CN101159253A - Under bump metallurgy structure, wafer structure and method for forming the wafer structure - Google Patents

Abstract

The invention discloses an under bump metallurgy structure, a wafer structure and a method for forming the wafer structure. The under bump metallurgy structure includes an adhesion layer, a barrier layer and a wetting layer. The adhesion layer is arranged on a contact pad of a wafer, the barrier layer is arranged on the adhesion layer, and the wetting layer is arranged on the barrier layer. The adhesion layer is made of nickel containing boron, the barrier layer is made of cobalt, and the wetting layer is made of gold.

Description

UBM layer structure, wafer structure and method for forming the wafer structure

【Technical field】

The present invention relates to an under-bump metal layer structure, a wafer structure and a method for forming the wafer structure, and in particular to an under-bump metal layer structure and a wafer structure formed by electroless plating technology And a method for forming the wafer structure.

【Background technique】

In semiconductor packaging technology, common chip connection technologies include flip chip, wire bonding, and tape automated bonding to electrically connect the chip to the substrate. Among them, the flip chip bonding technology uses solder bumps as the medium for the electrical connection between the chip and the substrate. Compared with the wire bonding and automatic tape bonding methods, the flip chip bonding technology has a shorter electrical connection path. , and has better electrical connection quality, so that bumps are more and more widely used in semiconductor packaging technology.

In the traditional bump forming method, an Under Bump Metallurgy (UBM) is formed on the surface of the chip and covers the copper pads on the surface of the chip, usually by sputtering or electroplating. way to form the under bump metal layer. Steps such as coating a photoresist layer and photolithography are then carried out, so that the size of the UBM layer approximately corresponds to the size of the copper pad. Then, the photoresist layer is peeled off, and solder paste is printed on the UBM layer. Finally, the solder paste is reflowed, so that the tin particles in the solder paste are melted into tin soup, and cooled and solidified into bumps.

The traditional bump forming method has complicated process steps, which cannot effectively reduce the process cost. Therefore, the industry has developed a bump forming method that does not require photolithography. This method of bump formation without a photolithography step includes the steps of electroless plating a nickel layer on the copper pad, and electroless plating a palladium layer on the nickel layer, and then A wetting layer of eg gold material is formed. Then, bumps are formed through the steps of printing and reflow. Electroless nickel plating is a chemical reduction reaction in which nickel ions are reduced and deposited on a catalytic surface using a reducing agent (such as sodium hypophosphite) in solution. In terms of interfacial reactions, electroless nickel plating is widely used in electronic packaging to act as a diffusion barrier in solder bumps due to its good barrier effect on copper diffusion. In the step of electroless nickel plating, the wafer (wafer) is generally immersed in the plating solution, nickel ions are provided by nickel sulfate (NiSO4) in the plating solution, and sodium hypophosphite (NaH2PO2) is used as a reducing agent to make nickel ions It is reduced to nickel metal, and the nickel metal is used as a catalyst to carry out an autocatalytic reaction, thereby depositing a phosphorus-containing nickel layer (Ni-P) on the aluminum or copper pad. This electroless plating method has the advantages of uniform coating thickness, low porosity, fine crystallization, high hardness, and good solderability. However, since this electroless plating process is affected by parameters such as the composition of the plating solution and its concentration, operating temperature, and pH value, when performing high-temperature process steps such as reflowing solder paste, it is easy to be affected by high temperature. The interface between nickel layers forms a phosphorus-rich crystalline intermetallic phase (Inter-Metallic Compound, IMC). In the displacement reaction of electroless plating, when a nickel atom with a small size dissolves away (oxidizes), two relatively large gold atoms deposit (reduce), which will cause a comprehensive Pushing misalignment, thus causing a lot of voids and porosity in the interface between nickel and gold, even containing solution, which is likely to cause continued passivation and oxidation of the nickel layer, affecting the quality of the interface. In addition, when the phosphorus content in the nickel layer is high, it is easy to cause a decrease in weldability, so the general practice is to control the phosphorus content between 7% and 9%. The following is an example of the interface between the bump and the phosphorus-containing nickel layer. Please see Figure 1 and Figure 4 at the same time. Figure 1 shows a schematic diagram of the interface between the traditional bump and the electroless nickel layer. Figure 4 is a diagram 1 electronically scanned photograph. According to scanning electron microscopy (SEM) and component analysis, a phosphorus-rich crystalline intermetallic phase 102 is formed between the bump 103 and the phosphorus-containing nickel layer 101 . Since the phosphorus-rich crystalline intermetallic phase 102 is brittle, the joint strength between the bump 103 and the chip is reduced. During chip bonding, sealing or product testing, the crystalline intermetallic phase 102 is prone to fracture, which reduces the yield and reliability of the product.

