JEDEC JESD221-2011 Alpha Radiation Measurement in Electronic Materials.pdf

1、JEDEC STANDARD Alpha Radiation Measurement in Electronic Materials JESD221 MAY 2011 JEDEC SOLID STATE TECHNOLOGY ASSOCIATION NOTICE JEDEC standards and publications contain material that has been prepared, reviewed, and approved through the JEDEC Board of Directors level and subsequently reviewed an

2、d approved by the JEDEC legal counsel. JEDEC standards and publications are designed to serve the public interest through eliminating misunderstandings between manufacturers and purchasers, facilitating interchangeability and improvement of products, and assisting the purchaser in selecting and obta

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6、less all requirements stated in the standard are met. Inquiries, comments, and suggestions relative to the content of this JEDEC standard or publication should be addressed to JEDEC at the address below, or refer to www.jedec.org under Standards and Documents for alternative contact information. Pub

7、lished by JEDEC Solid State Technology Association 2011 3103 North 10th Street Suite 240 South Arlington, VA 22201-2107 This document may be downloaded free of charge; however JEDEC retains the copyright on this material. By downloading this file the individual agrees not to charge for or resell the

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9、cense agreement. For information, contact: JEDEC Solid State Technology Association 3103 North 10th Street Suite 240 South Arlington, VA 22201-2107 or refer to www.jedec.org under Standards and Documents for alternative contact information. JEDEC Standard No. 221 -i- ALPHA RADIATION MEASUREMENT IN E

10、LECTRONIC MATERIALS Contents Page Introduction ii 1 Scope .1 2 Terms and Definitions .1 3 Instrument Operation Parameters.2 3.1 Bias Voltage.2 3.2 Bias Point Selection .3 3.3 Discriminator .4 3.4 Gas Management and Maintenance .5 3.4.1 Gas Type5 3.4.2 Gas Purity 5 3.4.3 Optimal Gas Flow6 3.4.4 Gas T

11、ubing Specification.6 3.4.5 Active Area Determination6 3.5 Sample Distance 7 4 Calibration8 4.1 Calibration standard.8 4.2 Calibration Interval 8 5 Background Measurement9 5.1 Trays or sample stages.9 5.2 Method of Determining Background .10 5.2.1 Measure background before each analysis.10 5.2.2 Lon

12、g term average of statistically controlled background.10 5.3 Background measurement time requirement .11 6 Detection and treatment of systematic errors .11 6.1 Cumulative Density Function 11 6.2 F Statistic or ratio of variances 12 6.3 Treatment of outliers12 6.4 Method for determining count rate st

13、abilization and initial data rejection 12 7 Detection Limits .14 8 Secular Equilibrium Considerations14 9 Uniform Method of Reporting Data.15 10 References.15 Annex A (normative) Active area determination.16 Annex B (normative) Cumulative Distribution Function Example .18 Annex C (informative) Sampl

14、e Calculation21 JEDEC Standard No. 221 -ii- Introduction Soft error upsets in semiconductor devices are caused by energetic particle interactions with the sensitive nodes in the device. One source of these energetic particles is radioisotope impurities in the materials that comprise the device. Alph

15、a particles are of primary concern, and materials that have low alpha activity have been selected for critical applications to mitigate this effect. Measurement of the alpha flux is important to establish both the usability of these materials and the reliability of the semiconductor devices fabricat

16、ed from them. The measurement of alpha flux below 10 khr-1cm-2is complicated by the fact that the sample alpha flux is usually less than or equal to the background alpha flux in the detector. Achieving a reasonable degree of precision requires measurements lasting for many hours or days. The low sig

17、nal to background ratio also makes measurement results vulnerable to variations in techniques and methods. Elimination of or compensation for these sources of measurement variation allows for scientifically and statistically valid results that are reproducible between different laboratories. JEDEC S

