AN IMPROVED LASER ABLATEABLE MATERIAL

Abstract: This invention discloses a method for coating an integrated circuit (3) with a layer of plasma deposited polymers (PDP) ablateable by visible light laser radiation.

Description: AN IMPROVED LASER ABLATEABLE MATERIAL FIELD QE -CH--. INVENTION The present invention relates to materials which are ablateable by lasers of visual light and more particular¬ ly to methods of using such materials in the fabrication and customization of integrated circuits. BACKGROUND ___ _____ INVENTION Integrated circuits are usually manufactured in large runs. However it is frequently desirable to make small runs of a specific integrated circuit, typically for prototyping. US Patent 4,924,287, the disclosure of which is incorporated herein by reference, describes a customizable integrated circuit. Methods for customizing of such integrated circuits are shown in US Patent 5,329,152, the disclosure of which is also incorporated herein by reference. Customizable integrated circuits typically have predetermined portions which are adapted for modification before being supplied to the end user. Such modifications include, inter alia: (a) electrically programing memory locations; (b) cutting conducting links; and (c) creating conducting links. Customization by cutting of conductor links is preferred since this method does not require extra cir¬ cuitry on the integrated circuit as do electrically programmable logic devices. Furthermore, pre-produced links can carry a higher current density than created links. Two methods are mainly used to selectively cut links. One method is to cut each link directly with a laser beam. However, direct cutting with a laser may require high laser energy densities. Application of large amounts of laser energy to integrated circuit surfaces may damage the integrated circuit. A preferred method of customizing such circuits is to coat them with a layer of laser sensitive ablative material and to ablate the material at selected locations using a relatively small amount of laser radiation. After such ablation, the integrated circuit is etched using an etchant or other etching method that does not remove the ablative material, for example, by chlorine plasma etch¬ ing. Thus, only areas previously ablated by the laser are etched. Customizable areas typically include metal links so that etching the links modifies the interconnections, and therefore the function, of the integrated circuit. It is also known to use a photolithographic method wherein the integrated circuit is coated with a layer of radiation sensitive material and exposed to a pattern of ultra violet light, visible light, X-rays or to an elec¬ tron beam. The coating material is developed and the areas exposed to radiation are removed. The integrated circuit is then etched as described above. In practice, due to the characteristics required of them, very few materials are useful as laser ablative coatings. An efficient laser ablative material should be capable of absorbing a large portion of the laser energy and in response thereto be transformed directly and immediately to gas. Laser ablation sometimes causes the material to explode. Explosion transforms part of the material to gas, however, some of the material is also blown away as particles. Some of these particles may fall back on the chip and cover-up previously uncovered areas, counteracting the ablation/explosion at these areas. An effective laser ablative material should not form many particles. The term ablation means that the material is turned directly to gas, and very few particles are formed. It is also desirable that the resultant ablation pattern be as close as possible to the irradiation pat¬ tern and that only small amounts of energy leak into the surrounding area and into the integrated circuit. Other¬ wise, the definition of the geometry will be poor and the integrated circuit may be damaged. Additionally, the material should adhere well to substrates and provide good coverage of step geometries used in microelectronic circuits. Since the purpose of the coating is to protect coated areas while etching the uncoated areas, it is important that the material be resistant to at least one method of etching, preferably a metal etching method. An example of a material which has some but not all of the previous properties is Arsenic Sulfide. Arsenic Sulfide has most of the abovementioned properties, howev¬ er, since it does not cover uneven surfaces very well it is not as useful as other materials. Laser ablative materials which are ablated by ultra violet lasers are known in the art. For example, US Patent 5,302,547 shows covering an integrated circuit with a liquid polymer and ablating that polymer with ultra violet light. However, these polymers are transpar¬ ent to visible light and are not known to be ablateable by visible light lasers. Very few materials are known to be ablateable by visible light. Visible light is preferred to ultra violet light because laser technology supplies more efficient and less expensive lasers in visible light wavelengths. US Patent 5,329,152 discloses the use of amorphous silicon as a visible light laser ablative