Active Yield Management: New Trends in Advanced Optical Disc Production Rolf W. Hertling, Hongda Yue

Transcription

1 White paper Active Yield Management: New Trends in Advanced Optical Disc Production Rolf W. Hertling, Hongda Yue 1. Introduction Today, the market of optical disc is developing to new formats. The old standard formats CD-AUDIO/ROM, DVD-ROM and CD-R will still keep to have it s stable share of business, but there is rapidly increasing demand by end-users for the new recordable and rewritable DVD formats, as DVD+/-R, DVD+/-RW and DVD-RAM. This is a great chance for all disc manufacturers as well as line integrators. However, the processes are more sophisticated than for the standard formats of the last years, and the goal in our times is to keep the process parameters stable even for the high-density and high-speed formats like DVD+/-R 4X for example. In order to achieve this target of a stable yield which should be as close as possible to 100%, some new physical parameters are needed, and very tight, or even completely new specifications must be kept. In this paper, we will summarize our investigations about the topics, advanced optical disc production and disc testing, partially explained in detail in a series of previous papers ([1]-[5]). Here these results and some new features are presented with respect to the new aspect of Active Yield Management, which can be described as an active process regulation mechanism for the different process steps. Under this general new trend, there is no longer just a Good-Bad decision for the disc production rather than an automatic process regulation on the one hand, and the tools for real process development and optimization on the other hand. This paper is organized as follows: In section 2, we present the general concept of the Active Yield Management approach to some arbitrary process step, and then specify the various process steps in optical disc production. Section 3 will focus on the case of advanced recordable formats, and here we pick out the dye coating process as the most critical, as an example for successful closed-loop feedback control. For realization of this feedback control, the physics of dye and dye coating is essential, which will be briefly explained. Section 4 gives an overview over other closed-loop control of process steps, like sputtering of multiple layer stacks or single layers, or like DVD bonding. The conclusion in section 5 will summarize the closed-loop benefits for optical disc production. 2. Active Yield Management: The principle First, lets pick out any process step in general, without further specification which one it is. In Fig.1, some section of disc production in the manufacturer s line is shown, with numbers (1-9) as labels. The discs are processing from the left to the right of the picture (1) and are inspected at some station (2, Data aquisition). This inspection might be inline in the scanner of offline in some physical offline tester. Some values like birefringence are tested offline due to temperature reasons, other have to be tested inline (local defects, for instance) or have been shifted from earlier offline- to now Page 1 of 14

2 White paper inline-testing, as optical density or any layer and layer stack thicknesses. Some of the output of the offline tester is used as input (3) for the inline scanner. Then, during the production is running, the data are analysed (4) and visualized on some host PC (5). The analysis provides a good-bad decision for the disc (6) which is used by the line handling system for quality separation (7). This is the well-known conventional approach to disc production check. The new concept of Active Yield Management now is realized by an automatic feedback step (8), using the data analysis to adjust the process in production time. The feedback in production means an automatic process regulation, but for the process optimization before production, it means to enable the process engineers to understand the reasons of some disc failure, so that he is not dependent on secret knowledge from specialists, but can do his own process. Both is part of Active Yield Management. Fig. 1. : The concept of Active Yield Management This new concept can be applied to all of the various production steps. For example, in the case of DVD+/-R, we have basically four steps: Moulding, dye coating, sputtering and bonding. If we know the best process parameters for each step which are defined by the guarantee of good discs, and if we know that the process steps are absolutely stable, then we can be sure to achieve 100% yield (9). To come as close as possible to this yield, we must care for both targets: 1) Understanding the influence of physical parameters on the disc properties (electrical signals). 2) Adjusting the process parameters automatically in order to keep the good physical properties stable during production. Target 1) is a task for offline testing and process development, target 2) for the closed loop process control. Both require precise measurements as an absolutely must, like only real spectral dependent evaluation can provide them, and both 1) and 2) are needed for the highest possible yield of good discs. Now let us list the various process and testing steps for the example DVD+/-R (Fig.2) or DVD+/-RW and RAM (in this case, there is no dye coating): Page 2 of 14

