Semiconductor device manufacturing: process – Introduction of conductivity modifying dopant into... – Ion implantation of dopant into semiconductor region
Reexamination Certificate
2000-01-31
2001-10-16
Bowers, Charles (Department: 2823)
Semiconductor device manufacturing: process
Introduction of conductivity modifying dopant into...
Ion implantation of dopant into semiconductor region
C438S257000, C438S262000, C257S296000, C257S390000, C257S369000
Reexamination Certificate
active
06303474
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a read-only memory and to a method for fabricating the memory.
Read-only memories are widely referred to as ROMs. They are nonvolatile memories which cannot be erased. The so-called mask ROM is widespread in practice. The read-only memory is fabricated as an integrated circuit whose individual layers are fabricated by means of lithographic projection methods. In order to carry out a lithographic projection method, first of all a resist layer is applied to a planar layer of the integrated circuit. Afterwards, through a corresponding mask, the entire resist layer is exposed simultaneously at the locations left free by the mask. The mask is then removed and the resist layer is developed and its exposed parts are removed. The remaining parts of the resist layer are subsequently used for further processing of the integrated circuit, for example for an etching process or for a dopant implantation process.
The lithographic projection method outlined above is the most frequently used lithographic method for the fabrication of integrated circuits. Owing to the whole-area exposure (projection) of the resist layer, that method requires the production of the masks outlined above. Once the masks are available, however, they can be used to fabricate a large number of identical integrated circuits in a short period of time.
In addition to the lithographic projection methods outlined above, lithographic beam writing methods are also known. There, the exposure of a resist layer is effected by means of an electron beam or an ion beam. Such a beam which can be controlled by electric fields enables very fine structures to be fabricated and makes it unnecessary to produce masks of the kind necessary for the lithographic projection methods. The disadvantage of the beam writing methods is that the desired exposure of the resist layer is significantly more timeconsuming than in the case of the exposure carried out over the full area in the projection methods. Therefore, the lithographic beam writing methods are used only for fabricating very small structures within integrated circuits that are fabricated in small numbers.
Read-only memories are frequently used in smart cards, where they serve as memories for a processor likewise disposed in the smart card. For example, the microcode required by the processor is then stored in the ROM. In many applications (for example in a credit card or charge card context), it is additionally necessary, for security reasons, to store an individual identifier (for example a PIN number) in each smart card. Since the customary mask programming of the ROM does not permit storage of individual data for each smart card in the ROM, the individual identifier is stored in an EEPROM (Electrically Erasable Programmable ROM) that is additionally provided in the smart card for this purpose.
Due to the fact, however, that it can be erased, an EEPROM has the disadvantage that manipulation of the security-relevant data stored in it cannot be precluded.
SUMMARY OF THE INVENTION
The object of the invention is to provide a read-only memory and a related production method, which overcome the abovenoted deficiencies and disadvantages of the prior art devices and methods of this kind, and which render it possible to store both a first data record, which contains a large number of standard data (for example the microcode of a processor), and a second data record, which contains a small number of individual data (for example a PIN number).
With the above and other objects in view there is provided, in accordance with the invention, a method for fabricating a read-only memory in an integrated circuit, which comprises:
fabricating first structures of an integrated circuit by means of a lithographic projection method with the use of a mask and storing a first data record in the form of the first structures; and
fabricating second structures of the integrated circuit by means of a lithographic beam writing method without the use of a mask and storing a second data record in the form of the second structures.
In other words, the objects of the invention are satisfied in that the first data record to be stored by the formation of corresponding first structures and the second data record to be stored by the formation of corresponding second structures within an integrated circuit of a common read-only memory or ROM. In this case, the first structures are fabricated by means of a lithographic projection method with the use of a mask and the second structures are fabricated by means of a lithographic beam writing method without the use of a mask.
The use of the lithographic projection method (customary for the fabrication of ROMs) enables the first structures holding the first data record—to be fabricated rapidly and effectively. The first data record therefore involves data which are to be stored in the same way in a multiplicity of ROMs to be fabricated. The use of the lithographic beam writing method in the manner according to the invention makes it possible, on the other hand, for the data of the second data record to be individually different for each read-only memory to be fabricated. If the read-only memory is used in a smart card, by way of example, the second data record may be a PIN number, a serial number or a key which is necessary for the encryption (cryptology) of data to be stored in the smart card. In these cases, the second data record contains only a small number of data, with the result that the second structures take only a short time to fabricate despite the use of a lithographic beam writing method.
In accordance with an added feature of the invention, the first and second structures are fabricated with a common resist layer. The common resist layer is exposed through the mask for producing the first structures (projection method) and without the mask for producing the second structures (beam writing method). This procedure means that altogether only one resist layer is required for fabricating the first and second structures. This has the advantage that the first and second structures are produced in the same plane of the integrated circuit in directly succeeding fabrication steps.
In accordance with an additional feature of the invention, the resist layer is patterned after exposure, and the first and second structures are formed by implanting dopants via the patterned resist layer. In other words, the resist layer which has been successively exposed by means of the projection method and the beam writing method is patterned, and the first and second structures are formed by the implantation of dopants with the use of the patterned resist layer. In this way, it is possible, in an advantageous manner, for example to set the doping of channels of field-effect transistors for the purpose of storing the two data records, as will be explained further below using the exemplary embodiment.
In accordance with again another feature of the invention, lithographic beam writing methods to be taken into consideration are, in particular, electron beam writing and ion beam writing method. However, the resist layer can also be exposed using a laser beam writing method.
With the above and other objects in view there is also provided, in accordance with the invention, an integrated circuit read-only memory, which comprises:
first structures in the integrated circuit fabricated by means of a lithographic projection method with the use of a mask, the first structures storing a first data record; and
second structures in the integrated circuit fabricated by means of a lithographic beam writing method without the use of a mask, the second structures storing a second data record of the read-only memory.
In accordance with again a further feature of the invention, the second data record contains an individual identifier and/or an individual coding key.
The above-outlined integrated read-only memory is particularly suitable for incorporation in a smart card or the like.
Other features which are considered as characteristic
Bowers Charles
Greenberg Laurence A.
Infineon - Technologies AG
Lee Hsien-Ming
Lerner Herbert L.
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