Sunday, December 20, 2009

FCC regulations for unlicensed transmitters

What does the term unlicensed transmitter actually imply? What is it that one can do with this frequency band/bands? In fact, to start a wireless system design in the unlicensed band this has to be the first step in the design. To understand this more fully we took a look at the FCC website. It is a vast website and in spite of a search engine it still takes a bit of doing to locate the relevant articles, regulations, rulings, tips etc. We did manage to locate a couple of papers which we believe are helpful for people who may want to understand this concept of the unlicensed frequency band in the US. These can be accessed through our website at ( or, of course, through the FCC website!). Go to engineering_pages>engineer's corner and look for the unlicensed frequency band information. By the way, there are always updates to these, so it is a good idea to also check on the updates.

Saturday, October 17, 2009

VSWR, refelection coeffcients, s11, etc

The basic quantities such as VSWR, return loss, reflection coefficients, s11 and s22 etc are all basic to design. Their interactions and relationships are also basic and a brief note to document these relationships can be both informative and useful. There are numerous descriptions of these individually on the web. We prefer to publish the note in our website at>engineering pages>engineer's corner for interested parties.

Thursday, October 8, 2009

The relationship of Tf ( forward transit time) and ft in a bipolar transistor model

Someone recently asked how the ft of a bipolar is related to tf, the forward transit time of the bipolar. The tf is a model parameter while ft is not. Yet we always talk about the ft of the transistor. The answer to this question can be found in the spg website ( under engineering pages>engineer's corner for interested parties.

Friday, May 8, 2009

IP3 -- third order intermodulation product

IP3 is an important quantity in systems analysis of wireless/RF systems. It is a measure of the performance of an amplifier and is related to the 1 dB compression point of an amplifier. To put it simply IP3 can be interpreted as an unwanted signal that comes out of the noise level in an amplifier system ( or a mixer ) and climbs at a rate of 3X the amplifier input - output characteristic. It is, as mentioned above, an unwanted signal and therefore care is taken to calculate it as it sets a boundary on achieveable performance of the circuit. A good paper on this and other related quantities ( such as 1 dB compression point, IP2 ) is presented in the "Engineer's Corner" in our website located at>Engineering_Pages for interested readers. As young RF engineers it took us a while to completely understand the ramifications of IP3 and associated quantities. To be able to understand intermodulation intuitively is neccessary for good RF/wireless design.

Saturday, May 2, 2009

The Harmonic Balance algorithm.

The Harmonic Balance algorithm is now an established technique for CAD programs of various types, specially for RF/MMIC and analog. We felt we needed to understand the algorithm. This would allow us to be better at using it in simulations and more importantly be able to say if we wanted to purchase it in a CAD tool we wanted or not.

The implementation of these algorithms in the circuit simulator are fairly involved. However, luckily, compared to a couple of decades ago we as circuit designers do not really need to know its intricacies. What we want to know is at a higher level of abstraction. The expectation is that, if we do this we can do better at simulation and know when to use it effectively and when to not use it!

As a result of discussions internal to our design and CAD group a better understanding was gained and we decided to write a brief paper on it. This paper is now available on our website at Interested readers may follow the links>engineering pages>engineer's corner and read the paper if they wish.

Wednesday, April 29, 2009

An ADPLL for Clock generation in a mixed signal IC

Precise clock generation is required in a majority of mixed signal ICs. Generally a PLL of some sort is used. In a prior post the concept of clock distribution was explored. The actual clock was generated by an interesting PLL based on a DCO. There are some advantages to this technique when it comes to providing a clock to an AMS system. Interested readers may go to and then navigate to "Engineering_Pages>Engineer's corner" and look for the ADPLL design... paper.

Clock distribution in mixed signal IC designs

In relatively high speed analog and mixed signal IC designs, a challenge is to distribute the clock ( usually derived from a clock reference like a PLL) such that clock skew is either eliminated or minimized.In one of our designs, clock distribution was becoming a problem so we studied it and came up with a solution which is illustrated in this posting and its accompanying article under "Engineering Pages" in the website. For a detailed look at this technique, interested readers may go to and then navigate to "engineering_pages>engineer's corner>clock distribution strategy".

Saturday, April 25, 2009

References for diode design

Collecting the right references for the design of diodes became a fairly serious project. However, this was done and a reasonable collection was generated for both simple p-n junctions and pin diodes. Use of these references can make the job a little easier. The reference list is on the website under "engineer's corner" in engineering pages.