3 White paper Physical values Modus for testing - Substrate thickness - Birefringence - Tilt/Deviation - Dye material data n/k - Dye optical density Offline Offline (temperature!) Offline + Inline Offline - Dye profile: Offline, Inline and Closed Loop, feedback procedure Thickness In groove/on land and D(abs) Offline, Inline and Closed Loop, feedback procedure - Sputter material data n/k - Sputter thickness / layer stack configuration (RW) - Bonding layer thickness Offline Offline, Inline and Closed loop, feedback procedure Offline, Inline and Closed loop, feedback procedure Fig. 2: Process steps and physical values for DVD+/-R and DVD+/-RW and RAM From these four (Recordable) or three (Re-Writable) process steps, we will take a closer look to the dye coating in the next section 3. The sputtering and bonding will be the topic of section Dye coating process control The reading and writing wavelength of the DVD+/-R is approximately 650nm, compared to 780nm for CD-R. Thus, the material properties, refractive index n, and absorption index k, which are always functions dependent on the wavelength, must be completely different from CD-R. The dye solutions for DVD+/-R are now established, after several years of research and development. There is, however, still freedom with the exact region of maximal dye absorption. This is important as input for the dye coating process control since it affects the measurement drastically. We start from this point and progress to the explanation of dye process window, over the new physical features of DVD recordable dye coating and end up with an example of complete dye profile control, thus enabling total stability of the electrical signals which are related to the dye physical properties. 3.1 The material properties of Dyes We show one example for CD-R and another for DVD-R dye in Fig.3. All dyes must have a high refractive index n (top lines of Fig. 3), exactly at the writing/reading laser wavelength so that we can use thin dye layers for the process. But then, the absorption index k is relatively low at the same wavelength, which is 780nm for CD-R and 650nm for DVD-R. So, for a good check of dye layer distribution on the disc, this wavlength is not suitable. There are ambiguities or even completely wrong results about the dye coating process if this wavelength is used for optical density testing. The only possibility is to use the wavelength of the highest absorption, which is close to the wavelength of the highest absorption index k. It is shifted to shorter wavelength, about Page 3 of 14

4 Preprint for China Mediatech, Issue6 729nm for CD-R Supergreen type dye, but it might vary from 570nm to 620nm for DVD-R dyes. So we see that we must have freedom for the wavelength in order to measure precisely. Fig. 3. : Dye material property n/k for CD-R dye (left) and DVD-R dye (right) Page 4 of 14

5 Preprint for China Mediatech, Issue6 3.2 Process window and Optical density The process window for a typcial dye with now fixed material properties n/k is defined by the area of dye thickness in groove and dye thickness on land, where we have both high transmittance of the dye coated discs (for good writeability) and high reflectance (for good readability), this time again at the wavelength of the laser, which is 780nm for CD-R and 650nm for DVD-R. Fig. 4. : Process window for DVD-R dye layer A simple closed loop feedback system is based on the fact, that the transmittance and thus the optical density OD=-log 10 (T) is mainly dependent on the dye thickness on land, if we chose the correct wavelength for highest absorption, like 729nm for CD-R Supergreen dye. Under the assumption that the dye thickness on land an important parameter for the dye profile, we can use the Inline data for OD from the ETA-Scanner as input into the closed loop algorithm. This is done by adjusting the coating parameters of the coating program (Fig. 6) which will mostly influence the OD. Fig. 5. : Transmittance (OD is -log 10 (Transmittance)) and dye thickness on land Page 5 of 14

6 Preprint for China Mediatech, Issue6 Fig. 6. : Dye coating program (RPM = revolutions per minute) 3.3 New physical parameters and specifications for DVD+/-R dye coating process Now we take an even closer look into the physics of the dye profile. There is the laser incoming light I IN which will partially hit the grooves and partially the land area on the disc, from the bottom side (Fig.7). As the first one example about the influence of physical parameters to electrical signals, we chose the Radial Contrast before recording, RC b. The Orange Book parameter [6] Radial Contrast before recording, RC b is very important because recorders use it to I l I Diff I distinguish between land and groove and g then can write the data in groove. The Dye h l RC b must be more than 0.05 by the Sub h g Orange Book s specification, the w definition is RC b l g 2* I I I l I g (1) Fig. 7. Electrical signals without regard to Reflector Fig.7 shows the laser intensity parameters I l and I g. For better illumination we have ignored the Reflector Layer. The I l depends on dye thickness on land, and I g depends on dye thickness in groove. By spectral measurement for all wavelengths at the same time, we can obtain both dye on land and dye in groove, and thus also the levelling D(abs), which completes the full dye profile information. Another example is the reflectance after sputtering. There is an optimum for the dye thickness in the groove, for which we achieve the highest possible reflectance. More dye will not cause any effect, and we loose expensive dye then, less dye will result in too low reflectance (Fig.8). I IN I IN Page 6 of 14

