Transplanted magnetic random access memory (mram) devices on thermally-sensitive substrates using laser transfer and method for making the same

Description of equivalent WO 2004114312 A2

0001TRANSPLANTED MAGNETIC RANDOM ACCESS MEMORY (MRAM) DEVICES ON THERMALLY- SENSITIVE SUBSTRATES USING LASER TRANSFER AND METHOD FOR MAKING THE SAME

BACKGROUND OF THE INVENTION

0003Field of the Invention

0004The present invention generally relates to magnetic memory devices, and more particularly to MRAM devices, in which a magnetic tunnel junction (MTJ) device is transplanted onto a thermally-sensitive substrate.

0005Description of the Related Art

0006MRAMs are presently fabricated on Si for integration of magnetic tunnel junctions (MTJs) with complementary metal oxide semiconductor (CMOS) technology. MTJ devices can potentially provide nonvolatile, dense, high-performance storage elements. Lower performance applications of MTJs, such as in "smart cards", "wearable sensors", and the like, would require

0007fabrication of such MTJs on flexible, thermally-sensitive substrates, such as polymers and plastics. However, the thermal budget required (e.g., between about 250 to about 400° C) during processing of the MTJ devices precludes direct fabrication on these substrate materials. That is, the processing of such

0008devices are not compatible with the low-temperature substrates.

0009Additionally, present day smart cards made of plastic have one or more semiconductor chips, including memory, attached to a module embedded in the card. Having the circuit directly imprinted or bonded to the card would

0010make them more mechanically flexible, durable and lightweight. Cost is also an important factor for low performance application. Replacement of silicon chips with cheaper methods of fabricating memory circuits on plastics would be preferred. However, prior to the invention, no such method has been provided.

SUMMARY OF THE INVENTION

0012In view of the foregoing and other problems, drawbacks, and

0013disadvantages of the conventional methods and structures, a feature of the present invention is to provide a method and structure in which applications of magnetic memory devices (MRAM device arrays, MRAM stacks, MTJs, etc.) requiring fabrication of such devices (e.g., MTJs etc.) on flexible, standard (e.g., thermally-sensitive such as low-temperature) substrates, such as , polymers and plastics, is made possible.

0014Another feature is to provide a structure and method in which the MTJ device can be fabricated directly on such substrate materials.

0015In a first exemplary aspect of the present invention, a method of

0016forming a magnetic memory device (and a resulting structure) on a low-temperature substrate, includes forming the memory device on a transparent substrate coated with a decomposable material layer subject to rapid heating resulting in a predetermined high pressure, and transferring the memory device to the low-temperature substrate. In a second exemplary aspect of the present invention, a method of

0017making a magnetic tunnel junction (MTJ) device on a thermally-sensitive substrate, includes coating a transparent substrate, having a MTJ device thereon, with a nitride material, and transferring the MTJ device to the low-temperature substrate using a laser lift-off process. In a third exemplary aspect of the present invention, a magnetic memory device, includes a transparent substrate having decomposable material coated thereon, a device formed on the transparent substrate, and a low-temperature substrate for receiving the device, after the decomposable

0018material is subjected to a predetermined high pressure. In a fourth exemplary aspect of the present invention, a method of forming a magnetic memory device on a low-temperature substrate, includes forming a memory device on a transparent substrate coated with a nitride

0019layer, applying a laser light to the memory device, to subject the nitride layer to pressure, and transferring the memory device to a low-temperature

0020substrate.

0021Thus, with the unique and unobvious aspects of the present invention, magnetic memory devices (e.g., MRAM devices, MRAM stacks, MTJs, etc.)

0022can be formed advantageously on low-temperature substrates such as plastics,

0023polymers, and the like. Fabrication of memory circuits for low performance applications using the inventive method can lead to lower cost, flexible, durable and lightweight plastic circuits with potential applications for many devices, including smart cards and wearable sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

0025The foregoing and other purposes, aspects and advantages will be

0026better understood from the following detailed description of exemplary embodiments of the invention with reference to the drawings, in which:

0027Figure 1 illustrates an exemplary structure 100 according to an

0028exemplary embodiment of the present invention;

0029Figure 2 illustrates the structure 100 after pulsed laser-induced transfer of a magnetic memory device (e.g., an MRAM device array) to a low-temperature substrate, according to an exemplary embodiment of the

0030present invention; and Figure 3 illustrates a method 300 of forming an MTJ device on a thermally-sensitive substrate according to an exemplary embodiment of the present invention. DETAILED DESCRIPTION OF EXEMPLARY

EMBODIMENTS OF THE INVENTION

0032Referring now to the drawings, and more particularly to Figures 1-3, there are shown exemplary embodiments of the method and structures according to the present invention.

