four loaded oscillators.

Table 2-9: Summary of lumped load oscillators.

TypeLoad Type
Load Capacitance

(QuickCap) [pF]
Oscillation

Period [ps]
Unloaded (1)Minimum M1 14354
Simple (4)530 µm M1 86570
Simple (4)1218 µm M1 180825
Simple (4)1562 µm M1 229947
Simple (4)1906 µm M1 2811087
Simple (7)530 µm M2 73534
Simple (7)1218 µm M2 156764
Simple (7)1562 µm M2 200875
Simple (7)1906 µm M2 238980
Overlap / M3 Plane (5)530 µm M1/M3 Gnd 84567
Overlap / M3 Plane (5)1218 µm M1/M3 Gnd 188841
Overlap / M3 Plane (5)1562 µm M1/M3 Gnd 234966
Overlap / M3 Plane (5)1906 µm M1/M3 Gnd 2831090
Overlap / M2 Plane (3)530 µm M1/M2 Gnd 99603
Overlap / M2 Plane (3)1218 µm M1/M2 Gnd 211900
Overlap / M2 Plane (3)1562 µm M1/M2 Gnd 2661058
Overlap / M2 Plane (3)1906 µm M1/M2 Gnd 3281209
Finger / M3 (6)530 µm M1/M3 Gnd 87573
Finger / M3 (6)1218 µm M1/M3 Gnd 187844
Finger / M3 (6)1562 µm M1/M3 Gnd 233970
Finger / M3 (6)1906 µm M1/M3 Gnd 2861105
Finger / M2 (2)530 µm M1/M2 Gnd 94595
Finger / M2 (2)1218 µm M1/M2 Gnd 203876
Finger / M2 (2)1562 µm M1/M2 Gnd 2551027
Finger / M2 (2)1906 µm M1/M2 Gnd 3101169

Figure 2.23: Layout of all the unloaded and loaded ring-oscillators along with the load type. The structure numbers are cross-referenced in Table 2-9.

Differential Ring Oscillators with Distributed RC/RLC Load

On-chip wire topology is much more complex than a simple long wire. Standard options in present day design tools are to model the wire as (a) or (b) in Figure 2.24 [Kahn96]. With longer wire lengths, the delay is dependent on wire resistance too. These models start to fail if the total interconnect resistance is comparable to the driver output resistance.

(a) (b)

(c) (d)

Figure 2.24: Various types of interconnect models (a) lumped C (b) lumped R and C (c) distributed R and C (d) distributed R, L, and C.

This regime, where the wire delay is dependent on both resistance and capacitance, is quadratic in nature. When the wire lengths are short, the delay is linear with respect to the capacitance. The point of transition from linear to quadratic regime comes when the wire resistance begins to be significant with respect to the output resistance of the driver. In case of bipolar logic this point comes very early as the driver output resistances are in the 150 - 250 range. Based on calculations, M1 wires start showing the quadratic RC delay effect after a length of only 1.5-2 mm. The point where even the wire inductance becomes significant is starting to show up on long global wires. Assuming a lumped R and lumped C, as in (b), is optimistic as all the capacitance is shielded by the resistance. A distributed RC tree (c) indicates accurate delay as long as the inductance is not significant. As signal frequencies are increased, a full RLC model is needed to model these interconnects.

On-chip interconnections are fanned out in many ways and this makes the calculation of driver to receiver delay, also known as sink delay, even more complex. A number of algorithms, such as asymptotic waveform evaluation [Pill90] and Pade approximation [Lin92] have been put forward to approximate an RC tree. The earliest one used a simple RC time constant to approximate the 50% delay time [Elmo48]. These approximations can eat into the design margin of a high-performance design and therefore real SPICE based numbers need to be generated to get a measure of confidence in delay modeling before committing to an expensive run.

