Method for dual-layer polyimide processing on bumping technology

[0002] (1) Field of the Invention

[0003] The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method to create solder bumps for the interconnection of semiconductor devices.

[0004] (2) Description of the Prior Art

[0005] Semiconductor technology has, ever since its inception, improved semiconductor device performance by reducing device dimensions and, concurrently, increasing device packaging density. In the field of high density interconnect technology, it is necessary for many of the packaging approaches to fabricate a multilayer structure on a substrate in order to connect integrated circuits to one another. To achieve a high wiring and packing density, many integrated circuit chips are physically and electrically connected to a single substrate commonly referred to as a multi-chip module (MCM). Typically, layers of a dielectric such as a polyimide separate metal power and ground planes in the substrate. Embedded in other dielectric layers are metal conductor lines with vias (holes) providing electrical connections between signal lines or to the metal power and ground planes.

[0006] The methods that are used to interconnect a semiconductor device to an underlying substrate can be differentiated in methods of wire bonding (device interconnect points are provided around the periphery of the device, a factor that limits the number of input/output connections that can be made to the device), tape automatic bonding (device interconnect points are provided around the periphery of the device, again limiting I/O capability) and C4 or Controlled Collapse Chip Connection methods. The latter method of C4 interconnect offers the advantage of providing high input/output capability since the interconnect bumps can be placed in any location on the chip. The C4 technology offers, in addition to high I/O interconnect capacity, the advantage of short solder bumps (improving the electrical performance of the interconnect) while the process of reflow that is used to connect the solder bumps with the substrate allows the formation of a self-aligned interface between the solder bump and the contact point in the substrate to which the solder bump is connected.

[0007] For the formation of solder bumps, two different and well established technologies can be used, that is evaporation and electroplating. Other methods that are used are methods of adhesive applying, stud-bumping methods and the like. These latter methods will not be further discussed at this point.

[0008] The method of electroplating follows the processing steps of (over the surface of a substrate wherein a point of electrical contact, typically containing aluminum, has been provided, all steps of processing being centered around this point of electrical contact) depositing a layer of polyimide and etching an opening in this layer of polyimide that aligns with the point of contact, depositing (by vacuum evaporation) a layer of Under Bump Metallurgy (UBM, also referred to as Ball Limiting Metallurgy or BLM) over the layer of poly including the opening created in the layer of poly. A layer of photoresist is deposited over the layer of UBM and patterned, creating an opening in the layer of photoresist that aligns with that part of the layer of UBM that remains in place under the to be created solder bump. Next a layer of metal (typically copper) is electroplated over the layer of photoresist whereby the layer of UBM serves as the common electrode for the electroplating process, the electroplated metal is in contact with the layer of UBM. A layer of solder is next plated over the layer of electroplated metal. The layer of electroplated metal is centered around the opening that has been created in the layer of photoresist as is the layer of plated solder. The photoresist is removed using the solder bump as a mask, the layer of UBM is selectively etched and removed where this layer does not underlie the to be created solder bump in order to electrically isolate the solder bumps from each other. A critical step of the process is performed as a final step whereby a flux is applied to the plated solder and the solder is melted in a reflow furnace under a nitrogen atmosphere, creating a spherically shaped solder bump. The above summarized processing steps of electroplating that are used for the creation of a solder bump can be supplemented by the step of curing or pre-baking of the layer of photoresist after this layer has been deposited over the layer of UBM.

[0009] The process of evaporation also starts with a semiconductor surface wherein a metal point of contact has been provided. A layer of passivation is deposited and patterned, creating an opening in the layer of passivation that aligns with the metal point of contact. A layer of UBM is formed over the layer of passivation and inside the opening created in the layer of passivation. The UBM layer may be a composite layer of metal such as chromium followed by copper followed by gold in order to promote (with the chromium) improved adhesion of the UBM layer and to form a diffusion barrier layer or to prevent oxidation (with the gold over the copper). Solder is next selectively plated over the deposited layer of UBM and melted in a solder reflow surface in a hydrogen ambient for solder reflow, in this manner forming the spherically shaped solder bumps.