18、tandard No. 221 Page 1 ALPHA RADIATION MEASUREMENT IN ELECTRONIC MATERIALS (From JEDEC Board Ballot JCB-11-21, formulated under the cognizance of the JC-13.4 Subcommittee on Radiation Hardness and Assurance and the JC-14.1 Subcommittee for Reliability Test Methods for Packaged Devices.) 1 Scope This

19、 standard applies generally to gas proportional instruments and the use thereof in measuring materials with an alpha emissivity of less than 10 khr-1cm-2. The primary focus will be on materials used in semiconductor fabrication. The purpose of this document is to specify the recommended method for m

20、easuring alpha emissivity in materials utilized in the manufacturing of semiconductors. The method specifically applies to gas proportional instruments and designates recommended instrument settings. In addition, the method discusses operation of ionization counters. The document also recommends met

21、hods for determining sample size and for evaluating instrument background accurately. Treatment of data is also outlined, including identification and elimination of systematic errors. The calculation of results and detection limits is detailed with examples in the annexes. A standard format for rep

22、orting results is specified. 2 Terms and Definitions accuracy: A measure of how close the measured result is to the true value. bias voltage: Potential applied between the anode and cathode in a gas proportional counter. detector background: Signal measured by the detector in the absence of a sample

23、. discriminator: Signal rejection mechanism which eliminates low and high energy events. efficiency: The ratio between the number of alpha particles detected and the actual number of events occurring. This same value for the detector efficiency is used when measuring the detector background. emissiv

24、ity: The rate of emission of alpha radiation measured in counts per unit area per unit time. JEDEC Standard No. 221 Page 2 2 Terms and Definitions (contd) gas proportional counter: Instrument which detects radiation by measuring ionization of counting gas between two electrodes. The anode is usually

25、 comprised of a fine wire or grid of wires so that the local electric field near the wires is large enough to cause gas multiplication and thus a large signal. A proportional counter has an output pulse proportional to the number of primary ions produced in the counting chamber by the sample radiati

26、on. ionization counter: Instrument which detects radiation by measuring ionization of counting gas between two electrodes at lower voltages below what is necessary for gas multiplication. limit of detection: The minimum alpha flux emitted from a sample that is statistically significant above the bac

27、kground at a defined confidence level. poisson distribution: Statistical distribution that predicts discrete variable behavior, including alpha particle counting. precision: The degree of mutual agreement among data that have been obtained in the same way. quenching: The technique used to control io

28、nization in a gas proportional counter by addition of an electropositive gas which absorbs secondary electrons. systematic error: Spurious events detected by a counting system which do not originate in the sample and are not random. 3 Instrument Operation Parameters For a good reference on proportio

29、nal counter operation, see Knoll1. A discussion on a modern ionization counter, with pulse shape discrimination, and low background is given by Warburton2, and Gordon3. In the following sections, the discussion generally refers to proportional counter operation. 3.1 Bias Voltage A standard setting f

30、or the applied bias voltage is not practicable given the differences in individual detector design and construction. What is desired is that the electric field between anode and cathode in a gas proportional counter be large enough to ensure that multiplication occurs. Therefore, the bias voltage mi

31、ght vary instrument to instrument depending on the anode-cathode distance, the gas type, wire diameter, and gas pressure within the instrument. JEDEC Standard No. 221 Page 3 3.2 Bias Point Selection A recommended method for selecting the bias voltage for proportional counters is outlined by Schindlb

32、eck4. The absolute efficiency and background count rate of the instrument is measured as a function of increasing bias potential. These two values and their ratio (relative efficiency) are plotted in Figure 1. The optimum bias voltage occurs at the maximum of the ratio between the absolute efficienc

33、y and the background count rate. In this region the relative efficiency represents the best balance between increasing signal and decreasing noise. In this example the optimum voltage is 900 volts. 1101001000800 1000 1200 1400Voltage Level in VRelative DensityDashed line = counting efficiency dotted

34、 line = background counts solid line = relative efficiency (= counting efficiency / background counts) Figure 1 Example bias point selection graph.4Another method is to measure the pulse height from the amplifier, using an oscilloscope or a multichannel analyzer, as a function of the bias voltage. T