EXEMPLARY EMBODIMENT

0034As described below, in an exemplary embodiment of the invention, a method of transplanting MTJ devices onto a standard (e.g., thermally-sensitive) substrate is provided, in which an array of devices fabricated, for example, on a transparent substrate (e.g., quartz, sapphire, etc.), are transferred using an exemplary laser lift-off process.

0035It is again noted that any substrate can be used so long as the substrate is transparent to laser light. That is, depending upon the substrate, an adequate

0036wavelength of light would be found for transmission. Sapphire is preferable since it transfers over a much wider range of wavelengths.

0037Turning now to the Figures, and more specifically to Figure 1 , an exemplary MRAM device structure 100 is shown.

0038Structure 100 includes an underlying conductive (e.g., Cu or the like) wiring 110 along with an MTJ cell 120. The MTJ cell typically includes a bottom contact electrode, a bottom magnetic layer, an oxidized Al barrier, a

0039top magnetic layer and a top contact electrode. The underlying conductive (e.g., Cu or the like) wiring 110 is fabricated on a transparent (e.g., sapphire, quartz, etc.; hereinafter sapphire will be assumed for exemplary purposes only) substrate 130, preferably coated with a thin blanket layer of nitride material 140. The layer of nitride material 140 preferably includes one or more nitride films selected from the group of gallium nitride, aluminum nitride or indium nitride and/or their alloys.

0040An insulating layer 160 (e.g., SiO₂ , SiN, SiON, SiONF and/or the like) may be formed between the upper conductive (e.g., Cu) wirings 125 by etching or the like. Additionally, the upper copper wirings 125 may be formed on a conductive nitride material (e.g., TiN, TaN, ^"WN, and/or the like) or other conducting metal or oxide layer 170. Between the TiN 170/MTJ stack 120 is preferably formed an insulating nitride material 180 (e.g., SiN) or an insulating oxynitride (e.g., SiON).

0041It is known that laser irradiation of the surface of wide-gap InGaAIN films with above (or near)-band-gap photon energies results in the metallization of the nitride following the reaction:

0042XN = X + 1/2N₂, where X represent Ga, Al, or In.

0043This has been used advantageously in demonstrating that GaN thin films grown on the transparent (sapphire) substrate can be lifted-off from the

0044substrate by metallizing the interface with a laser probe 150 incident through the backside of the substrate (e.g., sapphire) 130.

0045This approach also has been extended to show that by using the GaN layer as a template to grow layers of Si, the metallization process can be

0046utilized to lift off overgrown Si layers to be deposited onto arbitrary substrates. Efficient film transfer using A1N, InN and their alloys is also

0047expected.

0048Thus, Figure 1 illustrates the application of the pulsed laser 150 applied to the MRAM device array, fabricated on a transparent (e.g., sapphire)

0049substrate 130 for transfer of the MRAM device array to a low-temperature

0050substrate (e.g., plastic, polymer, etc.) 190.

0051Figure 2 illustrates the structure after pulse laser-induced transfer of the MRAM device array from the transparent substrate 130 to the low-temperature substrate 190, has occurred. As shown, the magnetic

0052memory (MTJ stack) has been formed on the plastic substrate. Additionally

0053the decomposable material (e.g., the nitride material) has decomposed into its substituents via the release of nitrogen. Hence, in the example shown in

0054Figure 2, there is an aluminum layer 195, and interfacial layer 200 below the

0055nitride material (e.g., A1N) 140. Turning now to the method 300 of the invention, the decomposable

0056material (e.g., nitride such as A1N, GaN, and/or InN, and their alloys, etc.)

0057film is deposited on a transparent (e.g., sapphire, quartz, etc.) substrate (step

0058310). It is noted that the deposition can be performed at relatively low

0059temperatures (e.g., room temperature or near room temperature). However, higher temperature could be employed (e.g., to about for example, 250 °C).

