This paper reports on the use of microelectronic test structures to characterise a novel fabrication technique for thin-film electronic circuit boards. In this technology, circuit tracks are formed on paper-like substrates by depositing films of a metal-loaded ink via a standard lithographic printing process. Sheet resistance and line width are electrically evaluated and these quantities are compared with optical and surface profiling measurements

Evans, PSA

Fallon, M

Harrison, DJ

Ramsey, BJ

Stevenson, JTM

Walton, AJ

English

Brunel University Research Archive

Ptoc. IEEE I998 I n t .  Chitcrcnce 011 Microelectronic Test Sliuctiirch. Vol I I ,  March 1998. 39 A. J. Department University of Edinburgh, Test Structures to Characterise a Novel Circuit Falirication Technique that uses Offset Lithography Wzlton, J .  7: M. Stevenson, M. Fallon?, P. S. A. Evans$, B. J. Ramsey$, D. Harrison$ of Electrical Engineering $Department of Design ?National Semiconductor Edinburgh, Brunel University Lark$eld Industrial Estate Egham, Surrey, TW20 OJZ, U K  Greenock, PA1 6 OEQ, U K  EH9 3JL, UK. echnique that has the t lithographic process all aspects of the image. Contact with an ink and a moistening roller allows the printing plate to ;if tract buth water and ink as required, and to form the image to be printed. The image is not printed directly onto the substrate material (e.g. paper), but is instead tramferred to an intermediate or blanket cylinder that has a yielding surface. The blanket cylinder then presses the ink film onto the surface of the substrate, which is now supported on a separate impression cylinder. The printed substrates rely on evaporation andlor oxidation of the ink film to fix the image. Initial investigations of the technology had indicated that sheet resistance and linewidth were functions of the type of "paper substrate" used and test structures have been employed to help quantify both parameters and their variability. 2. Test Structures and Measurement A number of test structures have been designed and are illustrated in figure 1. m m m  method. Cr tical elements iitclude the ink characteristics, the printing plate, the fountain solution and t he printing machine. In short, the lllh . ,mm of the process relies on the action of two on the surface of a smooth plate. The plate the printed image an oil-based ink to adhere. A Figure 1. Test structures printed using CLF technology. the ink in light regions of the 98CH36157 40 Paper tY Pe Mean Gloss Values Poly Standard Gloss Deviation , Poly Greek Cross Greek Cross Box Cross Linewidth Linewidth (100 " (300 m) (100 m) (300 P I  Q/O Q/o Q/o P m 0.156 0.225 0.203 129.4 324.4 0.104 0.130 0.125 130.5 314.5 0.0146 0.0161 0.0185 12.2 22.1 I 0.0067 , 0.0037 I 0.0043 I 10.3 I 16.1 I Table 1. Summary of electrical measurements made on the test structures (20 samples). 1 e e. e ' b  e 8 e e *e e 0.085 0.09 0.095 0.1 0.105 0.11 0.115 0.12 0.125 Sheet Resistance (OhmslSquare) 340 - v) 320 C.- $ E 300 C 5 280 b l , i l l , , l l ) , l l l l l l l l I I , i ~ ~ I J 1 , ~  0.09 0.095 0.1 0.105 0.11 0.115 0.12 Sheet Resistace (OhmslSquare) 80 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 Sheet Resistance (Ohms/Square) 0 e 380 360 C .! 340 v 5 320 al -1 .- 300 e e e 0 e 280 1 I I I ~ l l l l ~ " i l ~ l l l ' ~ ' l l l ~ l l ' l ~ i l l ' ~ i l l l ~ l l l l ~ l l ' l ~  0.15 0.155 0.16 0.165 0.17 0.175 018 0.185 0.19 0.195 0.2 0.205 Sheet resistance ( Ohms/Square) (c) ( 4  Figure 2. Scatter plots of electrical linewidth and sheet resistance for different substrates and nominal track widths. (a) Poly art (lOOpm), (b) Poly art (300pm), (c) Gloss art ( lOOp) ,  (d) Gloss art ( 3 0 O w ) .  These included two Greek cross structures (arm widths of 100 and 3 0 0 p ) ,  a box cross, and two linewidth structures (nominal widths of 100 and 3 0 0 p ) .  Twenty samples were printed on both polythene and gloss art paper and these were then electrically measured. The cross structure resistivity measurements have been averaged as detailed in reference [I] and the linewidth structures were measured using the procedure outlined in [2].  Table 1 sum- marises some the measurements from which it can be observed that the printed sheet resis- tance depends upon the substrate used with the gloss paper resulting in higher resistivities than the poly paper. In addition the measured linewidths are considerably larger than the nominal dimensions given in brackets, indicat- ing ink spread. The standard deviations Paper Gloss Nominal track width (p) 100 300 100 300 S tdev/mean S tdev/mean S tdev/mean sheet resistance linewidth AV 0.103 0.097 0.057 0.073 0.070 0.054 0.073 0.0075 0.034 0.060 0.052 0.049 Table 3. voltage difference Comparison between the Standard Deviation/Mean for sheet resistance, linewidth and the between the taps for both paper types. of the a significant tance and The linewidt dent on the resistance. linewidth and the linewidth for analysis. between these showing the observed thai:, of correlatior Gloss 0.833 0.689 I Poly I 1 0.916 I 0.640 measurements also indicate that there is variation in both the sheet resis- linewidth measurements. n structure measurement is depen- accurate measurement of sheet For the measurements made on the structure both the sheet resistance measurements were available Table 2 gives the correlation two measurements with figure 2 associated scatter plots. It can be as expected [3], there is a degree between the measurements. Paper tY Pe The linewic assume that and homogl - 100 p nominal 300 p nominal linewidth linewidth th and resistivity measurements the film is of uniform thickness neous. The linewidth extraction p nominal 300 p nominal linewidth Gloss Poly Table 2. tance and cross bridge To determine tion was due resistance between the variability 0’ table 3 and i: variability of fact due sheet resistace. which shows between the the voltage taps on the 0.401 0.199 Correlation between the sheet resis- linewidth extracted from the same structure. how much of the linewidth varia- to the variability of the sheet measurement the voltage drop taps was extracted. The relative the three factors is presented in can be observed that some of the the linewidth measurements is in vaiation in the extracted value of This is confirmed by table 4 a much reduced correlation sheet resistance measurements and difference observed between the linewidth structure. 