【Content of invention】

The main purpose of the present invention is to provide an UBM layer structure, a wafer structure and a method for forming the wafer structure, which can improve the strength of the joint, and further improve the reliability and quality of the product.

To achieve the foregoing objectives of the present invention, the present invention provides an UBM layer structure, including an adhesion layer, a barrier layer, and a wetting layer. The adhesive layer is arranged on a bonding pad of a wafer, and the material of the adhesive layer is nickel containing boron. The barrier layer is arranged on the adhesion layer, and the material of the barrier layer is cobalt. The wetting layer is arranged on the barrier layer, and the material of the wetting layer is gold.

The invention also proposes a wafer structure, including a wafer, a pad, a passivation layer and an UBM layer. The pads are disposed on the wafer, and the passivation layer covers the wafer and exposes part of the pads. The UBM layer includes an adhesion layer, a barrier layer and a wetting layer. The adhesive layer is disposed on the pad, and the material of the adhesive layer is nickel containing boron. The barrier layer is arranged on the adhesion layer, and the material of the barrier layer is cobalt. The wetting layer is arranged on the barrier layer, and the material of the wetting layer is gold.

The present invention further proposes a method for forming a wafer structure. Firstly, a wafer is provided, the surface of the wafer is provided with a pad and covered with a passivation layer, and the passivation layer exposes a part of the pad. Secondly, an adhesion layer is electrolessly plated on the contact pad, and the material of the adhesion layer is nickel containing boron. Then, a barrier layer is electrolessly plated on the adhesion layer, and the material of the barrier layer is cobalt. Then, a wetting layer is formed on the barrier layer, and the material of the wetting layer is gold.

Compared with the prior art, the UBM layer structure, the wafer structure, and the method for forming the wafer structure in the present invention use boron-containing nickel, cobalt, and gold as materials for the adhesion layer, barrier layer, and wetting layer, respectively. After thermal cycle steps between the bump and the pad of the wafer, no brittle intermetallic phase will be generated, the mechanical strength of the contact is improved, and the reliability of the product is further improved. Secondly, since the adhesion layer, the barrier layer and the wetting layer are formed by electroless plating, the process steps can be reduced and the manufacturing cost can also be saved. Furthermore, using cobalt as the material of the barrier layer can reduce the cost and improve the electrical performance compared with the traditional way of using palladium as the material of the barrier layer.

In order to make the above content of the present invention more obvious and understandable, the preferred embodiments are specifically cited below, together with the accompanying drawings, and are described in detail as follows:

【Description of drawings】

Figure 1 shows a schematic diagram of the interface between a conventional bump and an electroless nickel plating layer;

2A shows a schematic diagram of disposing pads and passivation layers on the surface of a silicon wafer (silicon wafer) according to a preferred embodiment of the present invention;

FIG. 2B shows a schematic diagram of forming an adhesion layer on the wafer of FIG. 2A;

FIG. 2C shows a schematic diagram of forming a barrier layer on the adhesive layer of FIG. 2B;

FIG. 2D shows a schematic diagram of forming a wetting layer on the barrier layer of FIG. 2C;

2E is a schematic diagram of forming a solder layer on the wetting layer of FIG. 2D;

FIG. 3 shows a schematic diagram of a wafer structure according to a preferred embodiment of the present invention;

FIG. 4 is an electronic scanning photo of FIG. 1; and

FIG. 5 is a scanning electron photograph of the interface between the UBM layer and the bump according to a preferred embodiment of the present invention.

【Detailed ways】

A wafer structure according to a preferred embodiment of the present invention includes a wafer, a pad, a passivation layer, and an UBM layer. In this embodiment, the pads are disposed on the surface of the wafer, and the passivation layer covers the surface of the wafer and exposes part of the pads. The UBM layer is disposed on the pad and includes an adhesion layer, a barrier layer and a wetting layer. The method for forming the wafer structure according to this embodiment is described in detail as follows.

Please refer to FIGS. 2A to 2E at the same time. FIG. 2A shows a schematic diagram of a wafer according to a preferred embodiment of the present invention; FIG. 2B shows a schematic diagram of forming an adhesion layer on the wafer of FIG. 2A; FIG. 2C is shown in FIG. 2D is a schematic diagram of forming a wetting layer on the barrier layer of FIG. 2C ; FIG. 2E is a schematic diagram of forming a solder layer on the wetting layer of FIG. 2D . According to the method for forming a wafer structure in a preferred embodiment of the present invention, a wafer 12 is firstly provided, as shown in FIG. 2A . In this embodiment, the wafer 12 is preferably a silicon wafer, and a pad 14 and a passivation layer 16 are disposed on the surface thereof. The pads 14 are made of, for example, copper or aluminum, and serve as electrical contacts on the wafer 12 . The passivation layer 16 covers the wafer 12 and has a contact window 16a to expose a portion of the pad 14 .