[0010] Some of the problems that have over time been experienced using the various processes are:

[0011] electroplating suffers from the problem that the etching of the layer of UBM may affect the solder that has been deposited since this solder is readily attacked by an acid solution; any acid that is used for the etching of the layer of UBM may adversely affect the deposited solder

[0012] the previous problem of the solder being affected during the UBM etch can be reduced by the application of a second mask that protects the deposited solder during the UBM etch; this however adds complexity and expense to the process of the formation of solder bumps while the additional mask complicates the processing sequence

[0013] the polyimide that is used during the electroplating procedure is difficult to completely remove from the opening that is etched in the layer of poly that aligns with the point of electrical contact; this creates a poorly defined opening in the layer of poly, which in turn results in a low quality contact between the solder bump and the underlying point of electrical contact

[0014] the process of electroplating is relatively complex and requires a significant number of processing steps

[0015] the process of evaporation is generally more expensive than the process of electroplating, and

[0016] the process of evaporation is, for decreased device dimensions, more difficult to control and therefore does not lend itself well to the era of sub-micron devices.

[0017] The present invention addresses concerns of a prior art method of forming solder bumps on a semiconductor surface. This prior art method is highlighted in FIG. 1, which contains the following sub-components:

[0018] 10 is a semiconductor surface over which the solder bump is to be created

[0019] 12 is the metal contact pad, typically containing aluminum, which is to be brought into contact with the solder bump and over which the solder bump therefore is to be created

[0020] 14 is a patterned layer of Plasma Enhanced silicon nitride, which serves as an etch stop layer for the etch of an overlying layer

[0021] 16 is a layer of insulation that contains polyamide (a polyamide insulator or PI coating)

[0022] 18 is a layer of Under Bump Metallurgy (UBM), and

[0023] 20 is the created solder bump.

[0024] The prior art processing sequence for the formation of the solder bump that is shown in FIG. 1 is as follows:

[0025] depositing a layer of PE Si₃N₄over the semiconductor surface 10 thereby including the surface of the aluminum pad 12

[0026] patterning and etching the deposited layer of PE Si₃N₄, creating an opening in the layer of PE Si₃N₄that aligns with the aluminum pad 12

[0027] applying a coating 16 of polyamide insulator (PI) over the surface of the layer 14 of PE Si₃N₄including the exposed surface of the aluminum contact pad 12

[0028] patterning and etching the layer 16 of PI, creating an opening in the layer 16 of PI that aligns with the aluminum pad 12

[0029] curing the etched layer 16 of PI

[0030] depositing a layer 18 of Under Bump Metallurgy (UBM) over the surface of the etched and cured layer 16 of PI

[0031] selectively depositing a layer (not shown in FIG. 1) of Pb/Sn to form solder bumps

[0032] selectively etching the deposited layer 18 of UBM to electrically isolate the solder bumps from each other using the deposited layer of Pb/Sn as a mask, and

[0033] melting the selectively deposited Pb/Sn in a reflow furnace creating the spherically shaped solder bump 20.

[0034] The problem areas that appear as part of the prior art processing sequence that is shown in FIG. 1 are the regions 22. Layer 16 of polyamide insulator is used for isolation and for planarization during the processing sequence of forming solder bumps. Polyamide however is prone to water absorption, the layer 16 is in contact with the aluminum pad 12 in regions 22. Even minute traces of water that are present in the layer 16 of polyamide insulator causes corrosion of the surface of the aluminum pad 12 in regions 22, resulting in poor adhesion between the layer 16 of polyamide insulator and the aluminum pad 12.

[0035] U.S. Pat. No. 5,946,590 (Satoh) shows a bump process using 2 organic layers for plating.

[0036] U.S. Pat. No. 5,492,235 (Crafts et al.) shows a bump creation process using UBM, and creating the bump by evaporation.

[0037] U.S. Pat. No. 5,903,058 (Akram) and U.S. Pat. No. 6,028,011 (Takase et al.) show other related bump processes.

[0038] A principle objective of the invention is to create a solder bump whereby no corrosion occurs in the surface of the aluminum contact pad to which the solder bump is connected.

[0039] Another objective of the invention is to provide improved adhesion between the structure of the solder bump and the underlying contact pad of aluminum by creating a layer of negative photoresist that is in contact with a portion of the surface of the underlying aluminum contact pad.

[0040] Yet another objective of the invention is to provide a method that improves the insulation of an aluminum contact pad and the regions that are adjacent to the aluminum contact pad.