coating materi¬ al. Amorphous silicon is ablated by visible light lasers and is partially resistant to etching by chlorine plas¬ ma, which is used to etch metals. Thus, an integrated circuit with exposed metal links can be customized by using amorphous silicon as the ablative material. One problem with amorphous silicon is its high vaporization temperature (2355 * C) - 1000 * C over its melting point which increases the tendency to explosion and particle generation. Plasma deposited polymers (PDP) , which are described in "Plasma Polymerization", by H. Yasuda, Academic Press, Inc. 1985, have properties such as crack-filling, chemical inertness and selective permeability which make them useful for a variety of uses such as surgical pros- thetics and semipermeable membranes. US Patents 5,320,875, 5,312,529, 5,283,119 and 5,308,649, the dis¬ closures of which are incorporated herein by reference, disclose methods of manufacturing and uses of PDPs. PDPs are typically manufactured as follows: First, a substrate is placed in a plasma chamber. The chamber is then filled with a gas, such as methane, at a low pressure, typically on the order of 1 torr. Plasma is then created in the chamber, typically using a radio frequency (RF) electric field which ion¬ izes the gas. Consequently, a polymer layer is continual¬ ly deposited on the substrate. It should be understood that a PDP is not a direct polymer of the gas used in the process. It is believed that the gas breaks down in the plasma and gas precursors and their compounds form the PDP which is then deposited on the substrate (and on the walls of the chamber) . The deposition process is a combination of two processes, one in which molecules hit the substrate and cling, and another in which they do not cling, and may even cause some material to be etched off the substrate. The temper¬ ature of the substrate dictates the types of molecules which are likely to cling to the substrate and the manner in which they will be attached to the PDP already deposi¬ ted. The gas usually flows through the chamber at a rate which determines the types of molecules that form in the plasma and, consequently, the type of PDP deposited. There are many other parameters which may affect the deposited PDP, such as the distance of the substrate from different portions of the plasma and the RF power used to create the plasma. The gas used is typically an organic compound. However, some inert gases, such as argon, may be added in order to speed up the deposition process. It has also been observed that similar polymers can be created from different starting materials. SUMMARY QF ____. INVENTION The inventors have discovered a method of depositing a Plasma Deposited Polymer (PDP) which is laser-ablative by laser light having a wavelength longer than ultra¬ violet light, such as visible light. In a preferred embodiment of the invention, the PDP is deposited in the following manner: (a) providing a plasma vacuum chamber; (b) placing the substrate (typically, a silicon wafer) on the cathode of the chamber; (c) introducing a plasma into the chamber, said plasma generated by applying Radio Frequency (RF) radia¬ tion to an active gas, the power density used is between .08 Watt*cm "2 and 1.59 att*cm~ 2 ; and (d) terminating the process when the PDP is deposit¬ ed on the substrate to the desired thickness. Preferably, an inactive gas such as argon is added to the active gas at a ratio between 3:7 and 7:3, prefer¬ ably, at a ratio between 4:6 and 6:4; most preferably, at a 1:1 ratio. Preferably the gas mixture is introduced through the anode and turned into a plasma in the cham¬ ber. Alternatively, the gas is first turned into a plasma and then introduced into the chamber. The inactive gas may be added before or after the active gas is turned into a plasma. The substrate is preferably maintained at a tempera¬ ture below 70 β C. A pressure of between 0.1 and 2 torr is maintained, preferably, between 0.5 and 1.5 torr. The inventors have found that a pressure of about 1 torr is most preferred. The active gas used is preferably organ¬ ic, preferably, a hydro-carbon; typically, ethylene is used. The gas is introduced into the chamber at a flow rate between 1 seem and 50 seem, preferably between 10 and 35 seem and most preferably, about 20 seem. Prefera¬ bly, . Ionization of the gas is achieved with an RF power density between 0.15 Watt*cm and 1 Watt*cm~ 2 ; most preferably, the power density is between 0.24 Watt*cm~ 2 and 0.48 Watt*cm~ . Preferably, the RF radiation is continuous at 13.6MHz. However, other methods of applica¬ tion of RF enegry may be used, such as pulsed RF radia¬ tion at a frequency of 400MHz. Preferably the cathode is 8" in diameter. The distance between the cathode and the anode is between 1.5 and 10 cm, preferably 3 cm. A typical deposition time of approximately 10 minutes will produce a polymer of a typically desired thickness of approximately lμ. These parameters can be adjusted to create a polymer having predetermined characteristics. Specifically, a material with low specific heat, low heat conductivity, a small difference between its evaporation temperature and its melting temperature and