Thursday, April 23, 2009

More about diodes!

Need a pn junction diode that has a high reverse breakdown, very low capacitance ( so its fast) and low resistance in the forward direction? If this is the case then a simple pn junction diode may not provide the answer. The reason is, that as the breakdown voltage goes up, the forward resistance goes up, capacitance goes down. If you a need a lot of current then this diode will not provide it. As the resistance goes down the breakdown goes down and the capacitance goes up. So sometimes a simple pn junction diode cannot meet specifications.

Yet if this type of performance is required, either for purposes of high current
(read low resistance ) or high frequency applications then a different type of diode is needed.

This is the P-I-N or N-I-P diode. The I stands for "intrinsic". This diode has a heavily doped p region and n region, just like in a ordinary pn diode. However, the resemblance ends there. In a P-I-N diode there is a high resistivity ( or
"intrinsic" region) sandwiched between the n and p heavily doped regions. The inclusion of the intrinsic or high resistivity region imparts some very useful characteritics to this structure. These characteristics are explored heuristically in this post.

Resistance: The resistance of the P-I-N diode is inversely proportional to the forward current through the diode and can be controlled by it. Very flat resistance characteristics can be generated this way. The reason for the low resistance with current is that as the high resistive region has very few carriers for recombination, any injected minority carriers coming from the heavily doped p and n regions do not die quickly but persist for "long" lifetimes in the I region. Thus the higher the current, the more free carriers in the I region and the lower the resistance. In the ultimate limit the forward resistance reaches the contact resistance which can be made very low.

Capacitance: The pn junction zero bias capacitance in the P-I-N diode is very low ( or relatively low compared to the ordinary pn junction diode). The reason is that the depletion region ( the region that is completely depleted of carriers with increasing reverse bias or zero bias) forms the "insulator" of a parallel plate capacitance. The parallel plates are, of course, the heavily doped p and n regions of the diode. The higher the resistivity of the I region the wider the depletion region and the lower the capacitance. Also the capacitance is very flat over a wide band of high frequencies so matching with other circuits becomes easier. As a result of the low capacitance the P-I-N diode can switch very fast and can be used in high frequency applications.

Reverse breakdown voltage: The breakdown voltage is high since the breakdown electric field drops voltage across a wider depletion region. As the depletion region becomes wider and wider with reverse voltage the breakdown increases.

Thus if one wants to reconcile high breakdown with low resistance and low capacitance then a P-I-N diode is a great choice. Both power diodes and RF diodes can be made with this technology.

Some disadvantages in the usage of the P-I-N diode are that (a) Its performance can only be predicted accurately if the lifetime of the minority carriers in the I region are known accurately. There are not a lot of analytical techniques to calculate this, therefore for precise usage, measurements need to be made. ( See the previous posts). (b) Most circuit simulator programs such as PSPICE do not provide a mathematical model ( empirical or physics based) so circut simulation is difficult. (c) The fabrication of the diode is slightly more complex. However most vendors provide the parameters and application notes for their P-I-N diodes so usage is made fairly easy. However, designing one from scratch can be quite involved because of the above factors.

Sunday, April 19, 2009

Minority carrier lifetimes in silicon

As semiconductor designers we grew up with the concept of lifetimes of minority carriers in silicon. Our task was to take the process parameters and design rules from the foundry and fashion a chip. However, once we venture beyond this safe boundry and pit our skills against device design from scratch, a number of issues come up with which we are not too familiar with. One such came up for me this weekend. I was trying to calculate minority carrier lifetimes for specific conditions. I found out that this is a very difficult thing to do. Minority carrier lifetimes vary quite broadly and are dependent on a number of factors. Among these are Auger recombination, band to band recombination and Shockley-Read-Hall (SRH)recombination.

The lifetime is a strong function of the doping concentration of the silicon. It is easier to use analytical formulas for lifetime calculation when the concentration is high ( > 1E17).

High resistivity material is harder to handle analytically. The lifetimes in these materials can be a function of the construction of the crystal(CZ versus FZ). In addition various processing steps can have an impact on the lifetime.

Nevertheless analytical formulas do exist for estimation of lifetimes. The one that I am now using is: lifetime = 5E-7/(1.0 + 2.0E-17)N, where N is the doping concentration in cm**3.

