INVESTIGATION OF INNOVATIVE PROCESSES FOR MANUFACTURE OF PRECISE METAL SCALE GRATINGS

Transcription

1 6th International DAAAM Baltic Conference INDUSTRIAL ENGINEERING 2-26 April 2008, Tallinn, Estonia INVESTIGATION OF INNOVATIVE PROCESSES FOR MANUFACTURE OF PRECISE METAL SCALE GRATINGS A. Barakauskas, A. Kasparaitis, G. Zakas Abstract: Selection of a particular method for metal scales manufacture is validated. The scheme of the selected method realization in terms of working principles, process parameters and required equipment is described. The main research activities during this project included investigation of the processes for transfer of scale gratings from the master scale made of glass to a metal scale, and determination of the errors occurring during the exposure. Methods of minimizing inaccuracy have been suggested. The experimental samples of linear metal raster scales and the results of their testing are presented, which assure a successful completion of the project. The research project has been realized by JSC PRECIZIKA METROLOGY with the support from EU structural funds. Key words: Precise Metal Scales Gratings, Producing processes, Investigation. 1. INTRODUCTION Measurements of linear and angular displacements are performed, as a rule, by means of photoelectric measuring systems. Displacement is counted off by means of glass or metal gratings scales. In certain application cases metal scales are superior as compared with glass scales. Such superiority features include the convenience of mounting because they do not occupy additional space, and their coefficient of linear expansion is close to the coefficients of linear expansion of other metal equipment into which they are installed. The main requirements of the raster structure of metal scales are as follows: High accuracy of the raster pattern elements position along the whole length of the scale, because it has effect on the magnitude of the systematic error of the measuring system; Accuracy of the dimensions and form, especially edges, of the raster elements, which influences the form of the primary electric signal of the measuring system and the magnitude of high frequency systematic and random errors; Considerable difference of the reflection from a raster element and the remaining scale, which has effect on the magnitude of the constant and variable components of the primary electric signal, and thus, on the complexity of their processing scheme and the magnitude of high frequency systematic and random errors. Several well-known methods are widely used for production of linear metal scales. They can be arranged into the following groups in accordance with the raster image formation method: Photolithographic methods, in which the photolithographic technology is used; Laser methods, when the raster is formed by a laser beam; Mechanical methods, when the result is achieved by mechanical action upon the work piece of the scale, e.g. by imprinting, cutting and the like. JSC Precizika Metrology has created innovative glass scale gratings

2 manufacturing technologies which, due to particularity of the metal scale production, cannot be used for production of these scales, although the overall experience will be evaluated and used by all available means. Therefore, the main task of this research consists in offering a method of production of precise metal scales the error of which does not exceed 3µm/m, and in determining and examining this method s peculiarities, conditions and parameters that ensure the necessary quality of the scale. 2. THE METHOD UNDER EXAMINATION AND THE TECHNOLOGICAL PROCESS The lithographic raster formation method is selected as a basic method of the research, using a glass master scale of the same length as that of the scale to be produced. The main stages of the technological process under examination and optimization are as follows: Production of a work piece of the necessary shape and surface quality; Preparation of the surface of the scale work piece for coating and the procedure of coating with the materials necessary for realization of the photolithographic process; Raster pattern exposure; Thermal and chemical processes of raster pattern formation; Evaluation of raster quality. To realize the selected method the following specific technological processes of producing precise metal scales have to be examined: Selection of materials for the formation of scale work piece and raster, and optimal parameters of raster formation processes; Research into the exposure process and optimization of parameters of this process; Calibration of scale errors and evaluation of dimensions and shape of the raster elements. 3. EXAMINATION OF MATERIALS AND PARAMETERS OF THE RASTER FORMATION PROCESSES Materials of optimal properties for scale work pieces and solvents for raster formation (etching) were examined and searched for. Scale work piece material. The main evaluation criteria for selection of scale material were established as follows: Resistance to corrosion under manufacturing conditions; Surface reflection coefficient; Coefficient of thermal linear expansion; Dimensional (strip length) stability in time; Strip form divergence, surface roughness, defectiveness; Strip dimensional uniformity lengthwise and edgewise. The element composition of materials was determined using X-ray spectrum microanalysis by EVO 50 XVP scanning electron microscope microanalyzer After examining 8 materials selected in accordance with technical literature and information provided by potential suppliers the AISI 30 stainless steel 0.3 mm thick strip was accepted as one having the best properties. It is very important that this material has a sufficiently good coefficient of reflection and resistance to corrosion without additional coating thereof by precious metals. In accordance with literature information about etching of stainless steels [1-3] 6 solvents FeCl 3, CuCl 2, KMnO ; K 2Cr 2O7 and oxalic acid-base solvents were selected at the initial stage of the research. The following etching process evaluation criteria were accepted: Etching rate, Micro profile of the etched surface, Geometric parameters of an etched line, Influence of ambient factors. Pursuant to the analysis results further detailed tests were performed using