7 Preprint for China Mediatech, Issue6 Fig. 8. : Spectral refletance as a function of dye thicknes in groove The third reason for real spectral (multiple-wavelength) measurement of both reflectance R and transmittance T is the new dp/d specification, which is introduced for DVD+R ([7]: Philips/Eindhoven) and is expected to come true for DVD-R as well very soon. This value is defined as the first derivative with respect to wavelength of the writing power P into the dye, which is itself a function of wavelength, because it is proportional to the inverse absorption of the dye, P( ) ~ 1/A( ), and this absoprtion function is defined as 1 R T, so both R and T (Fig.9) must be known for all wavelengths. The benefit of the new specification is to keep the dye coating stable with respect to variations of drive laser power, which are caused by laser production and temperature effects. The dp/d must be kept in the limits between 0 and 18. As can be seen from Fig.9, both reflectance R and transmittance T must be used for the calculation of A and thus dp/d. And from Fig.10, it follows, that these values on the other hand are process-dependent parameters: The reflectance R is changing in a chaotic way between higher and lower values, for fixed wavelength. This is caused by an overlap of interference and absorption effects at the same time. Measuring both R and T for many wavelengths will offer the possibility to keep the A and so the dp/d stable at the best value during disc production. Page 7 of 14

8 Preprint for China Mediatech, Issue6 Fig. 9. : The dp/d specification (DVD+R-Spec. V1.1, Annex J.2, Philips/Eindhoven). Note: We just write R instead of R and T instead of T Fig. 10. : Change of reflectance and transmittance for different dye layer thicknesses, neglecting the groove influence for illustration 3.4 Example of Inline Dye profile control We have seen how important the real spectral measurement for R and T is. Now let s come back to practice and show an example of a real field test of dye profile closed loop control. The system ETA-ProCon is integrated as shown in Fig. 11: The blank half discs are coated in the dye coater and the are inspected by the ETA-Scanner with an integrated spectral measurement system ETA-CD-I, which can check the Optical Density, the Dye in groove and on land and the D(abs). The data are transferred to the ETA-ProCon module to obtain the necessary settings of the Page 8 of 14

9 Preprint for China Mediatech, Issue6 coating parameters for the PLC. The n/k date and the best wavelength for OD measurement are measured using the offline tester ETA-RT. The result of such a kind of process control can be seen in Fig.12. After switching on the ETA-ProCon, the cup-to-cup variation of the OD is considerably reduced (Fig.12, top picture). And temperature changes are compensated by ETA-ProCon so that the OD is very stable regardless how variable the environmental conditions in the production area are (Fig.12, peaks in bottom picture are the forced temperature changes, OD is going back to normal by the use of ETA-ProCon). Fig. 11. : Setup and environment of a closed loop process control ETA-ProCon for dye coating Fig. 12. : History chart (time chart) by ETA-ProCon in real production Page 9 of 14

10 Preprint for China Mediatech, Issue6 4. Other process steps for Active Yield Management For DVD+/-R, the production steps after dye coating are sputtering and bonding. We show examples of inline measurement and process control in the next two sections. However, for DVD+/-RW and DVD-RAM, as well as CD-RW, the critical procedure is the multi-layer-stack sputtering, which would replace the dye coating as the most essential process step. Measuring many thin films in the [nm]-thickness range at the same time requires the full and photometric exact spectral reflectance evaluation after sputtering. We start the three examples of other process steps with this multi-layer stack layer evaluation, invented and introduced by Steag Eta-Optik more than 5 years ago and successfully running both offline and inline in about 200 installations worldwide. 4.1 Sputter layer stacks and sputter layers The principle how to obtain many thin film thicknesses from only one measurement is explained in Fig.13. The blue line is the measurement of reflectance for a final DVD+RW disc, the red line is the simulation, using an exact physical model and precisely determined material data n/k for each of the layers in the stack: Buffer1, Barrier1, Phase Change, Barrier2 and Buffer 2. Fig. 13. : Single point evaluation of DVD+RW layer stack by spectral fitting For many radii on the disc, such a spectral fitting procedure provides the complete layer stack in fast speed, and so this algorithm can run inline, as standalone-unit or in the ETA-Scanner. Fig.14 shows the result of a layer stack history plot. At the same time as the layer stack, also the groove depth is evaluated. So, changes of groove formation during moulding will not affect the layer thickness measurements, but we can obtain the information about moulding problems in addition Page 10 of 14