[0041] In accordance with the objectives of the invention a new method and processing sequence is provided for the formation of solder bumps that are in contact with underlying aluminum contact pads. Using the process of the invention, a patterned layer of negative photoresist is interposed between a patterned layer of PE Si₃N₄and a patterned layer of polyamide insulator. The patterned negative photoresist partially overlays the aluminum contact pad and prevents contact (that was created using the prior art processing sequence) between the layer of polyamide insulator and the aluminum contact pad. By forming this barrier (between the polyamide insulator and the aluminum contact pad) no moisture that is contained in the polyamide insulator can come in contact with the aluminum contact pad, therefore no corrosion in the surface of the aluminum contact pad will occur.

[0042] FIG. 1 shows a cross section and the therefrom following processing flow of the creation of a prior art solder bump.

[0043] FIG. 2 shows a cross section and the therefrom following processing flow of the creation of a solder bump of the invention.

[0044] It must be noted that the use of polyimide films as interlevel dielectrics has been pursued as a technique for providing partial planarization of a dielectric surface. Polyimides offer the following characteristics for such applications:

[0045] they produce surfaces in which the step height of underlying features is reduced, step slopes are gentle and smooth

[0046] they are available to fill small openings without producing the voids that occur when low-temperature CVD oxide films are deposited

[0047] cured polyimide films can tolerate temperatures of up to 500 degrees C. without degradation of their dielectric film characteristics

[0048] polyimide films have dielectric breakdowns which are only slightly lower than that of SiO₂

[0049] the dielectric constant of polyimides is smaller than that of silicon nitride and of SiO₂

[0050] the process used to deposit and pattern polyimide films is relatively simple.

[0051] A layer of polyimide can be deposited by spin-on coating and can be cured after deposition at for instance 400 degrees C. for 1 hour in a vacuum or nitrogen ambient.

[0052] Referring now specifically to FIG. 2, there is shown a cross section of the formation of a solder bump using the process of the invention.

[0053] Layer 10 is the surface of a semiconductor layer, a contact pad 12 has been created on surface 10. Surface 10 will typically be the surface of a semiconductor substrate, the surface of an interconnect substrate and the like. The essence of surface 10 is that a contact pad 12 has been created on this surface, electrical contact must be established with contact pad 12 by means of an overlying solder bump. Contact pad 12 therefore serves as interface between the solder bump and electrical interconnects that are provided in the surface of layer 10.

[0054] A layer 14 of Plasma Enhanced silicon nitride (PE Si₃N₄) is deposited over the surface of layer 10 and of contact pad 12.

[0055] Insulating layers such as silicon oxide and oxygen-containing polymers are deposited over the surface of various layers of conducting lines in a semiconductor device or substrate to separated the conductive interconnect lines from each other, the insulating layers can be deposited using Chemical Vapor Deposition (CVD) techniques. The insulating layers are deposited over patterned layers of interconnecting lines where electrical contact between successive layers of interconnecting lines is established with metal vias created for this purpose in the insulating layers. Electrical contact to the chip is typically established by means of bonding pads or contact pads that form electrical interfaces with patterned levels of interconnecting metal lines. Signal lines and power/ground lines can be connected to the bonding pads or contact pads. After the bonding pads or contact pads have been created on the surfaces of the chip package, the bonding pads or contact pads are passivated and electrically insulated by the deposition of a passivation layer over the surface of the bonding pads. A passivation layer can contain silicon oxide/silicon nitride (SiO₂/Si₃N₄) deposited by CVD. The passivation layer is patterned and etched to create openings in the passivation layer for the bonding pads or contact pads after which a second and relatively thick passivation layer can be deposited that further insulation and protection of the surface of the chips from moisture and other contaminants and from mechanical damage during assembling of the chips.