35、he voltage should be large enough to ensure that the pulse height varies exponentially with applied voltage. JEDEC Standard No. 221 Page 4 3.3 Discriminator The discriminator defines the range of alpha particle energies that are counted, and affects the measured count rate and the calculated emissio

36、n rate from the sample. At energies less than 1.2 MeV background noise from beta radiation becomes significant, so the recommended discriminator threshold is 1.2 MeV. For vendors providing raw materials this alpha energy range (higher than 1.2 MeV) is recommended as it makes no assumptions about the

37、 final use of the material in a product. If specific instrument limitations result in significant noise above 1.2 MeV, the lower threshold will be adjusted accordingly, and the revised energy range stated in the report. The lower level discriminator could be set by measuring the count rate and energ

38、y spectrum of a thick source (NIST traceable) using a standard solid state detector (i.e., silicon) in a 2 geometry configuration. Once the energy spectrum and count rate over the applicable energy range is determined, the same source is placed in the proportional or ionization counter and the lower

39、 discriminator is adjusted to give an equivalent count rate response. For many semiconductor products, alpha emission from the packaging materials (external to the silicon chip) is one component of soft error upsets. Alpha particles emitted from the packaging materials must traverse many metal and d

40、ielectric layers to reach the active silicon circuitry, and lose energy in proportion to the material density and distance traveled. Semiconductor device manufacturers may desire to account for the fact that lower energy alpha particles (above the discriminator energy) will be harmlessly absorbed in

41、 the chip layers before reaching the device silicon. In such cases the discriminator setting can be set higher so that only those alpha particles able to cause a soft error in the actual chip are counted. Figure 2 shows the alpha particle range as a function of energy through copper and SiO2and the

42、threshold at which a soft error is initiated. The energy to be used should be defined by a conservative estimate of the minimum energy that could traverse the chip layers without being absorbed. For a conservative estimate, normal incidence is used (since at higher angles the effective distance trav

43、eled is greater) and the alpha reaching the silicon with 0 MeV remaining is the threshold (when in actuality, an alpha particle needs some energy to create a soft error). To keep the estimate conservative and to account for the fact that minimum metal coverage due to interconnect is usually 20% we c

44、an calculate an effective thickness of metal as the product of the actual metal thickness in the chip and the coverage fraction. For the dielectric layers one can use the actual thickness of the layers. This is not very accurate but does ensure that the minimum energy defined is conservative with no

45、 possibility of underestimating the alpha particle flux reaching the device surface. More accurate methods based on ray tracing through actual layers with interconnect layout considerations can be done with simulations to get a better estimation of the minimum discriminator setting. Irrespective of

46、the actual technique used the minimum discriminator energy must be recorded in the final report of the alpha emission for the sample. JEDEC Standard No. 221 Page 5 3.3 Discriminator (contd) 010203040500123456789Alpha Particle Energy (MeV)Range (um)SiO2CuDevice Si7 umRange (um)Figure 2 The range of a

47、lpha particles in semiconductor materials with sample chip cross-section demonstrating the range of minimum discriminator energies for a 7um stack that is 100% SiO2or 100% Cu.53.4 Gas Management and Maintenance 3.4.1 Gas Type The gas type used shall be reported. Typical proportional counting gases i

48、nclude P-10 and P-5. There is a negligible difference in results between these two gases. If other counting gases (Ar-CO2, Isobutane) are used, differences in quenching must be compensated for when comparing settings from instruments utilizing P-10 or P-5. In ionization counters, counting gases can

49、include Ar or N2without the need for a quench gas. 3.4.2 Gas Purity Standard high purity counting gas is recommended over ultra high purity gas because the difference in performance of gas counters is negligible for these applications. It is recommended that counting gas quality be verified periodically to eliminate the possibility of using radon contaminated counting gas. A recommendation to quantify gas quality consistency is to measure background or a check sample after each cylinder change, and examine the before and after results for shifts in count rate. A gas cylinder