high light absorption is desirable, and is achieved utilizing the above parame¬ ters. A PDP deposited under the abovementioned conditions has one or more of the following properties: (a) the material is stable enough to act as a passi¬ vation layer; (b) when ablated by laser, preferably a visible light laser, the material absorbs enough of the incident laser energy directed at it so that underlying layers are not damaged by laser energy; (c) only areas directly illuminated by laser energy are ablated; (d) the material is ablative, i.e., it vaporizes rather than explodes, so that very little debris is formed on the substrate when the material is ablated; (e) the material is ablated in response to relative¬ ly low levels of energy; (f) the material has good filling qualities, so that it can be deposited evenly over non leveled geometries; .(g) the material is insulative; (h) the material adheres well to the substrate; (i) the material is resistant to at least some forms of metal etching; (j) the material is ablateable by e-beam; and (k) the material is etchable by some means, which preferably do not etch the structure of the underlying integrated circuit. Alternately, a PDP having a selected absorption coefficient can be deposited by properly selecting the power density used to generate said plasma. There is thus provided according to a preferred embodiment of the invention a method of customizing integrated circuits: providing an integrated circuit; depositing a PDP on the integrated circuit; and ablating the PDP at preselected locations using a laser beam, preferably a visible light laser beam. Preferably, the integrated circuit is then etched, preferably by reactive ion etching, at locations underly¬ ing said ablated locations. There is further provided in accordance with a preferred embodiment of the invention a PDP ablateable by visible light. Also provided in accordance with a preferred embodi¬ ment of the invention is an integrated circuit coated with a PDP ablateable by visible light. There is further provided in accordance with a preferred embodiment of the invention a PDP deposited according the above described deposition parameters. PRIEF DESCRIPTION -OF THE DRAWINGS Fig. 1 is a schematic of a vapor deposition chamber as used in forming an ablative layer in accordance with a preferred embodiment of the invention; Figs. 2A-2C show an integrated circuit customized in accordance with a preferred embodiment of the invention; and Figs. 3A-3D show an integrated circuit customized in accordance with an alternative preferred embodiment of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a plasma vacuum chamber 1 suitable for depositing a Plasma Deposited Polymer (PDP) on a sub¬ strate, for example, a Plasma-Therm model 790 vacuum chamber. A substrate 3, to be coated with a PDP is placed in chamber 1, preferably on a cathode 4. The temperature of substrate 3 may be controlled by a heater/cooler 5 such that during deposition the substrate is maintained at a substantially constant temperature. Preferably, the substrate is maintaining at 20 β C while the walls of the chamber are heated. A pump 2 reduces the gas pressure in chamber 1. Preferably, the pressure in the chamber during the deposition is approximately 1 torr, however, pres¬ sures between 0.1 and 2 torr are useful in carrying out the invention. A gas is supplied to chamber 1, preferably through a plurality of nozzles 8 formed in an anode 6. The gas is typically a mixture of an active gas, preferably an organic compound such as ethylene or another hydro-carbon and an inactive gas such as argon. Preferably, the ratio between the active gas and the inactive gas is between 7:3 and 3:7, preferably, at a ratio of about 1:1. The rate at which the gas mixture is supplied is called the "flow rate" and is preferably between 1 seem and 50 seem, preferably 20 seem. Since many organic compounds and in particular: C 2 H 2 , C 6 H 6 , C 6 F 6 , C 2 H 4 , C 2 F 4 , Styrene, Cyclohexane, Ethylene oxide, Acrylic acid, Propionic acid, Vinyl acetate, Methyl acrylate, Hexamethyldisilane, Tetrame- thyldisiloxane, Hexamethyldisiloxane, Divinyltetramethyl- disiloxane and many hydrocarbon compounds are known to be interchangeable with ethylene in PDP deposition, a PDP ablateable by visible laser light can be obtained if any of the preceding organic materials are used, with appro¬ priate changes in the deposition parameters, if required. An RF generator 7, preferably operating at 13.6 MHz, is connected between anode 6 and cathode 4. For a cathode diameter of 8" the power output of RF generator 6 is typically between 25W and 500W, preferably, between 75W and 150W. Preferably, anode 6 and (the walls of) chamber 1 are grounded and a voltage differential appears between them and cathode 4. The distance between anode 6 and cathode 4 is between 1.5 and 10 cm, preferably, 3 cm. Alternatively, the active gas is turned into a plasma in a separate compartment within the chamber. The inactive gas may be added to the active gas before or after it is turned into a plasma. In a chamber configured as described above, a layer of PDP is continuously deposited on substrate 3. Prefera¬ bly, a layer