3 solvents containing Fe (III) ions. The main purpose of steel etching tests with Fe(III) compounds consisted in determining the ranges of solvent component concentrations that would optimally coordinate the rate of etching and maximum roughening of surface, so that to achieve an optical contrast as deeper as possible, which is necessary for obtaining a maximum electric converter signal. The composition of solvents was corrected in a manner enabling to achieve steel etching rate of no more than 0.2 µm/min and ensuring maximum equivalence of surface roughness. Over 20 solvents have been tested. In order to optimize the effect of Fe (III) solvents the main reagent s Fe (III) concentration and solvent acidity were changed, and additional components were put in. The surface micro profile was examined in a contact mode with a scanning atom probe microscope Topometrix Explorer SMP. The frequency of vibrations of Z-coordinate piezoscanner is 5 MHz. Chemical composition of etched surfaces was analyzed with Escalb MK II device by method of X-ray photoelectron spectroscopy (RES). Changes in the surface micro roughness after etching with solvent 00g/lFeCl ml/l HCl + 50ml/l C 2H 5OH for 10 min. at +20 C temperature are illustrated in Fig a. Image of none etched surface µm b. After 10 min in immersion leach µm 10 Fig. 1 Surface micro roughness epures (3-D images) before and after etching. RESEARCH INTO THE EXPOSURE PROCESS The complex set of these investigations comprised: Direct investigation into the exposure process, whereby investigators intended to create the methods and equipment enabling to transfer the pattern of the master scale onto the photoresist layer of the lightsensitive material applied to the surface of the scale workpiece. This process is to ensure the necessary scale error, no more than 3 µm/m; Analyses and selection of light-sensitive materials that guarantee good quality of lines using photolithographic method for raster formation. Research into the exposure process was conducted using a 5 mm thick float glass master scale with a raster pattern formed in the chromium layer on its surface comprising transparent and nontransparent elements (lines). The lines layout error did not exceed 2 µm within the total length (1000 mm). A workpiece (metal strip) was placed on the copying machine table with the photoresist layer upwards; a master scale was placed on top of the workpiece with the pattern downwards. The workpiece-to-master scale position was fixed with special holders. The

4 exposure equipment is shown in a diagram form in Fig P 5 P a Fig. 2 Special holder with rolling bearings. 1- Machine table; 2- workpiece; 3- master scale; - bearing; 5- lamp; 6- glass strip; 7- plastic cables. The exposure is performed by moving a lamp along the master scale thus gradually and equally illuminating the whole length of the master scale. Light passing through the transparent windows reaches the photoresist layer on the surface of the workpiece, later on, during the development process the exposed photoresist areas dissolve whereas the unexposed areas remain covered with a protective film. The research has shown that the plane divergence of the glass master scale and the metal workpiece created conditions for light interference between them and the interference pattern is clearly visible. This phenomenon negatively affects the exposure process and causes additional scale errors.to eliminate the negative influence of this phenomenon the method of pressing the master scale to the workpiece was changed and analyzed. To this end a special carriage with roller bearings was made; and it was moving along the scale in the course of exposure pressing together only a certain place of contact between the master scale and the workpiece (about 60 mm in length) which at that moment was illuminated. It is shown in Fig. 2a. It was clearly seen in the location of compression P b 6 7 that the interference image was being changed and was moving together with the lamp, which enabled to eliminate the above-mentioned effect. However, another effect was observed. As a result of friction, despite the fact of being minute, between the bearings and the master scale the latter slightly slipped against the workpiece at 9 µm/m. Due to this effect the resultant pattern of the scale was longer by the same value, i.e. the scale error was greater than the master scale error by nearly 9 µm. The results of calibration of a 1 m long master scale (diagram 1) and the scale produced by the above-mentioned method (diagram 2) are graphically represented in Fig. 3. The scale error was calibrated with a laser comparator [6] in a thermostable laboratory wherein temperature changes within the range of 20 ± 0.1 C. 12 δ, µm l, mm Fig. 3 Error calibration diagrams of the master scale (1) and other scales (2, 3, ) In order to avoid the effect of this phenomenon the master scale s end, in the direction of which the scale tended to slip, was securely fixed, then the experiment was repeated. The resultant displacement amounted to 2.6 µm. This result was better (diagram 3 in Fig. 3), however it still failed to meet the scale precision requirements.