Roulston has published a curve that also shows the approximate variation of lifetime with concentration. Both of these techniques are just approximations. I compared calculations of the lifetime for various concentrations using the analytical formula with Roulston's curve. The fit became very close as the concentrations increased, but was poor at low concentrations (highly resistive silicon).

My conclusions are that if the need is simply to estimate the lifetime to a rough order of magnitude then by all means one can use the analytical formula given above or Roulston's curve. However, if precise numbers are required then measurements must be made on samples of doped silicon under the conditions of operation. There is no shortcut here for that kind of accuracy!

Saturday, April 18, 2009

Microstrip on silicon design

Microstrip is the preferred style for designing passive circuitry for MMICs, RF and high speed digital circuits. If the substrate is a board or GaAs the task is simpler and the design can be pretty much cookbook. However, if the designer has to do this on a silicon substrate ( just an ordinary one, say for a SiGe process or fine line CMOS) then it becomes complicated. Why?

The reason is that standard silicon substrates are very lossy for high frequency signals and the design of microstrip ( specially the initial hand calculation/engineering judgement type designs) become a chore. If one is fortunate to have expensive CAD tools that one can use extensively then it is less of a grind. However, one still has to understand how microstrip behaves on silicon and what one has to do to make the right corrections.

A while ago I wrote an article on this precise subject. It is available on the SPG website under the engineering pages> engineer's corner for interested colleagues. Feedback on this will be greatly appreciated since some of the issues were expounded based on personal observation and experience.

The intricacies of CRC encoding

Recently, we at SPG got involved in high speed data transmission issues and in particular the CRC algorithm. The algorithm itself has been around forever it seems, yet its simplicity is very appealing. Anyone involved in it, or about to get involved in data transmission is probably very familiar with it. In any case I found it very interesting.

The basic scoop on it is as follows: ( Interested readers may view the details on our webpage:>engineering_pages>engineer's corner and look for the detailed article and hardware implementations.)

The CRC procedure can be explained as follows: You have a data message you want to transmit which is k bits long. You can use the CRC to generate another sequence of bits that is n bits long. The latter sequence is called the frame check sequence. What you have to do is actually trasmit both the original k bits of your message and the FCS that is n bits long. Therefore the total length of your transmitted message becomes k + n bits. This k+n bits should be exactly divisible by some predetermined number.

At the receiver the received k+n bit long message is divided by the same predetermined number. If there is no remainder then the message has been received without errors. If there is a remainder then the message has errors. Its as simple as that!

Estimating performance parameters for a current source DAC

An ever present issue in the design of analog circuits is the challenge of estimating power dissipation, size on silicon, cost etc. We would all like to know these factors as early on as possible. Both designers and customers can benefit from this information. These parameters are very dependent on the specifications and therefore the technology chosen for implementation. As we got to pondering this, a customer did, very bluntly, ask for these estimates for a current source "high speed" DAC. As a result we had to go and look at these design factors. Ultimately it turned out that the results of that little study turned into a report. We published the report and it proved to be very useful indeed, not only for that particular item but for a broad class of analog devices. The report is available at for interested viewers.

Tuesday, April 14, 2009

This month we got involved in the detailed design of a diode. For most analog and RF ASIC designers diodes are pretty trivial as far as design is concerned. The reason is that we get the parameters from the foundry and use the scaling for diodes already fully characterized by the foundry. However, when we get a specification like: " Need a diode with a series resistance of 1 ohm, a capacitance of 0.2pF, with a clamping voltage of 25 volts with a 5 Amp current", things get a little more sticky.

Where do we start? I suppose one set of answers are: (1) Start with the substrate.
(2) Use Irwin's curves to calculate sheet resistance, (3) Use a manual like the Semiconductor QRM design manual to get an initial design for the required capacitance. This involves extracting (a)The concentration gradient (b) The built in voltage (c) zero bias capacitance... Once all of these preliminary parameters are calculated, we use a simulator like Athena or Atlas ( or other simulation tools like the Stanford University TCAD set) or SYNOPSYS. This is the tricky part where optimization becomes so important and it takes a long time!

Friday, February 20, 2009

Sampling rate conversion in digital signal processing

Multirate processing, sampling rate conversion, or interpolation and decimation as it is known, is a clever technique in DSP. As analog and mixed signal design engineers we have learned to use this technique in various product designs for our customers. It offers an added degree of freedom in the design of mixed signal integrated circuits that may be of help to other professionals such as us.