having a thickness of about 0.6μ is deposit¬ ed within less than 15 minutes. It is to be appreciated that some other combinations of values for the above parameters may also result in a PDP being deposited and in some cases, the resultant PDP may be ablateable by visible laser light. A PDP deposited according to the abovementioned configuration will be ablateable by visible laser light and will have some or all of the following properties: (a) low thermal conductivity; (b) low evaporation temperature; (c) an absorption coefficient of at least 3*10 3 cm "1 11 in the mid-visible range; (d) stability in Chlorine and Fluorine plasmas, generally used to etch metal layers and insulation layers respectively; (e) ablateability at ambient conditions by visible laser radiation (at 532 nm or 527 nm) at pulse energy densities below 4 J/cm 2 ; (f) etchability in an oxygen plasma; (g) capability of filling cracks which are approxi¬ mately as wide as the layer of PDP; (h) even deposition on the substrate; (i) insulating capability; (j) ablation without forming many particles at laser energy above 1 J*cm , • (k) good conformance between the ablation pattern and the laser radiation pattern; (1) good adhesion to the substrate; and (m) stability over a period of more than a year in storage, so that the PDP can be used as a passivation layer. The inventors have also discovered that a PDP depos¬ ited according to a preferred embodiment of the invention has an evenly monotonicly decreasing absorption coeffi¬ cient in the visible light range. Thus, it is easily ablateable by ultraviolet light lasers and also ablate¬ able by laser light in the near infra red. A preferred use of a PDP having the abovementioned properties is in customization of integrated circuits. Figs. 2A - 2C show customization of an integrated circuit in accordance with the present invention. Fig 2A shows an integrated circuit which includes a metal layer 901, separated from a second metal layer 902 by an insulation layer 904. Layer 902 is also covered by a second insula¬ tion layer 905, and another metal layer 903 covers layer 905. The entire top of the integrated circuit is prefera¬ bly covered with a passivation layer 436. Since customi- zation preferably includes disconnecting metal links in layers 902 and/or 903, a plurality of apertures 422 are preferably formed through passivation layer 436 and through any intervening layers down to a metal link 438 that is to be disconnected. US Patent 4,924,287 to Orbach and US Patent application serial number 08/290,550, filed August 15, 1994 and titled "A Customizable Logic Array Device", the disclosures of which are incorporated herein by reference, both describe an integrated circuit pre¬ ferred for customization. A first step of the customization process, as shown in Fig. 2A, includes depositing a layer 900 of PDP over the integrated circuit. Preferably, the layer is lμ thick. Since PDP layer 900 is preferably stable at ambi¬ ent room conditions, the coated integrated circuit is preferably manufactured in large runs and stored until needed. A second step of the customization process, as shown in Fig. 2B, includes ablating PDP layer 900 at selected locations. Preferably these locations are over apertures 422. Preferably, a NdYAG frequency doubled laser at 532nm or Nd YLF at 523nm is used for ablating PDP layer 900. Alternatively an Argon laser at 514nm or at 488nm is used. Preferably, the pulses are approximately 100 nano¬ seconds long and have an energy density of approximately 4 J/cm . The ablations can be performed using a laser micromachining system, preferably, a QS650 available from Chip Express (Israel) LTD. A third step of the customization process, as shown in Fig. 2C, includes etching the integrated circuit, preferably, using Chlorine plasma. Preferably, the remaining PDP is removed by Oxygen plasma and a passivation layer is formed over the cus¬ tomized integrated circuit. Figs. 3A-3D show an alternative preferred customiza¬ tion process similar to the abovedescribed process. In this embodiment, the integrated circuit is first coated with a planarization layer 906, preferably silicon diox¬ ide or other insulating planerizing materials as well known in the art, before the step of depositing PDP layer 900 thereon (Fig. 3A) . Fig. 3B shows the device after layer 900 is ablated at selected locations. An additional step of etching planarization layer 906, preferably with CF 4 plasma is preformed (Fig. 3C) before etching the integrated circuit itself (Fig. 3D) . The abovedescribed customization processes can be carried out on most kinds of integrated circuits, includ¬ ing CMOS devices, gate arrays, multi-chip packages and memory chips. Additionally, the inventors have found that the ablation pattern of the PDP is very similar to the irra¬ diation pattern used, so that very precise micromachining of the PDP is possible. The depth of the cut is con¬ trolled with a precision of 0.3μ or better. The radius of curvature at the corners of rectangular cuts is 0.2μ or less, even though a wavelength of .532μ was used. This is attributed to the threshold nature of the ablation proc¬ ess and to the properties of imaging with