A test structure was designed using an 8 mm long tapped delay M1 line as shown in Figure 2.25. It has a very compact layout and simplifies testing by using a special ring-oscillator configuration. There are 8 ring oscillators formed out of 8 taps on the delay line. All 8 oscillators can be measured in one probe touchdown unlike 4 only in the previous scheme described for unloaded oscillators. The layout of the structure is shown in Figure 2.26. The inputs to the structure are 3 DC signals to a 3-to-8 decoder which generates 8 select lines. These lines in turn select one of the ring oscillators.

Table 2-10: Summary of RC structure.

Size1.55 mm x 1.6 mm
Device Count302 Standard Q1 HBT, 20 Diodes, 104 resistors
Current Level / Output Swing2 mA / 250 mV
Number of Stages8
Outputs / Inputs3 (1 High-speed) / 3

Figure 2.25: Tapped delay line.

Figure 2.26: Layout of the distributed RC structure.

Device Characterization Structures

Since the ring oscillator structures involved devices, another way for measuring these device characteristics in the vicinity was provided with special probe de-embedding sites. There were both de-embedded transistors and de-embedded schottky diodes on the chip. A standard compact structure was used to characterize these devices with two-port s-parameter measurements, as shown in Figure 2.27 .

Figure 2.27: Layout of the device characterization structure.

The upper half of the structure contains a device connected in a common-emitter configuration with the B (base), E (emitter), and the collector (C) terminals as labeled on the pads. The lower half of the structure contains the same geometry but without the device. The measurements from the lower half of the structure are used to de-embed the data from the upper half. The structure is probed in two steps with a coplanar high-speed probe. The measurement scheme and results are described in the next chapter.

Device Yield Structure

This structure contains eight 30-stages oscillators to test the unloaded gate delays of different types of transistors under different biases. The structure has a very compact layout and contains more than a 1000 transistors in an area of 1.6 mm2 and is used as an yield indicator. An augmented testing scheme measured oscillation frequencies, voltage swing, and current levels of all eight oscillators in just one probe touch down.

Figure 2.28: Schematic of the device yield structure.

Table 2-11 summarizes the characteristics of this structure. The schematic and layout of the structure are shown in Figure 2.28 and Figure 2.29 respectively. Four ring oscillators feed each of the 4-to-1 multiplexers which in turn feed a 2-to-1 multiplexer. The output of this multiplexer drives a 50-ohm driver. The input to the structure are 3 DC select signals to select one of 8 oscillators.

Table 2-11: Summary of the device yield structure.

Size1.0 mm x 1.6 mm
Device Count1057 Transistors, 8 Diodes, 782 Resistors
Device TypeStandard and Non-Standard Q1
Current Level / Stages2 mA, 1.6 mA, 0.8 mA / 30 Stages
Output Swing250 mV
Outputs / Inputs4 (1 High-speed) / 3

Figure 2.29: Layout of the device yield structure.

Other Structures on the Reticle

There were other chips and chiplets on the reticle as shown in Figure 2.5. These were: RPI Test Chip, Voltage Controlled Oscillator, and Boundary Scan Test Chip. These chips contained much more complex circuits which were verified by applying results from the structures described herein. The RPI test chip, tests a 200 ps 32 word x 8 bit SRAM and a 1-ns delay carry chain. The SRAM is a sensitive indicator of the accuracy of capacitance extraction because its wiring is extremely intricate and the memory is heavily wiring-capacitance limited in its operating speed. The carry chain similarly tests the raw speed of the devices in minimal load environment The voltage controlled oscillator [Camp97] tested the limits of the transistor speed by trying to achieve a 20 GHz oscillator. The boundary scan chip contained a scheme to test complex chips at high-speed.

Summary

A test chip containing passive and active test structures was designed to characterize an AlGaAs/GaAs HBT process. The structures were developed, in particular, to investigate the process for high-density digital applications. Another requirement in the design of these structures was the calibration of parasitic extraction tools for this process which are mostly 3-D in nature. Both passive and active structures were designed for this purpose. The measurement methods and test results are described in the next chapter.