0060Regardless of the host substrate used, it is important that the deposition

0061temperature be below the temperature at which the host substrate would begin

0062to degrade or delaminate. It is noted that the nitride film should be of sufficient thickness so that the laser light (e.g., depending upon the laser light used) is absorbed in the nitride film. It is noted that some of the nitride may remain on the MRAM after the transfer, and can be removed at that time or left behind. That is, the nitride should not be so thin that the MRAM stack is damaged by the laser light being completely absorbed in the MRAM. Thus, preferably the nitride film has a thickness of about 100 A to about 2000 A

0063In step 320, the magnetic memory device (e.g., MRAM arrays, MRAM stack, MTJs etc. depending upon the desired applications) along with the conductive (e.g., Cu, etc.) wiring are formed on top of the nitride surface. The

0064MRAM device arrays preferably are complete such that substantially no further processing (or minimum processing) is required after the transfer. That is, preferably, the MRAM device is completely fabricated and processed on the sapphire substrate. During the transfer process, the top copper wiring (with any additional insulating top layer) is mounted face down onto a low-temperature (e.g., polymer or plastic) host substrate in very close proximity. It is preferable to coat a thin layer of an elastomer material, such as polydimethylsiloxane (PDMS) on the host substrate to enable intimate contact between the two substrates during the transfer process. The two surfaces can also be bonded using an epoxy or other mechanism.

0065In step 330, preferably a single pulse of laser radiation, preferably near

0066or above the band gap of the decomposable material (e.g., nitride), will result in a metallization of the interfacial nitride. It is noted that the laser light may be a patterned beam of laser light on the order of 200-400 mJ/cm² (e.g., a fairly large beam) useful for the local transfer.

0067As a result of such metallization, in step 340, the nitride/MRAM structure will detach from the transparent (e.g., sapphire, quartz, etc.) and adhere to the host substrate (e.g., plastic, polymer, etc.). For the laser, preferably an excimer laser is employed, for producing a pulse having a duration on the order of tenths of nanoseconds (or less).

0068Along these lines, a laser having a wavelength of 193 nm or 248 nm could be used. By the same token, any short pulse laser could be used, which allows the light to be absorbed in the nitride material could be used. Thus, in a very short time period, the thin layer of nitride material is heated up and subsequently decomposed (e.g., broken up into its constituent elements).

0069The debonding occurs due to the pressure build up with the release of nitrogen from the nitride metallization. It is noted that, by optionally rastering the laser beam, different

0070MRAM arrays can be transferred using single pulses (step 350).

0071Additionally, by optionally growing an Si layer on top of the nitride

0072(step 360), depending upon the geometry involved, the MRAM devices can also be integrated with transistors to control operation of the cells, which are then be transferred simultaneously. The silicon layer can be epitaxial, polycrystalline, or amorphous depending on the growth temperature of the silicon and the underlying nitride material. Epitaxial-quality wide band gap nitrides can be grown on sapphire. Subsequent growtli of silicon on this nitride layer will be substantially single crystal in nature and of better crystalline quality than the amorphous or polycrystalline silicon. Thus, transistors fabricated in the grown silicon with the MRAM devices on top can be transferred at the same time.

0073Alternatively, optional organic transistors can be fabricated on the host substrate (e.g., the low-temperature substrate such as plastic, polymer, etc.) prior to transfer of the MRAM arrays (step 370) and interconnected with the transferred MRAM structure.

0074After the transfer process, optionally (e.g., depending on the designer's requirements) the overlying nitride layer can be etched away using established dry or wet etching techniques (step 380).

0075An issue of the inventive laser transfer process is the temperature rise of the MRAM structure and also that of the host substrate. The temperature rise can be estimated based on the known thermodynamic and thermal properties of the nitride film and the sapphire substrate.

0076For example, for an energy fluence of ~ 300 mJ/cm² from an excimer

0077laser at 248 nm, the maximum temperature rise at a distance of 0.5 μm is

0078about 150°C for conduction through sapphire (and about 350°C for

0079conduction through quartz). The temperature is rapidly quenched to below

0080100°C (within 0.1 μs) for the case of sapphire. Thus, the temperature rise is

0081not very severe, and can be controlled by the thickness of the nitride layer.

0082Thus, the invention provides a method (and resulting structure) in which a magnetic memory device (e.g., MRAM device array, MRAM stack, MTJ cell, etc.) is formed on a substrate transparent to laser light, with a thin nitride layer on the bottom which absorbs the laser light and which decomposes due to the high pressure, etc., thereby facilitating the transfer of the MRAM device array to a low-temperature substrate.

0083While the invention has been described in terms of several exemplary embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.

0084Further, it is noted that Applicant's intent is to encompass equivalents of all claim elements, even if amended later during prosecution.