11 Preprint for China Mediatech, Issue6 to the layer stack evaluation. The thickness values for each of the stack s layers now can be transferred to the sputter machine via profibus system and the feedback loop is closed by adjusting the power used for the sputtering. For feedback, it is important to measure all the layer thicknesses independently, which can only be done by precise, multiple wavelength measurement and caring about the groove influence during the running process. The spectral fitting method for such an inline monitoring and closed-loop system was patented by Steag Eta-Optik many years ago. Fig. 14. : Layer stack (Buffer1, Phase Change, Buffer2) and groove parameter history chart for DVD+RW The Re-Writable formats are the case of many sputter layers in a stack. But also for Recordable and Pre-Recorded formats, there is sputtering, for reflector and/or semireflector layer, respectively. All these kind of single sputter layers show a simpler optical spectrum than multi-layer-stacks, so the transmittance is enough for layer thickness measurement, as soon as the n/k values for the sputter materials are known. Steag Eta-Optik has developed an inline system ETA-STS for monitoring and closed-loop control of this kind of metallic or semireflective layers (Fig.15) which can be integrated in any production line. The n/k values necessary for the very fast inline evaluation are determined one time only for this particular sputter layer, using the offline system ETA-RT. After the layer thickness is measured for each radius, the values are provided to the sputter machine to enable the closed-loop adjustement of the sputtering process. Page 11 of 14

12 Preprint for China Mediatech, Issue6 Fig. 15. : Transmission sensor ETA-STS for reflective and semireflective single sputter layers (left) and radial scan of inline system ETA-STS for sputter layers (right) 4.2 Bonding layer For all DVD formats, bonding of the reading and the dummy side is another critical step, either due to optical reasons, as for DVD9, or mechanical and cosmetic reasons (DVD+/-RW, DVD+/-R). Especially the DVD9 case requires a very precise measurement of the bonding layer thickness, since a failure of the specifications will lead to bad discs immediately. The problem quite often is to keep the maximal variation on one revolution lower than 4 µm. So the measurement accuracy should be in the order of 0.1 µm to check this specification. If we use a single-wavelength system like a laser to determine the bonding layer thickness, we run into big trouble as soon as there is some radial or tangential tilt of the disc. This is really the case at the outer radius 57-58mm, so here one must be careful to avoid cross-effects of tilt and bonding thickness measurements. The only way to do so is the multiple-wavelength method again, because then the presence of a complete interference pattern (Fig.16, left) will make the thickness measurement very stable against tilt effects, as well as local defects or reflectivity changes, because the bonding thickness is calculated from the distance of the interference minima and maxima in Fig.16. And these stuctures do not depend on any of the effects mentioned! The result is an extremely precise and stable Page 12 of 14

13 Preprint for China Mediatech, Issue6 bonding thickness measurement, which is ideally suited for inline use (Fig.16, right) and feedback control. Fig. 16. : Interference pattern of DVD9-bonding layer measurement (left) and ETA-Scanner history chart of deviation (left) and bonding layer (right) 5. Conclusion There were two main topics of this paper, summarized under the general terminus Active Yield Management. These two topics are: 1) Presentation of some effects which the physical values have on the electrical signals and thus the disc quality. Spectral measurement can provide the knowledge of new physical parameters like the complete dye profile, and can offer the possibility of process development and adjustement for everybody. There is no longer need to rely on some experts for any process. 2) Automatic process stabilization to target values by a closed-loop process control. Also for this procedure, we need the spectral measurement, to keep the disc quality absolutely stable. We have seen many examples of inline-monitoring and closed-loop feedback, which are proven to save many trouble in production and thus lead to constant disc quality and highest yield. Return on invest of such intelligent process-control solutions is within a few months, due to less labor costs and operating expenses. Thus, the Active Yield Management can be said to be the new trend in optical disc production. The necessary condition for such tools is precise measurement, which means spectral measurements in many cases.. Page 13 of 14

14 Preprint for China Mediatech, Issue6 References [1] Dye OD closed loop control for CD-R production,china Mediatech, 2003, Issue 1 [2] Study on Dye Profile in Groove and on Land, China Mediatech, 2003, Issue 2 [3] Closed Loop Control for DVD-R Production, China Mediatech, 2003, Issue 3 [4] Eccentricity control in Optical disc production, China Mediatech, 2003, Issue 5 [5] Inline control for bonding layer in DVD9, China Mediatech, 2003, Issue 5 [6] CD-R System Description (Orange Book), Dec-1998, Philips Electronics N.V. [7] DVD+R Specification v1.1 (DVD+R Book), 2003, Annex J: Philips Electronics N.V. Page 14 of 14

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