laser coherent light. The inventors have found that the PDP is more versatile than using standard photo-resists because it does not need to be developed and washed away. Further¬ more, the inventors has discovered that the optical properties of the PDP, in particular absorption, can be controlled by changing the RF power used to ionized the gas, therefore, a PDP layer with the correct absorption can be produced for any chosen laser wavelength and any desirable film thickness. Additionally, the etch resist¬ ance properties of the PDP are similar to those of stand¬ ard photo-resists. It will be appreciated by persons skilled in the art 14 that the present invention is not limited to what has been thus far described. Rather, the scope of the present invention is limited only by the following claims:

Claims: Ql______ 1. An integrated circuit coated with a layer of PDP ablateable by visible light laser radiation. 2. An integrated circuit according to claim 1, wherein said PDP is ablateable by a laser energy density lower than 4 J/cm 2 . 3. An integrated circuit according to claim 2, wherein said PDP is partially resistant to at least one method of metal etching or one method of oxide etching. 4. An integrated circuit according to claim 3, wherein said at least one method of metal etching comprises plasma etching. 5. A method for customizing a device comprising: providing a device; depositing a layer of PDP on said device; and ablating said device at preselected locations using a laser beam. 6. A method according to claim 5, wherein said laser beam is a visible light laser beam. 7. A method according to claim 6, further comprising etching said device at the ablated locations. 8. A method according to claim 6, wherein said device is an integrated circuit. 9. A method according to claim 6, wherein said device is a customizable gate array. 10. A method according to claim 6, wherein depositing a PDP comprises: placing said device in a plasma chamber; and providing a gas plasma in said chamber. 11. A method according to claim 10, comprising activat¬ ing said gas plasma with a radio frequency electric field of between .08 and 1.59 Watt*cm~ 2 ; 12. A method according to claim 10, comprising activat¬ ing said gas plasma with a radio frequency electric field of between .24 and .48 Watt*cm~ 2 ; 13. A method according to claim 10, further comprising maintaining a chamber pressure between .1 and 2.0 torr. 14. A method according to claim 10, wherein said plasma has a flow rate between 1 and 50 seem. 15. A method according to claim 14, wherein said flow rate is between 15 and 25 seem. 16. A method according to claim 10, wherein said gas plasma is created from a mixture of an organic compound and at least 30% argon. 17. A plasma deposited polymer (PDP) ablateable by visible laser radiation. 18. A PDP according to claim 17, which is ablateable by n a laser energy density lower than 4 J/cm . 19. A PDP according to claim 18, which is partially resistant to at least one method of metal etching. 20. A method of producing a Plasma Deposited Polymer (PDP) which has a selected absoption coefficient on a substrate, comprising: 17 providing a plasma chamber; providing a substrate in said chamber; and providing a gas plasma in said chamber by activating a gas comprising an organic gas to form said gas plasma by a radio frequency electric field, wherein said field has a power density selected to result in a PDP having the selected absoption coefficient. 21. A method of producing a Plasma Deposited Polymer (PDP) which is ablateable by visible light radiation on a substrate, comprising: providing a plasma chamber; providing a substrate in said chamber; and providing a gas plasma in said chamber by activating a gas comprising an organic gas to form said gas plasma by a radio frequency electric field having a power density of between .08 and 1.59 Watt*cm~ 2 . 22. A method according to claim 21, wherein said power density is between .15 and 1 Watt*cm . 23. A method according to claim 21, wherein said power density is between .24 and .48 Watt*cm ~2 . 24. A method according to claim 23, wherein said organic gas is a hydro-carbon compound. 25. A method according to claim 23, wherein said gas comprises at least 30% inactive gas. 26. A method according to claim 23, wherein said organic gas comprises ethylene. 27. A method according to claim 21, further comprising maintaining a chamber pressure between .1 and 2.0 torr. 28. A method according to claim 21, wherein said plasma has a flow rate between 1 and 50 seem. 29. A method according to claim 21, wherein said plasma has a flow rate between 15 and 25 seem. 30. A PDP produced by the method of claim 21. 31. A PDP produced by the method of claim 24. 32. A PDP produced by the method of claim 26. 33. A PDP produced by the method of claim 28.

Inventors: CASSUTO YORAM, JANAI MEIR, SILVERSTEIN MICHAEL STEPHEN, CASSUTO, YORAM, JANAI, MEIR, SILVERSTEIN, MICHAEL, STEPHEN
Applicants: CASSUTO YORAM, CHIP EXPRESS CORP, JANAI MEIR, SILVERSTEIN MICHAEL STEPHEN, CASSUTO, YORAM, CHIP EXPRESS CORPORATION, JANAI, MEIR, SILVERSTEIN, MICHAEL, STEPHEN
Application year: 1996
Main classification: H01L29
IPC codes: H01L21, H01L23