Composition and principle of formation of standard evaluation circuit as simulator of gate arrays and semicustom very large scale IC based on them for radiation tests
Ensuring the requirements of radiation resistance for semi-custom IC on the basis of GA in the current engineering practice has a number of features:
levels of radiation resistance of IC are determined primarily by design and technological parameters of GA, it is assumed that they are "laid" during developing of GA, confirmed by the qualification tests during R&D and guaranteed by the controlling of "base" wafers batch in accordance with the requirements of the subgroup "E" of ОСТ В 11 0998 and ОСТ В 11 1010;
for confirmation of radiation resistance of GA the special test "lining" (so-called "zero") is developed, which compiles and simulates standard lining of GA for her certification and qualification, and in most cases – for control of wafers batch [3, 4]. Such test lining is a standard evaluation circuit (SEC) for GA;
the SEC includes some simple (to facilitate monitoring of performance in tests) functional blocks on the basis of a standard library elements, while it is considered that all library elements of GA have the same or at least similar levels of radiation resistance;
within the same batch (group of batches) of a "base" various working linings for semicustom ICs on GA can be produced, and it is assumed that the control of resistance of "base" according to "E" subgroup (ОСТ В 11 0998) guarantees a given level of resistance for all working linings realized on it, regardless of their functional complexity and fabrication time, therefore, there is no need to carry out radiation tests of working linings [3, 5].
The above approach to ensuring radiation resistance for ICs on GA has a number of disadvantages, which limit the accuracy and informativeness of test results of SEC and possibility of their unconditional use for the working linings:
during the R&D of GA, as a rule, there are no data of comparative studies of radiation resistance of all the basic library elements of GA taking into account their possible mutual influence in a joint application;
there is no clear separation according to the levels of radiation resistance of the digital, analog-to-digital (digital-to-analog) and analog basic library elements of GA, which even at proximity of design and technological implementation have a completely different systems of parameters-criteria of validity, and, therefore, different levels of radiation resistance;
the structure of the test "zero" SEC lining, as a rule, does not contain all of the basic library elements and is formed using only a few types, which are selected generally by the criterion of simplicity and efficiency of measurements ;
as a result of radiation test, the differences of the levels of radiation resistance both between different working linings of GA, and between working linings and SEC are revealed, although a full statistical analysis of the significance of these differences isn't made so far;
the accepted approach to assessing the stability of parameters of resistance on the basis of the control of the batches of the wafers is not effective enough for occasional production of large batches of the wafers with subsequent long-term (not time-limited) realization on their basis of a wide range of working linings, a custom ICs.
In view of the foregoing, at the request of customers, in the most critical cases the qualification radiation testing of each working lining of GA is carried out with the control of all important for the functioning of the product parameters-criteria of validity, in all informative modes and conditions of operation, which ensures unconditional compliance of its radiation resistance level with the specified requirements. This radical approach definitely reduces the risks of the consumer, but significantly increases the cost of small batches of products, which reduces technical and economic performance and overall competitiveness.
The objectives of the present work are the study of the criteria for the choice of a method of assessment of radiation resistance of semicustom circuits on GA (according to test results of SEC or working linings), as well as development of requirements for structure, principle of formation and testing procedure of SEC to ensure rational combination of the reliability and informativeness of the obtained forecast of the level of resistance of working linings while reducing the cost of testing.
Rational structure and features of SEC are defined by used element-technological basis and the level of functional complexity of created on its basis products.
The standard evaluation circuit must meet the following requirements (according to ОСТ 11 0999-99):
the number of gates on a SEC chip must be at least half the maximum number of gates on a chips semicustom IC based on GA;
structure of SEC should include all the standard library elements used in the design of semicustom IC based on GA. The association of library elements of similar assignment (functional classes) in the functional blocks is possible;
architecture of SEC should provide the possibility of diagnosing failures in all functional classes of library elements (ideally, each type);
design and technical solutions of SEC must comply with the basic values of design rules for semicustom IC based on GA;
the structure of the SEC must be uniform for all tasks of evaluation of radiation resistance during qualification tests of GA, and also for control of wafers batches and monitor the process of their manufacture;
electric modes at the testing of SEC shall be established based on considerations of the worst combinations of design standards and loads of construction elements;
for manufacturing of the SEC it is necessary to use the standard (certified) materials, technology systems, and cases (if necessary).
SEC for the evaluation of radiation resistance of GA should provide the possibility to determine (control) the following parameters [6–10]:
the degree of degradation of characteristics under the impact of accumulated dose of ionizing radiation with special attention to radiation-induced leakage in power supply;
• thresholds of occurrence of catastrophic failures and latch-up under the influence of pulse ionizing radiation and individual nuclear particles;
the nature of the response of parameters, malfunction-free operation level and time of loss of operability in the conditions of pulse ionizing radiation;
sensitivity to single effects (including crashes) under the influence of individual nuclear particles – heavy charged particles, high energy protons and single neutrons;
the degree of degradation of parameters in conditions of radiation-induced structural damages from the influence of neutrons and protons (for bipolar elements of GA).
The main problem in the development of the SEC is to provide depth diagnostics of the types and mechanisms of failure of different library elements at the limited number of outputs in the package of SEC. To construct rational SEC it is recommended to combine library elements with close functional purposes into the functional blocks with the decoding of access and buffering of output information (Fig.).
