active yield management: new trends in advanced optical ... · yield management”, which can be...

White paper

www.eta-optik.com

of 14

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

White paper

www.eta-optik.com

of 14

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):

White paper

www.eta-optik.com

of 14

Physical values

- Substrate thickness

- Birefringence

- Tilt/Deviation

- Dye material data n/k

- Dye optical density

- Dye profile:

Thickness In groove/On

land and D(abs)

- Sputter material data

n/k

- Sputter thickness / layer

stack configuration (RW)

- Bonding layer thickness

Modus for testing

Offline

Offline (temperature!)

Offline + Inline

Offline

Offline, Inline and Closed

Loop, feedback procedure


Loop, feedback procedure

Offline


loop, feedback procedure


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 4.

3. 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

Preprint for China Mediatech, Issue6

www.eta-optik.com

of 14

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)


www.eta-optik.com

of 14

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=-log10(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 -log10(Transmittance)) and dye thickness on land


www.eta-optik.com

of 14

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 IIN 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, RCb.

The Orange Book parameter [6]

Radial Contrast before recording, RCb is

very important because recorders use it to

distinguish between land and groove and

then can write the data in groove. The

RCb must be more than 0.05 by the

Orange Book’s specification, the

definition is

l g

b

l g

2*I I

RCI I

(1)

Fig.7 shows the laser intensity parameters

Il and Ig. For better illumination we have ignored the Reflector Layer.

The Il depends on dye thickness on land, and Ig 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).

w

lh

gh

Dye

Sub

IINIIN

IlIg

IDiff

Fig. 7. Electrical signals without regard to Reflector


www.eta-optik.com

of 14

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.


www.eta-optik.com

of 14

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


www.eta-optik.com

of 14

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


www.eta-optik.com

of 14

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


www.eta-optik.com

of 14

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.


www.eta-optik.com

of 14

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


www.eta-optik.com

of 14

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.

.


www.eta-optik.com

of 14

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.

active yield management: new trends in advanced optical ... · yield management”, which can be...

Documents