Second, an adhesive layer 22 is electroless plated on the pad 14 . Before the electroless plating of the adhesion layer 22 , the pads 14 are preferably surface treated. Remove the oxide (such as copper oxide) and organic and inorganic substances on the surface of the pad 14, and use materials such as zinc (zinc) or cobalt (cobalt) to activate the surface of the pad 14, and then the wafer 12 is immersed in a nickel-boron plating solution for electroless plating of the adhesion layer 22 . In this embodiment, in the electroless plating bath, nickel metal (Ni—B) containing boron is plated on the surface of the activated contact pad 14 by utilizing the self-catalytic reaction of nickel metal. The nickel-boron material layer formed here is the adhesion layer 22 , and its thickness is about 1-15 micrometers (um).

Next, as shown in FIG. 2C , a barrier layer 24 is electrolessly plated on the adhesion layer 22 to prevent outward diffusion of the nickel metal of the adhesion layer 22 . The thickness of the barrier layer 24 in this embodiment is about 0.15-7.5 microns, and preferably cobalt (Co) is used as the material of the barrier layer 24, compared to utilizing palladium as the material of the barrier layer 24, the cobalt material The barrier layer 24 has a lower material cost and can improve the electrical contact characteristics between the bumps and the pads 14 formed in subsequent processes.

As shown in FIG. 2D , a wetting layer 26 is then formed on the barrier layer 24 . The wetting layer 26 is used to prevent the oxidation of the barrier layer 24 and improve the wettability of the bumps. In this embodiment, the material of the wetting layer 26 is gold (Au), and is preferably formed on the barrier layer 24 by electroless plating, and its thickness is about 0.05-0.15 microns. However, the wetting layer 26 can also be formed on the barrier layer 24 by, for example, immersion plating. After forming the wetting layer 26 , the adhesion layer 22 , the barrier layer 24 and the wetting layer 26 form an under bump metallurgy (UBM) 20 .

Next, a solder layer 30 is formed on the wetting layer, as shown in FIG. 2E. In this embodiment, the solder layer 30 is printed (printing) on the wetting layer 26, and its material is preferably gold, but the material of the solder layer 30 can also be tin (Sn), lead (Pb), nickel, gold , silver (Ag), copper or combinations thereof.

The method for forming the wafer structure in this embodiment is followed by a step of reflowing the solder layer 30 , and the solder layer 30 forms a bump after reflowing. Those skilled in the art of the present invention can understand that the method of forming the bump is not limited to the above-mentioned method of printing and reflowing, and the bump can also be formed on the UBM layer by direct ball planting. The ball planting step can be performed, for example, by using a ball planting machine or a robotic arm, which directly places the bumps on the UBM layer correspondingly, and uses flux to bond the bumps to the UBM layer. Alternatively, the stencil can also be used to align the bumps, and the bumps are correspondingly placed on the UBM layer, and then the bumps are also bonded to the UBM layer by using flux. However, other methods of bonding bumps to the UBM layer commonly used in this field can be applied here.

After the bumps are formed, the wafer structure according to the preferred embodiment of the present invention is completed. Please refer to FIG. 3 , which shows a schematic diagram of a wafer structure according to a preferred embodiment of the present invention. Wafer structure 100 includes wafer 12, pads 14, passivation layer 16, UBM layer 20, and bumps 30'. The UBM layer 20 includes an adhesion layer 22 , a barrier layer 24 and a wetting layer 26 .

In this embodiment, the adhesion layer 22, the barrier layer 24, and the wetting layer 26 are all electroless plating layers. When the electroless plating of the material layers of the UBM layer 20 is performed in the plating solution, these material layers have approximately the same width. After the UBM layer 20 is formed, the steps of coating photoresist, photoresist and etching are not required. In addition, the material of the adhesive layer 22 in the UBM layer 20 is nickel containing boron, and when the wafer structure 100 undergoes heat treatment-related process steps, for example, when the solder layer 30 is reflowed to form the bump 30 ′, it can avoid A phosphorus-rich crystalline intermetallic phase is formed between the bump 30 ′ and the contact 14 . Please also refer to FIG. 5 , which is an electronic scanning photograph of the interface between the UBM layer and the bump according to a preferred embodiment of the present invention. According to the results of experiments and component analysis, there is no brittle phosphorus-rich crystalline intermetallic phase formed between the UBM layer 20 and the bump 30 ′, and the junction structure is smooth, so that the UBM layer 20 and the bump The blocks 30 ′ have good bonding properties, which further improves the stability of bonding between the bumps 30 ′ and the pads 14 .