[0056] Various materials have found application in the creation of passivation layers. Passivation layer can contain silicon oxide/silicon nitride (SiO₂/Si₃N₄) deposited by CVD, passivation layer can be a photosensitive polyimide or can comprise titanium nitride. Another material often used for passivation layer is phosphorous doped silicon dioxide that is typically deposited over a final layer of aluminum interconnect using a Low Temperature CVD process. In recent years, photosensitive polyimide has frequently been used for the creation of passivation layers. Conventional polyimides have a number of attractive characteristics for their application in a semiconductor device structure, which have been highlighted above. Photosensitive polyimides have these same characteristics but can, in addition, be patterned like a photoresist mask and can, after patterning and etching, remain on the surface on which it has been deposited to serve as a passivation layer. Typically and to improve surface adhesion and tension reduction, a precursor layer is first deposited by, for example, conventional photoresist spin coating. The precursor is, after a low temperature pre-bake, exposed using, for example, a step and repeat projection aligner and Ultra Violet (UV) light as a light source. The portions of the precursor that have been exposed in this manner are cross linked thereby leaving unexposed regions (that are not cross linked) over the bonding pads. During subsequent development, the unexposed polyimide precursor layer (over the bonding pads) is dissolved thereby providing openings over the bonding pads. A final step of thermal curing leaves a permanent high quality passivation layer of polyimide over the substrate.

[0057] The preferred material of the invention for the deposition of layer 14 of passivation is Plasma Enhanced silicon nitride (PE Si₃N₄), deposited using PECVD technology at a temperature between about 350 and 450 degrees C. with a pressure of between about 2.0 and 2.8 Torr for the duration between about 8 and 12 seconds. Layer 14 of PE Si₃N₄can be deposited to a thickness between about 200 and 800 Angstrom.

[0058] Layer 14 of PE Si₃N₄is next patterned and etched to create an opening in the layer 14 that overlays and aligns with the underlying contact pad 12.

[0059] The etching of layer 14 can use Ar/CF₄as an etchant at a temperature of between about 120 and 160 degrees C. and a pressure of between about 0.30 and 0.40 Torr for a time of between about 33 and 39 seconds using a dry etch process.

[0060] The etching of layer 14 can also use He/NF₃as an etchant at a temperature of between about 80 and 100 degrees C. and a pressure of between about 1.20 and 1.30 Torr for a time of between about 20 and 30 seconds using a dry etch process.

[0061] A layer 15 of negative photoresist is next deposited over the surface of the layer 14 of PE Si₃N₄. The layer 15 of photoresist can be deposited to a thickness of between about 300 and 800 Angstrom.

[0062] The methods used for the deposition and development of the layer 15 of negative photoresist uses conventional methods of photolithography. Photolithography is a common approach wherein patterned layers are usually formed by spinning on a layer of photoresist, projecting light through a photomask with the desired pattern onto the photoresist to expose the photoresist to the pattern, developing the photoresist, washing off the undeveloped photoresist, and plasma etching to clean out the areas where the photoresist has been washed away. The exposed resist may be rendered insoluble (positive-working) and form the pattern, or insoluble (negative working) and be washed away.

[0063] Before layer 15 of negative photoresist is patterned and etched, a layer 16 of polyamide insulator is deposited over the surface of the layer 15 of negative photoresist. Layer 16 can be deposited using methods of spin-on coating whereby this layer 16 of PI can be deposited to a thickness between about 300 and 800 Angstrom.

[0064] The layer 15 of negative photoresist will, upon patterning and etching, remain in place over the surface of pad 12 in an area that has the shape of the perimeter of the aluminum pad 12 and that is slightly removed from this perimeter toward the center of pad 12. Negative photoresist will, upon exposure to UV light, harden which is a quality that may be of value in selectively creating a firmer layer 15 of negative photoresist overlying the contact pad 12.

[0065] After both layers 15 (of negative photoresist) and 16 (of PI) have been deposited, both layers are patterned and etched forming a stacked layer of negative photoresist and PI overlying the aluminum pad 12.

[0066] Polyamide insulating layer 16 can be anisotropically etched with a plasma gas containing carbon tetrofluoride (CF₄) as an etchant using a commercially available parallel plate RIE etcher or an Electron Cyclotron Resonance (ECR) plasma reactor. The preferred processing conditions for the etching of insulating layer 16 are as follows: etchant used: CF₄/CHF₃, flow rate about 25 sccm, temperature about 40 degrees C., pressure about 225 mTorr, time of the etch between about 160 and 220 seconds.