Difficulties are compounded by the presence in the libraries of modern GA of analog-to-digital (digital-to-analog) and analog elements, to which it is desirable to provide access, bypassing the digital buffer elements or minimizing the impact of the latter.
In order to obtain reliable estimates, the structure of the SEC for the characterization of radiation resistance of microchips based on GA (at the dominant radiation effects) should contain the following functional units:
basic digital elements (combinational and serial) of logic, decoders, multiplexers, ALU, triggers, registers, counters;
analog, analog-to-digital (digital-to-analog) elements;
library macroelements and IP blocks.
During the development of SEC it is necessary to verify the functional significance of each of the library elements and node, that is, that the radiation failure of any library element during the test leads to the failure of a functional unit or SEC in general.
We consider it appropriate, in addition to a set of standard functional blocks on the basis of library elements of similar functionality, to include in the structure of SEC one or more functional units with the highest possible levels of functional complexity and performance, which are provided by this GA and characterize its limiting capabilities.
With the aim of correctly assessing the level of radiation resistance of products it is necessary to choose informative parameters of validity, which reflect the main functional features of the product and changes due to radiation exposure. At the same time it is necessary to consider that if the range of parameters of digital components of GA is rather limited (functional control, the levels of logic zero and one, current consumption, input current), then for analog components it is very wide (table). [11–14].
During the production of IC on GA the regulatory documents (ОСТ 11 0998) require to control production batches of wafers for "base". Radiation testing of the SEC at the stage of control of a production batches of wafers should meet the following objectives:
Ensuring and monitoring the safety of resistance factors of GA during controlled changes in the production process, including confirmation of the absence of the influence of the variation of electro-physical and electrical parameters of a chips on a wafer within a batch, and also from batch to batch, on the stability of parameters of radiation resistance;
Ensuring prompt and accurate evaluation of radiation resistance of finished products (working linings) in the process of manufacturing of chips on the basis of continuous and periodical monitoring of resistance factors of SEC .
SEC, intended for control of process of manufacturing of wafers in the part of radiation resistance, should provide the possibility of assessing the sustainability of all used variants of constructive-technological solutions to a given minimum level of radiation exposure, at the same time the topology of the SEC should include all the standard library elements used in the design of the chips, which are produced in this process. In general, in our opinion, it is necessary to provide the maximum unification of SEC for characterization of GA and for control of batches of wafers.
It should be noted that the objective of the evaluation (assurance) of radiation resistance of IC on GA basically allows the monitoring during a radiation test of only that part of the library elements and blocks in the SEC, which are used in the working lining, so a level of resistance can be significantly higher than specified, for example, if the lining does not include analog elements. In practice, however, such a reduction of the range of controlled functional units is highly undesirable, as the limitations of the "cut-down" set of data will not allow to provide reliable control of technological process and stability of radiation resistance level according to the results of the statistical analysis and regulation. Therefore it is necessary to carry out a comprehensive radiation testing of SEC in the process of control of batches of wafers, and to use for assessment of the actual level of resistance of worked lining only those test data that are related to library elements involved in the working lining. If the testing of SEC showed no reserve by the level of resistance of the particular working lining regarding the specified level of resistance of GA, then for providing guarantees to consumers it is expedient to carry out radiation tests of working linings.
In conclusion, we will note that to guarantee the level of radiation resistance of working linings of GA that are manufactured on a common "base" for a long time, it is advisable to include the radiation tests of SEC in periodic testing that is provided by assumptions on development of ОТУ.
As a result of the analysis of the current engineering practice the basic features of providing the requirements of radiation resistance for IC on GA by developing and testing of standard evaluation circuit that simulates IC are defined. The shortcomings that limit the accuracy and informativeness of test results of SEC and a possibility of their unconditional use for the working linings are analysed:
absence in structure of SEC of all basic library elements of GA;
absence in structure of SEC of possibilities to separately determine the levels of radiation resistance of the digital, analog-to-digital (digital-to-analog) and analog basic library elements of GA with regard to their controlled parameters;
lack of reliable statistical information about the ratio of the levels of radiation resistance of various working linings of GA and also of working linings and SEC;
ineffective control at occasional production of large batches (groups of batches) of the wafers with subsequent long-term (not time-limited) realization on their basis of a wide range of working linings (custom ICs).
The principle of formation and structure of SEC to assess the control of radiation resistance of GA and semicustom ICs based on them allowing to overcome the above mentioned disadvantages are proposed. It is proposed to include in structure of SEC of all basic library elements. To ensure the required depth of diagnostics of the types and mechanisms of failures of a wide range of library elements in terms of a limited number of outputs in the package, it is recommended to combine library elements with similar functionality into functional blocks with the decoding of access and buffering of output information.
It is offered to unify the SEC as much as possible for characterization of GA, control of batches of wafers, analysis of features of control of stability of technology and estimation of radiation resistance of working linings using results of tests of SEC.
If the testing of SEC showed no reserve by the level of resistance of the particular working lining regarding the specified level of resistance of GA, then for providing guarantees to consumers it is expedient to carry out radiation tests of working linings. ■
This paper was created with the financial support of the Ministry of Education and Science of the Russian Federation. Unique identifier RFMEFI58015X0005.