0.404 0.563 Table 4. Correlation between the sheet resis- tance and the voltage difference between the taps extracted from the same cross bridge structure. also assumes a rectangular cross-section of the track being measured. The film thickness and track cross-section were checked using a Dek- tak 8000 profilometer and the results for both paper types are shown in figure 3. From these results it can be deduced that the tracks appear to be printed with a surface roughness of a few microns resulting in a very non-uniform cross-section. Since these films had not been profiled previously the nature of the cross-section of the track and the degree of surface roughness was neither known nor fully suspected. The structures were also examined optically to check the electrical linewidth measurements and to determine the edge roughness. Figure 4 shows examples of optical photographs of a Greek cross with 100 pm wide arms in both reflective and transmissive modes for both types of samples. The pictures obtained using reflected light show the granular nature of the surface and, while they do illustrate the rough- ness of the edge of the tracks, this does not stand out to the same degree as the the pictures taken using transmitted light. These illustrate that the edge acuity is probably a function of the roller direction. 41 42 Figure 3. Example of a surface profile of 1OOpm track. (a) Poly art, (b) Gloss art. (c) ( 4  Figure 4. Optical photographs of 1 O O p  crosses. (a) Poly art (reflective), (b) Poly art (transmissive), (c) Gloss art (reflective), (d) Gloss art (transmissive). The linewidths of the tracks were also mea- the track edge was so rough two measurements sured optically using a Vickers CSS image were taken. One assumed that all material con- shearing microscope. This measurement is tributed to the full width of the track while the potentially subjective as the operator effec- other measured the minimum width or the tively determines the edge of tracks. Because "solid" portion of the track. Substrate Structure (P> (100 P> (300 m> (100 m> Poly Maximum Minimum Edge roughness ( P I  (Pm1 ( P I  222 178 22 (15) 3 69 417 24 (24) 203 229 13 (18) Gloss (300 m) 429 380 24.5 (33) Microelectro to help evalu3te tive lithograi that the edg: the film affect linewidth. I i  sheet resistance observed to /Table 5. Summary of electrical measurements made on the test structures iic test structures have been used the new technology of conduc- hic films. It has been determined roughness and cross-section of the electrical measurement of addition the variability of the measurements have been correlate with linewidth measure- s not so surprising given er indication of the true sit- 3. Conclu$ons Figure 5. Transmissive photo of a horizontal poly track (arrows indicate edge of the track). Light patches indicate thin regions. ments. This issue needs further investigation to see whether the variability is due to non- uniformity of the conducting layer caused by the printing process or is related to geometrical variations in the structure [4]. Additionally, a fuller investigation into the most robust cross resistor design for CLFs needs to be under- taken [ 5 ] .  This work has also highlighted the importance of measuring an accurate value of sheet resis- tance if linewidth is to be measured [3]. This is especially important if nanometer scale mea- surements are to be attempted [6]. Electrical measurements have many attractions over the optical ones in that they are performed along the whole length of the track rather than on a very small portion and are also very easily automated. Hence, if the electrical measure- ment of linewidth can be calibrated to the opti- cal ones then there will be more confidence in using it to both optimise and then control the full production process. However, it should be remembered that it is the electrical perfor- . 43 44 mance of the conducting tracks which defines the ultimate performance of the CLFs making the implementation of test ‘structures a neces- sary part of any process control strategy. References 1. M.G. Buehler and W.R. Thurber, “An experimental study of various cross sheet resistor test structures”, J. Electrochemi- cal Soc - Solid State Technology, vol. 125, no. 4, pp. 645-650, April 1978. 2. M.G. Buehler, S.D. Grant, and W.R. Thurber, “Bridge and Van der Pauw sheet resistors for characterizing the line width of conducting layers”, J. Electrochemical Soc - Solid State Technology, vol. 125, no. 4, pp. 650-654, April 1978. 3. M.W. Cresswell, Private communication, 1997. 4. H Tuinhout, Private communication, 1996. 5 .  M.I. Newsam, A.J. Walton, and M. Fal- lon, “Numerical Analysis of the Effect of Geometry on the Performance of the Greek Cross Structure”, ZEEE Znterna- tional Conference on Microelectronic Test Structures, pp. 247-252, Trento, Italy, March 1996. 6. M.W. Cresswell, N.M. P. Guillaume, R.A. Allen, W.F. Guthrie, and L.W. Lin- holm, “Extraction of Sheet-Resistance from van-der-Pauw Resistors Replicated in Monocrystalline Films Having Non- Planar Geometries”, Proceedings of IEEE International Conference on Micro- electronic Test Structures, Kanazawa, March 1998. 

An experimental study of various cross sheet resistor test structures”,

Bridge and Van der Pauw sheet resistors for characterizing the line width of conducting layers”,

Extraction of Sheet-Resistance from van-der-Pauw Resistors Replicated in Monocrystalline Films Having NonPlanar Geometries”,

Numerical Analysis of the Effect of Geometry on the Performance of the Greek Cross Structure”,

Test structures to characterise a novel circuit fabrication technique that uses offset lithography

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Test structures to characterise a novel circuit fabrication technique that uses offset lithography

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