5 Seeking to achieve a higher precision of exposure an additional element was introduced into the exposure chain, i.e. one more glass strip which was placed on the master scale. Two plastic 0.25 mm cables were stretched between the additional strip and the master scale. Presser bearings rolled on this additional glass strip. The front projection of this version of the testing unit is shown in Fig. 2b. This decision showed to be right. Practically, only slight additional errors, as compared with the master scale, were brought in at the time of exposure (diagram in Fig. 3). This scale and the master scale s error calibration curves differ by the value equal to the difference of temperature deformations, which originated as a result of different coefficients of linear expansion of glass (master scale) and metal (a scale workpiece). Temperature of the master scale and the other scale at the time of experiment was equal to C..1 Analysis and selection of lightsensitive materials This research was aimed at choosing a photoresist of optimal properties and finding optimal methods of surface preparation for coating with photoresist, a photoresist coat application method, and optimal parameters and conditions of the process. After evaluating the existent experience of glass scale production and reference source information, 9 methods of metal strip preparation for coating with photoresist were subjected to analysis. The best results were obtained in the cases where the following methods of preparation of strips for the coating operation were applied: Polishing with Polirit suspension, then washing with water and cleaning with acetone using a napkin; Cleaning with Vienna lime, washing with deionized water and drying by compressed air. While selecting a photoresist of optimal properties and optimizing the method of applying a photoresist coating, 5 brands of photoresist and 5 methods of applying a photoresist coating on steel strips were analyzed. These analyses enabled to find out that the best properties were shown by a coating of FP 9120 photoresist (produced by AB Niopik, Russia), obtained by applying it with special rubber rollers. 5. RESULTS OF THE RESEARCH A precise metal scale specimen was produced in accordance with the methods presented in this article, by using the described testing equipment, selected materials of optimal properties and determined optimal technological parameters. This scale s error calibration diagram is presented in Fig. 3, diagram. The depth of the etched raster grooves is equal to about 800 nm, whereas the surface roughness in the etched areas is considerably more definite than in the nonetched areas. It is especially obvious in the case of scanning along the grooves (Fig. ). Here the root-mean-square deviation of micro roughness (Rpm) is.5 times larger than on the non-etched surface of the strip. 73 nm nm a Fig.. Plane image of the raster structure (a), profilograms of non-etched (b) and etched (c) surface micro roughness and results of their evaluation. 3-D raster images are presented in Fig. 5 to enable evaluation of the possible influence of thus designed raster structure on the b c

6 formation of the primary electric signal of the measuring system. 10 a b 5 2 Fig. 5 3-D images of the raster structure The images contain various scaling factors that are used on the working plane and perpendicularly to it 6. CONCLUSION µm366.8 µm µm An innovative method for production of precise metal scales has been created and specific processes of this method have been studied. Materials of optimal properties necessary for the realization of the method have been selected and technological process parameters have been optimized. A precision metal scale specimen has been produced in accordance with the results of the research, and the specimen s raster error does not exceed 3. µm. The metal scale raster created by way of the checked up method has a sufficiently strong contrast of reflection from the lines and the remaining surface of the scale, which contrast enables via the converter to form the primary electric signal of sufficient parameters. A technological process of industrial production of precise metal scales can be created, and specifications of the equipment to be used for the realization of serial production can be drawn up according to the results of this research. 7. REFERENCES 1. Mineta, T. Basic characteristic of an electrochemical etching of Ni-Fe containing corrosion resistant alloys. Sensors and Actuators. A 11 (200) p Gorobets, S., Donchenko, M., Gorobets, O., Goiko, I. Effect of a magnetic field on the etching of steel in nitric acid solutions. Russian Journal of Physical Chemistry. 80 (2006) p Varlamova, O., Kostache F., Reif, J., Bestehorn, M. Self-organized Pattern Formation upon Femtosecond laser Ablation by Circularly Polarized Light. Applied Surface Science, 252, 2006, p Kaušinis, S., Jakštas, A., Barauskas, R., Kasparaitis, A. Investigation of Dynamic Properties of Line Scales Calibration Systems. Proceedings of IMECO World Congress Rio de Janeiro. September 17 22, CORRESPONDING AUTHORS Albinas Kasparaitis, Professor Faculty of Mechanics, Vilnius Gediminas Technical University, J. Basanavičiaus st. 28, LT-0322, Vilnius, Lithuania a.kasparaitis@precizika.lt A. Barakauskas, G. Zakas JSC Precizika Metrology Žirmūnų 139 LT-09120, Vilnius, Lithuania a.barakauskas@precizika.lt g.zakas@precizika.lt