In addition, the material of the barrier layer 24 in this embodiment is cobalt, which has a lower material cost compared with the conventional material using palladium as the material of the barrier layer. Furthermore, according to experimental measurement results, the sheet resistance of the UBM layer with the traditional Ni/Pd/Au structure is approximately 8.6% higher than that of the UBM layer with the Ni/Co/Au structure. Therefore, compared with the conventional barrier layer of palladium material, the barrier layer 24 of cobalt material in this embodiment has better electrical performance.

On the other hand, in the method for forming the wafer structure according to the preferred embodiment of the present invention, one pad 14 and the corresponding formation of one UBM layer 20 are taken as an example for illustration. However, in practical applications, the surface of the wafer 12 preferably has a plurality of pads 14 arranged in an array, and before the wafer 12 is singulated (wafer sawing), the wafer level (wafer level) process is used to A plurality of UBM layers 20 are correspondingly formed on the pads 14 . Furthermore, the method for forming the wafer structure according to the preferred embodiment of the present invention is, for example, applied to Wafer Level Chip Size Package (WLCSP) technology and flip chip package technology.

The above-mentioned UBM layer structure, the wafer structure and the method for forming the wafer structure according to the preferred embodiment of the present invention respectively use boron-containing nickel, cobalt and gold as materials for the adhesion layer, barrier layer and wetting layer , so that the brittle intermetallic phase will not be generated between the bump and the pad of the wafer after the thermal cycle step, the mechanical strength of the contact is improved, and the reliability of the product is further improved. Secondly, since the adhesion layer, the barrier layer and the wetting layer are formed by electroless plating, the process steps can be reduced and the manufacturing cost can also be saved. Furthermore, using cobalt as the material of the barrier layer can reduce the cost and improve the electrical performance compared with the traditional way of using palladium as the material of the barrier layer.

Claims (10)

1. An UBM structure comprising: an adhesion layer, a barrier layer and a wetting layer, wherein the adhesion layer is disposed on a pad of a wafer, the barrier layer is disposed on the adhesion layer, and The wetting layer is arranged on the barrier layer, and it is characterized in that: the material of the adhesion layer is nickel (Ni-B) containing boron, the material of the barrier layer is cobalt (Co), and the material of the wetting layer is gold (Au ). 2. The UBM layer structure according to claim 1, wherein the adhesive layer is an electroless plating layer with a thickness of approximately 1-15 microns (um), and the barrier layer It is an electroless plating layer with a thickness of about 0.15 to 7.5 microns, and the wetting layer is an electroless plating or immersion plating layer with a thickness of about 0.05 to 0.15 microns. 3. A wafer structure comprising: a wafer, a pad, a passivation layer and an UBM layer, wherein the pad is disposed on the wafer, the passivation layer covers the wafer and A portion of the pad is exposed, the UBM layer includes: an adhesion layer, a barrier layer, and a wetting layer, wherein the adhesion layer is disposed on the pad, the barrier layer is disposed on the adhesion layer, and the wetting layer It is arranged on the barrier layer, and is characterized in that: the material of the adhesion layer is nickel containing boron, the material of the barrier layer is cobalt, and the material of the wetting layer is gold. 4. The wafer structure according to claim 3, wherein the adhesive layer is an electroless plating layer with a thickness of about 1-15 microns, and the barrier layer is an electroless plating layer with a thickness of about 0.15-7.5 microns. Micron, the wetting layer is an electroless plating layer or an immersion plating layer, with a thickness of about 0.05-0.15 microns. 5. wafer structure as claimed in claim 3, is characterized in that: this structure also comprises: a bump (bump) that is arranged on this wetting layer, and the material of this bump is tin (Sn), lead (Pb ), nickel, gold, silver (Ag), copper, or combinations thereof. 6. A method for forming a wafer structure, comprising: providing a wafer, the surface of the wafer is provided with a pad and covered with a passivation layer, and the passivation layer exposes a part of the pad; Electroplating (electroless plating) an adhesion layer; electroless plating a barrier layer on the adhesion layer; and forming a wetting layer on the barrier layer, characterized in that: the material of the adhesion layer is boron-containing nickel (Ni -B), the material of the barrier layer is cobalt (Co), and the material of the wetting layer is gold (Au). 7. The method for forming a wafer structure as claimed in claim 6, wherein the thickness of the adhesion layer is approximately 1-15 microns, the thickness of the barrier layer is approximately 0.15-7.5 microns, and the thickness of the wetting layer About 0.05 to 0.15 microns. 8. The method for forming the wafer structure as claimed in claim 6, wherein in the step of forming the wetting layer, the wetting layer is electroless-plated or immersion-plating on the barrier layer. 9. The method for forming the wafer structure as claimed in claim 6, further comprising: printing a solder layer on the wetting layer; and reflowing the solder layer to form a bump. 10 . The method for forming the wafer structure according to claim 6 , further comprising: disposing a bump on the wetting layer by direct ball placement. 11 .

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