Multirate processing finds use in signal processing systems where various sub-systems with differing sample or clock rates need to be interfaced together. At other times multirate processing is used to reduce computational overhead of a system. For example, an algorithm requires k operations to be completed per cycle. By reducing the sample rate of a signal or system by a factor of M, the arithmetic bandwidth requirements are reduced from kfs operations to kfs/M operations per second. fs is the sampling rate. M is the decimation factor.

In other applications, resampling a signal at a lower rate will allow it to pass through a channel of limited bandwidth. In another application a high accuracy delta-sigma A/D converter can be made with very high modulation rate at the front end followed by a decimator ( down converter) to reduce the sampling rate and provide converted samples at or near the Nyquist rate.

Applications for this technique abound, if understood by the practitioner. The challenge is that it is not easy to pick up a book or a paper on DSP and understand Decimation and Interpolation to an intuitive extent. This causes hesitation in usage.

A tutorial paper has been written to aid in further understanding of this fascinating topic. Read it at

Tuesday, February 17, 2009

Developing Specifications for an analog/RF/MMIC ASIC

It is true that the quality and success of an analog ASIC generally depends on the specification that is developed for it. A good specification with requirements clearly defined may account for more than 80% of the success, of not only the ASIC device, but also the entire process of development including the business and technical relationship that develops between the customer and analog ASIC vendor.

For it is true that the quality of an analog ASIC is defined by not only how well the device meets the specifications, but also the experience the customer has with the very process of working with the vendor.

It behooves us then, to at least define some basic ground rules for the generation of specifications. It is also true that each analog ASIC will be unique and have its own features, but it is usual for certain items to be included in the specification and follow certain formats.

These issues are explored in this post. We hope this post will be of help to those involved in specifying or implementing an analog ASIC.

This post follows the following outline:

Types of specifications required
Suggested format for the specifications
Challenges in building specifications

1.0 Types of specifications required

In general the specification of an analog chip should be in two parts. The first part is the functional specification and the second part is the test specification.

The functional specification contains a comprehensive description of the chip and all the detailed functionality required by the user. It includes the interaction of the chip with the board ( or substrate ) along with all external components.

The test specification contains the test methods, test options, reliability test options, burn in, thermal operational tests. These numbers and descriptions are usually specified at the I/O of the chip since no other part of the device is available to the outside world.

2.0 Suggested Formats

2.1 Functional Specifications:

2.1.1 The cover page should be the part number of the chip, approvals, revisions and any other high level information.

2.1.2 The next section should provide a clear but brief conceptual level description of the function.

2.1.3 Following the functional description a fairly detailed block diagram with the pin I/O clearly marked should be provided.

2.1.4 A table of pin descriptions should be included which provides clear information on the pin number, the pin name, the pin symbol, whether input or output, and a
succinct description of the function of the pin.

2.1.5 Also included are the absolute maximum ratings for current, voltage, temperature, etc. the chip may be exposed to in extreme cases in a tabular format.

2.1.6 The next section of the specification should clearly describe the principle of operation, timing, flow charts, relevant technical data, operational characteristics etc. in reasonable detail.

2.1.7 Specify the DC operating conditions of the chip including logic levels, power dissipation, supply currents, operating temperature, supply voltages etc. in this section. Minimum, typical and maximum values are preferred along with the symbols of the parameters being specified and the conditions under which the specification has been made.

2.18 Specify the transient operating conditions of the chip, including all delay times, rise and fall times, hold times, setup times, clock frequencies etc. Include timing diagrams if more clarity is required for each parameter. Include symbols for all parameters being specified and the conditions under which the specification is made.

2.19 Specify AC operating conditions. Specify gains, noise levels, input and output impedances, input and output analog voltage and current levels, frequencies, analog accuracies and tolerances etc. Include symbols for all the parameters being specified and the conditions under which the specification is made.

2.110 In the last section include some typical application circuits and/or applications hints that allow the user/designer to understand the operation of the overall system including the role of external components and any test signals. Also include in this section, the suggested board layout for accurate operation of the device. If possible include a specification of the board material or other substrate being recommended for usage.

2.2 Test Specifications:

2.2.1 Cover page is almost identical to that of the functional specifications with all the nomenclature indicating revisions, dates, initiators, approvals and title.

2.2.2 Provide a block diagram of the test architecture showing all external components and any switching relays, matrices of other auxiliary test structures to be used.

2.2.3 Provide a complete pin I/O description. Note that in many cases the test pin I/O list may be more extensive that the functional pin I/O list since there may be test pins included on the device. The package for test may or may not have the same number of pins. Provide a clear description of the pins and their functions.