[0067] Layer 15 of negative photoresist can be etched by applying O₂plasma and then wet stripping by using H₂SO₄, H₂O₂and NH₄OH solution. Sulfuric acid (H₂SO₄) and mixtures of H₂SO₄with other oxidizing agents such as hydrogen peroxide (H₂O₂) are widely used in stripping photoresist after the photoresist has been stripped by other means. Wafers to be stripped can be immersed in the mixture at a temperature between about 100 degrees C. and about 150 degrees C. for 5 to 10 minutes and then subjected to a thorough cleaning with deionized water and dried by dry nitrogen. Inorganic resist strippers, such as the sulfuric acid mixtures, are very effective in the residual free removal of highly postbaked resist. They are more effective than organic strippers and the longer the immersion time, the cleaner and more residue free wafer surface can be obtained.

[0068] The photoresist layer 15 can also be partially removed using plasma oxygen ashing and careful wet clean. The oxygen plasma ashing is heating the photoresist in a highly oxidized environment, such as an oxygen plasma, thereby converting the photoresist to an easily removed ash. The oxygen plasma ashing can be followed by a native oxide dip for 90 seconds in a 200:1 diluted solution of hydrofluoric acid.

[0069] After the two layers 15 (photoresist) and 16 (PI) have been patterned and etched, both layers are cured. The curing and cross-linking of layers 15 and 16 provides extra protection to the device circuitry and can be performed in a N₂gas ambient at a temperature of between about 300 and 400 degrees C. for a time period between about 1.5 and 2.5 hours and a pressure of 760 Torr.

[0070] At this point in the processing sequence, there exist two layers, that is a layer of patterned negative photoresist (layer 15) and a layer of PI (layer 16), both layers overlying the patterned layer 14 of PE Si₃N₄, both layers 15 and 16 have been cured and an opening has been created through both layers over which the solder ball must be formed. The process is, at this point, ready to create the UBM layer and the solder bump.

[0071] The next step in the process of the invention is therefore the creation of the layer 18 of UBM by vacuum evaporation, this layer is as yet to be patterned and etched. The surface of layer 18 of UBM is, after its blanket deposition and before patterning of the layer of UBM, selectively plated with solder. This solder plating is aligned with the aluminum pad 12. UBM layer 18 is then etched using the plated solder as a mask. This removes the layer of UBM from areas adjacent to the contact pad 12 and leaves only enough of the UBM in place to form an electrical interface between the solder bump and the contact pad 12. Standard RIE procedures, using Cl₂—BCl₃as etchant, can be used to etch the UBM layer 18.

[0072] A flux is next applied to the selectively plated solder and the solder is melted in a reflow surface under a nitrogen atmosphere, forming the spherically shaped solder bump 20 that is shown in FIG. 2.

[0073] It is clear from FIG. 2 that the surface of the aluminum pad 12 that underlies the layer 16 of polyamide insulator is separated from layer 16 by a layer 15 of negative photoresist. This separation prevents any moisture that is present in the layer 16 from coming in contact with the aluminum pad 12, thus preventing any corrosion of the surface of the aluminum pad 12.

[0074] The process of the invention can be summarized as follows:

[0075] deposit a layer of PE SiN over the semiconductor surface, including the surface of the aluminum pad

[0076] pattern the deposited layer of PE SiN forming an opening with a first diameter in the layer of PE SiN that aligns with the aluminum pad, partially exposing the surface of the aluminum pad

[0077] apply a coating of negative photoresist over the patterned layer of PE SiN, including the exposed surface of the aluminum pad

[0078] apply a coating of polyamine insulator over the surface of the negative photoresist

[0079] pattern and etch the layers of photoresist and polyamine insulator, forming a stack of photoresist and polyamine overlying the semiconductor surface while forming an opening with a second diameter through both layers that aligns with the aluminum pad; the above second diameter is smaller than the above first diameter, resulting in the negative photoresist partially overlying the surface of the aluminum pad and forming an interface between the aluminum pad and the layer of polyamide

[0080] curing the layers of photoresist and polyamine insulator

[0081] depositing a layer of UBM over the surface of the layer of polyamine insulator, the layer of UBM is in electrical contact with the aluminum pad

[0082] selectively depositing solder over the surface of the layer of UBM, the solder deposition is aligned with the underlying aluminum pad

[0083] etching the layer of UBM using the deposited solder as a mask, thus preventing electrical shorts between layers of UBM that are formed for different solder bumps

[0084] applying a flux or paste to the solder, and

[0085] flowing the solder, forming the solder bump.

[0086] Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. It is therefore intended to include within the invention all such variations and modifications which fall within the scope of the appended claims and equivalents thereof.