Static information storage and retrieval – Read/write circuit
Reexamination Certificate
2001-04-19
2002-05-21
Nelms, David (Department: 2818)
Static information storage and retrieval
Read/write circuit
C428S064400, C369S116000
Reexamination Certificate
active
06392934
ABSTRACT:
This application is a national phase of PCT/FR99/02804 which was filed on Nov. 16, 1999, and was not published in English.
FIELD OF THE INVENTION
The present invention relates to a method for writing and reading on an information medium comprising a recording layer with a succession of zones of material with at least a first and second physical state respectively.
“Physical State” implies a particular structure or composition of the material linked to a physical property that is characteristic of the state.
As an example, a physical state may imply particular doping of the material, a given ferroelectric structure or a crystalline structure of the material. The state of the material results in an electrical or optical property, such as a characteristic resistivity.
In particular, the recording layer may comprise a succession of regions in a crystalline and amorphous state respectively.
In general the invention has applications in recording information. For example, the information may be in the form of digital data, images or sounds.
In particular, the invention may be used in the fields of television and creating computer memories.
BACKGROUND ART
FIG. 1
is a schematic drawing of a system for writing and reading on an information medium according to a known technique based on the distinction between two physical states of a material.
The information medium bears the general reference number
1
.
It comprises a substrate medium
10
covered with a layer of an electrical conductor material that constitutes an electrode
12
.
A recording layer
14
covers electrode
12
. For example, this layer is of a material such as Ge
2
Sb
2
Te
5
that is in either a crystalline or an amorphous state.
Information is recorded in the recording layer using coding in a succession of zones in a first physical state (crystalline) and zones in a second physical state (amorphous).
In the figure the amorphous zones bear the reference
14
a
and the crystalline zones reference
14
c.
The information is recorded by the zones becoming more or less heated such that they change from an amorphous to a crystalline state or vice versa.
The heating is achieved, by means of an electric current that is more or less intense in the various zones of recording layer
14
. The current flows between a conductor point
20
applied against a write/read surface of the recording layer and electrode
12
. The current is supplied by a generator (not shown in the figure). Point
20
and information medium
1
are displaced relative to one another so as to scan recording layer
14
.
This recording technique is called “phase change recording”.
The recorded information on medium
1
is also read using conductor point
20
. It uses the electrical properties of the recording layer material that has a different resistivity in the amorphous and the crystalline states.
A source of voltage
22
is connected between conductor point
20
and electrode
12
such that a measuring circuit is created with a zone of the recording layer with which point
20
is in contact.
A current carried by the measuring circuit is detected by ammeter measuring means
24
and conductor point
20
is displaced on the surface of the recording layer in order to scan it.
Measuring current I may have two values depending on whether the zone of recording layer
14
in contact with point
20
is amorphous
14
a
or crystalline
14
c
. The resistance of recording layer
14
has two values respectively Ra and Rc depending on the amorphous or crystalline state of the material.
In practice, resistance Ra is stronger than resistance Rc, resulting in the following:
Ia=V/Ra if the recording layer is amorphous,
Ic=V/Rc if the recording layer is crystalline.
In these expressions V is the value of the voltage supplied by source of voltage
22
.
The higher the Ra/Rc relation, the easier it is to distinguish between these two states.
The above measuring method poses problems linked to the mechanical and electrical contact between the conductor point and the write/read surface of recording layer
14
. After a certain number of reading operations the displacement of point
20
against recording layer
14
causes mechanical wear of the point and the recording layer. The wear results in reading errors or inaccuracy.
FIG. 2
shows a second known technique for reading an information medium
1
, such as that described, that prevents the problems of mechanical wear mentioned above.
The reading technique uses the tunnel effect.
The equipment used to implement the second reading technique is more or less the same as that described with reference to FIG.
1
.
It comprises a data medium
1
consisting of a substrate
10
, an embedded electrode
12
and a recording layer
14
. The reading apparatus comprises a conductor point
20
, a source of voltage
22
and measuring means
24
. However, unlike the apparatus in
FIG. 1
conductor point
20
is held away from the surface of recording layer
14
by a distance d. Despite this distance a current is still capable of penetrating the recording layer by means of the tunnel effect.
A current-voltage diagram is used to determine the value of the intensity of the current. This type of diagram is shown in FIG.
3
. In this diagram voltage V is shown on the abscissas and current I on the ordinates. The current-voltage characteristic is a curve with an elbow, as in tunnel diodes. Three characteristics C
1
, C
2
, C
3
are plotted in FIG.
3
. They show three different distances, d
1
, d
2
, d
3
respectively, between point
20
and the write/read surface of the recording layer. For the same voltage applied between conductor point
20
and electrode
12
, the smaller the distance between the point and the read surface the greater the current carried by the measuring circuit.
In the example shown, distances d
1
, d
2
, d
3
with characteristics C
1
, C
2
, C
3
are such that d
1
<d
2
<d
3
.
In the diagram load lines are defined that relate current I to voltage V and that slope depending on the more or less resitive character of the circuit. When the tested zone is amorphous its resistance Ra is great and the drop in voltage is significant. In a crystalline zone resistance Rc is lower and the drop in voltage smaller. The load lines are therefore as shown in
FIG. 3
with references DCa and DCc for an amorphous zone
14
a
and a crystalline zone
14
c
respectively.
If characteristic C
2
is considered relative to a distance d
2
the point of operation of the measuring circuit is located at either Ma (if the zone is amorphous) or Mc (if the zone is crystalline). The current-voltage couple is then either Ia-Va (amorphous zone) or Ic-Vc (crystalline zone).
The variables Va, Vc, Ia and Ic are the voltages and currents respectively in amorphous zones
14
a
and crystalline zones
14
c.
Measuring means
24
are used to measure currents Ia or Ic in this second read mode to distinguish the physical state (amorphous or crystalline) of the scanned zones in order to read the information encoded on the recording medium.
The second read mode does not cause wear on the write/read surface of the recording layer or on the micro-point. It does, however, have the drawback of being extremely sensitive to the measurements of distance d separating the micro-point from the recording layer.
As shown in
FIG. 3
, for a given voltage the current carried in a given zone of the recording medium (amorphous or crystalline) is greatly dependent on
10
distance d (characteristics C
1
, C
2
, C
3
).
Therefore, a crystalline zone could very easily be mistaken for an amorphous zone if distance d between the point and the write/read surface were to be increased.
An error of this kind is likely to occur frequently if distance d is small and the recording medium is not perfectly flat.
The second read mode is also therefore subject to measuring errors.
DISCLOSURE OF THE INVENTION
The aim of the present invention is to provide a recording medium and a method for reading the recording medium that does not have the difficulties and limits of the read techniques desc
Bechevet Bernard
Saluel Didier
Burns Doane , Swecker, Mathis LLP
Commissariat a l'Energie Atomique
Lam David
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