2.2.4 Provide a complete list of tests to be carried out. Name each test with an appropriate name and number. Link a test description to each test number.

2.2.5 Provide detailed device specific test procedures for each of the tests specified in 2.2.4 above including the role of external supplies and other signals and expected results and tolerances for the results.

3.0 Challenges in building Specifications

It is one thing to say that specifications should be provided for a design to be done accurately and another to actually do it. This is specially the case if the device is a new device with very little functional or test history behind it.

In most cases no one really knows enough about the device to specify it completely. Typically, information that needs to be input into the specifications is non - existent before the device designed. This is the first hurdle or challenge faced by those who would specify the device.

Therefore it is common practice to have a “ preliminary “ specification which is a specification which has a considerable amount of information but also has a lot of “TBD’s” i.e. “ To be determined “ parameters.

The TBD’s can only be replaced by hard data after the chip has been designed and in some cases after the chip has been fabricated and evaluated.

The test specifications are also in a similar position. Since the operating parameters may be unknown the test specification suffers a similar fate with a TBD’s also.

There is a common practice in test specification development where a number of iterations may be performed on the specification. The first test specification may be “ Comprehensive”. This simply means that there is an overkill of tests included in it. These extra tests provide information for the final test specification after the device is designed and fabricated.

As more and more information becomes available the “ Comprehensive “ specification is trimmed downwards with fewer and fewer tests remaining until a final test specification can be approved.


Wednesday, February 4, 2009

ASIC success factors based on customer - vendor relationships

ASIC success is something that all customers and ASIC vendors pray for. For the customer the issue is one of his/her credibility and recognition within his/her own organization and potential gain/loss of funding. The issue for ASIC vendor is his/her reputation, revenue and long term prosperity. Whichever side one is on, any information on succeeding with an ASIC product can only come in handy.

Over the past 35 years, my involvement with ASICs, first as an employee of large systems companies and then latterly ( 21 years) in a smaller "ASIC - centric" company I have learnt some lessons which I thought might be useful for others. Therefore this post.

The following success factors are in no particular order. Yet all are equally important. If the interested person makes use of these ideas then his/her success with ASICs will be greatly enhanced. That is my earnest hope.

1.0 Underestimation of time and money: The success or failure of an ASIC actually starts right at the beginning, when the vendor is asked for a quotation. Many times the vendor thinks that by underbidding the project he may improve his chances of getting the business. As a matter of fact, this is true in many cases. A low bidder wins. However, the customer may not realize that two things are about to happen:

(A). The vendor cannot possibly complete the project in the time and money originally quoted so he asks for more time or money or both, in many ingenious ways.
(B) The vendor fails to do a complete job on the ASIC and releases the ASIC to fabrication anyway. Of course the probability of failure is very high, since due diligence may not have been done.

Both of these options lead to negative consequences. One of the most serious being loss of trust by the customer, and the death of a close friendly relationship between the two. It is very hard to salvage the project after this has happened.

Therefore for ASIC success, an accurate estimation of time and money is critical. If the vendor estimates a higher amount, he still has to live by it and quote it. In spite of this the wise customer will add at least 20% 0r more to both these quantities as contingency planning.

2.0 Schedule: This is a corollary to the first factor. Establishment of a reasonable workable schedule is one of the most important success factors for ASICs. Similar comments hold as above. When schedules become too tight or unworkable the project can continue for a while in cloud cuckoo land but the cuckoos will ultimately come home to roost as the time runs out. The result may be (i) ASIC not complete (ii) ASIC done badly to satisfy customer push to meet an unworkable schedule leading to failure of the ASIC at probe or bench test. After this it is very difficult to continue with the project because it may take a very long time to debug and fix the ASIC. Again a contingency plan should add more time to take into account any unknown factors from completely wrecking the schedule and hence the ASIC.

3.0 ASIC development agreements: A carefully thought out and written ASIC development and supply agreement is an absolute must for success. Agreements should contain, at a minimum, a clear and detailed SOW for each phase of the project, review process, Engineering Change Order( ECO) procedures, a program plan as accurate as can be at this initial stage, payment schedule and terms, communication procedures between the vendor and customer and any other legalities ( boiler plate). Functional and test specifications may or may not be included. If a separate conceptual/feasibility study was done before the actual ASIC project was started ( highly recommended) then both specifications should be part of the agreement.

4.0 Customer expectations: Customer expectations are a very important success factor for an ASIC project. Customer expectations are really set by his/her view of the vendor and the information that vendor supplies. It is imperative that a set of realistic ( or slightly pessimistic, [I daresay], expectations) be the rule. I know that if the expectations are very pessimistic then the customer will decide not to do the project. Conversely, if the customer expectations are too high then when reality sets in, the negative feelings may cause untold misery to both parties. I believe that one of the critical jobs that a vendor must do is to manage customer expectations in an effective and positive manner.

5.0 Design tools: Lets make sure that the appropriate design tools are available both to the vendor and customer. At a minimum, a reasonable system simulator, a circuit simulator, a layout tool, a debug tool and a documentaion tool is essential. Today this is not a problem, since CAD tools are freely available under a variety of license options. A corollary to this, is that the vendor must know how to make effective use of the tool. Currently CAD tools have become so complicated and massive that sometimes this very fact causes problems. I think that the rule should be, to use the simplest and most user friendly tool appropriate to the job. The customer needs to have some way to review the work being done and to help if any issues arise. Therefore the customer should also have access to some tools which allow him/her to do his bit.

6.0 Fabrication models and design rules: This is very important and has major ramifications. As shown in an earlier post, one of the ways to realise a successful ASIC to through extensive simulation, clean layout and verification of the layout before submission to a fabrication facility. Accurate device models and design rules must be supplied by the fabricator and packaging houses. Any inaccuracies in this data will cause untold problems, during and after the fabrication of the ASIC and may render a ASIC completely useless. So lets make sure we pick a good fabricator and packaging house who can supply accurate models.

7.0 Design expertise: The vendor should make sure that the design expertise exists in the company for a particular type of ASIC. Competence is what is required. The augmentation of expertise with CAD tools is great and perhaps competence with the CAD tools is part of the expected competence. If the appropriate design expertise for a certain part of the ASIC does not exist within the company, then ask for help within the larger ASIC vendor or consultant community to fill the gap.

8.0 Full disclosure to the customer: It is essential that the vendor and customer disclose any issues that may be troubling or which may impact the success of the ASIC. Colloquialy, "lets be up front" with each other to get the maximum benefit from each others expertise. Anything that is hidden will eventually be found out and usually at the worst possible time.

9.0 Customer - vendor relationship: This is one of the most important success factors for ASIC success. A close, cordial, mutually respectful and friendly relationship between the customer and vendor, in my opinion, is a very important success factor. Even in times of stress ( for whatever reason) a close relationship will help to get over any issues being generated by the project. A close friendly relationship is so important that I rate it as the number one success factor for ASIC success.

10.0 Communications between the customer and the vendor: Another important success factor is the level of communications between the customer and vendor. Regular reviews, informal or formal conversations between the appropriate members of the customer/vendor team make all the difference in the world. Specially to catch any problems in their infancy, before they become big problems. E-mail, video teleconferencing, telephone, etc. whatever means are most effective should be used often and regularly to achieve a close communication link.

11.0 Use of ECOs ( Engineering change orders): In some cases, even though due diligence was done, a change may be required by the customer after the project starts. I strongly recommend the use of ECOs to prevent problems due to "MISSION CREEP", the biggest problem in some major projects. Obviously it is not easy to eliminate this, but the ECO allows both parties to do the needful in a friendly and professional manner. The change to be done is discussed, a new quote for time and money is generated and the program plan is amended in such a manner that both customer and vendor are satisfied.

12.0 Multiple iterations for large ASICs: The rate at which there are first pass successes for ASICs has risen steadily. However, when the ASIC is complex for any reason ( Large analog and digital content, different types of simulation conditions, unknown design parameters etc), multiple iterations should be factored in right at the beginning and customer and vendor should clearly understand that this is the case. The number of iterations required varies from project to project. Sometimes the vendor may fail to inform the customer of this fact and thereby fail to manage customer expectations as mentioned above leading to a failure of the project.

13.0 Post fabrication analysis tools: If the ASIC is small and uncomplicated, then the probability is, that it will be a first pass success and no other work will be needed post fabrication. However, when the ASIC is complex ( as described above), it is very probable that multiple iterations will be required to get it to production status. In order to effectively analyze performance problems or debug the ASIC some analysis tools and equipment is neccessary.
An analytical prober with a low capacitance, high impedance probe capable of probing down to
1 micron is neccessary. Access to a Focused Ion Beam ( FIB) resource is becoming more and more popular. Appropriate laboratory test equipment is neccessary. A PCB design, layout and fabrication resource is required. This is of course not a comprehensive list. Some of this equipment and tools may be acquired internally. However some expensive items like the FIB tool may be rented as needed.

14.0 Conclusions: The above musings are a result of my experience. The success factors listed above are by no means exhaustive. I would welcome comments from peers on their experiences and permission to add their recommendations to this list. Finally I sincerely hope that this post is useful and leads to better ASICs and great ROI for our customers.

Monday, January 5, 2009

First pass analog and RF/MMIC ASIC success

For customers of analog, RF/MMIC ASICS a first pass success is a consummation devoutly to be desired. But what is a "first pass success"? In the strictest sense, a first pass success for an ASIC of any kind means: (i) It works functionally right out of the fab. (ii) It not only works functionally but also meets all the electrical and environmental specifications.

The question then is: Is it possible to develop and fabricate devices which will be first pass successful as per the above definition? I think the answer to this question is quite complicated.

The following set of posts will address this issue.

Anyone who knows about the process of device development, fabrication, testing , packaging and applications knows that each of these steps have their own perils. Therefore to meet the above definition of a first pass success each of these hurdles must be overcome successfully.

Let us first take a look at the device development phase.

Device development:
During the device development phase, a specification is agreed to, after conceptual deliberations and feasibility studies. The integrity of this specification is very important as this document will be the guide to the rest of the execution. Therefore it is imperative that the specification should be as good as can be with no T.B.D's. Any T.B.D's will lead to risk.

After this, the entire chip will be designed from the top down using various design tools appropriate to the type of device. ( MATLAB? ). Once the top level design has been been verified on a functional block basis ( or behavioral basis) the various functional blocks will be converted to circuit schematics.

Each block will then be designed and simulated using industry standard simulation tools ( PSPICE, ADS, CADENCE etc). These simulations will be performed for various ( specified) environmental conditions such as temperature. ( Industrial range, -40 to 85 degrees C or military -55 to 125 degrees C etc). Multiple design reviews with all concerned parties will be held to make sure that these simulations are precise and essential. In this way all the functional blocks will be designed, simulated and finalized.

The next stage of the development will interconnect these functional blocks and attempt to simulate the complete chip over all the required operating conditions. This is a key step in the development and in the pursuit of first pass success. This is explained below.

Note that the simulations of the chip are done using electrical and geometrical models provided by the fabrication facility and the packaging facility. It is absolutely essential that a fab and a packager be picked that provides a complete set of these models and certifies that these models are up to date and accurate. The reason is simple. If the models are not accurate, the simulations will also be inaccurate and the device will fail to operate as required and first pass success will be thwarted!

Assume for the moment that the models provided are accurate. The next question is about the simulation tools being used and the nature of the device being simulated. The simulator tool can simulate very complicated circuits but it has some real problems when a certain set of conditions of simulation are met. For example convergence of DC and transient solutions can be a very real hazard. DC convergence problems can occur with the existence of very high impedance nodes or branches in the circuit. Transient non convergence can occur when there are very long and very short time constants involved in the circuit. Analog and digital circuits in the same circuit can be a big problem because they are very difficult to simulate.

There are analog simulators and digital simulators but a true mixed signal simulator is not really available. Analog simulators simulate time point ( or frequency point) by point and thus generate a very large number of data points .

Digital simuators generate a true or false data set. Therefore if there is significant digital content in the circuit the data generated by the analog simulator will be very large and swamp the computer memory. A digital simulator will be incapable of simulating the small analog steps required for precise analog simulation. In addition the time for simulation will be so long as to be really not practical. In general a fairly small mixed signal device can play havoc with the simulations! This is not a practical way of simulating this class of circuits. No one really knows currently of a practical way of simulating these type of circuits.

Having understood this, it is now possible to point to risk number one for the failure of the device upon first pass. If the complete chip cannot be simulated a 100% then the probability that the chip will be a first pass success will be lower than 100%.

How can one estimate the probability of success quantitatively?

The difference between 100% simulation of the entire chip and the actual depth of simulation will be the risk that the chip will not meet the specifications on the first pass depending on the circuit simulation issues alone. Therefore to avoid this risk, the chip must be 100% simulated. If it cannot, then one has to assume the risk mentioned above.

Following the simulation of the chip, layout will be done ( or even before the chip simulation is complete). The layout is the second most critical part of the process which will determine first pass success. There is no correct or incorrect way of doing layout in general, except insofar as all the foundry layout rules are obeyed and the layout is LVS compliant. ( LVS = Layout versus schematic verification).

However, for analog and RFIC/MMIC designs layout becomes a very critical activity, since shape, placement and interconnect type of the layout elements becomes important to performance of the chip. Matching of active and passive devices is dependent on how close these devices are on the layout. They need to be in the same orientation. For temperature critical elements, the devices ( resistors, capacitors, active devices) may not only have to obey shape and orientation rules but also lie on isothermals on the chip surface. Fringe capacitors can lead to unintended coupling of signals. For a high gain, wideband amplifier, input and output traces placed close together can lead to parasitic oscillation! Ground shielding must be used whenever there is a danger of unintended coupling for reasons of size or electrical performance.

Matching of devices is also important. Common centroid layout ( layout of sections of a passive device or a number of matched active devices around a common pivotal point) has to be used. For reduction of offset the usual differential pair may have to be split into a quad and cross-connected.

If the device is a radhard device a number of other techniques have to used, specially for CMOS type devices where threshold shifts with radiation will almost certainly kill the performance/device.

There are a multitude of layout techniques, beyond the scope of this post which have to be learned through experience. The point however, is this. Even if all these techniques are used the layout of the device is more susceptible to errors which lead to chip failure ( and thereby miss the first pass criterion) than schematic errors.

In spite of this a number of devices can indeed be first pass successful. However, in the author's experience these are fairly simple devices where the level of criticality is low. In such devices first pass success can be expected and many times, found.

Thus, there is a finite risk of failure due to layout issues.

The next step in the chip path is the fabrication.

The fabrication of the device is carried out in the foundry selected. Hopefully, the foundry will be a good one, providing precise models and design rules/process rules which will be certified.

The foundry will run its own DRC ( design rules checks) on the chip database sent to them for fabrication. If the DRC toolset at the foundry and that at the customer are correlated then no DRC errors will be found at this stage. However, the usual case is, that there will be some errors found at this juncture. These errors will have to be corrected ( or waived) by the customer. In the author's opinion waiving errors is not a safe option. All DRC errors should be corrected before fabrication is started. If not, this will lead to another risk that the chip may fail first pass success.

As the fabrication proceeds the customer will be given access to the WIP ( Work in Progress) database and when the fabrication ends the customer will receive the finished wafers and the process control module test results. The wafer test results must be scrutinized for compliance with upper and lower level limits of all parameters.

Again sometimes the fabricator cannot meet the process limits and may ask for a waiver. This should be considered very seriously as any parameters out of limits can cause a failure of the device.

The bottom line is this. There should be no DRC errors in the final DRC run by the fabricator and no waivers asked for before/after the processing, if we are to eliminate the risk of the fabrication causing a chip failure .

Finally if all goes well the wafers should be available for a probe test using a test program ( or a manual probe test) which will be the first evaluation of the device ( before packaging). If the simulations are accurate, the layout is accurate and the fabrication is done correctly, the probe test should yield first pass functional devices.

However, we are not there yet. The device must be packaged ( usually) and the package test must yield good devices. As is well known package parasitics can have severe effects on the device performance, specially if it is a high performance device. This problem can be avoided of course, by making sure precise package parasitics are available when the device is in the simulation stage. If this is not done, then there is a finite probability of the device failing the packaged test.

Finally if the device does pass the packaged test, it must be inserted into the board or the system it was designed to operate in. Here the device may be subjected to various forms of stress such as EMI, RFI, thermal. mechanical, noise, etc, etc. In order for the device to pass this test, it should have been designed to operate in the environment it is in now. This is why a great deal of attention must be paid to the deliberations and assessments during the conceptual stage of development. In the author's opinion the road to first pass success really starts at that point.

Significant attention during the conceptual phase is a good approach, which leads to a positive result at the end of the entire process. If this is not done then there is a finite probability that the device will fail at this late stage which is a really catastrophic event by any standards.

As can be seen, to ensure a first pass success a great many factors must be taken into account and due diligence paid to them. In spite of this approximately 5%-10% of devices fail to be first pass functional ( studies have shown) for one reason or the other and may have to be re-iterated with a reduced set of masks.. This factor should always be taken into consideration when planning a new high performance analog or RF ASIC or MMIC.

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