TWI234852B - Inspection method of flip-chip bonding surface by applying atomic force microscope (AFM) - Google Patents

Abstract

The present invention relates to one kind of inspection method of flip-chip bonding surface by applying atomic force microscope (AFM), which mainly obtains observation results of atomic force microscope with higher image contrast ratio. The present invention utilizes the technique of ion-beam etch to prepare the flip chip (FC) specimen, wherein while adjusting etch time and ion beam energy, incorporated with a tilting angle, the deformed portion between the solder ball and the pad caused by grinding and polishing can be removed to obtain better morphology in the specimen for the Ni-layer and Cu-layer of the pad, which in turn optimizes analyses of the atomic force microscope (AFM).

Description

1234852 V. Description of the invention (1) [Technical field to which the invention belongs] The present invention relates to a method for detecting a flip-chip interface surface applied to an atomic force microscope (AFM), and particularly to a method for preparing a flip-chip test piece. Better observation. [Previous technology] Traditional packaging is a single 1C package, which requires a lead frame or substrate, die attach, wire bonding, molding, and molding. (Tγ ^ and other systems, the size of 1 C after packaging is several times the chip (Ch ip). The flip chip is sighed with copper (cu) or solder bumps (Bump) on the jc chip. It is used for soldering j. The flip-chip is called flip-chip when it is mounted on the PCB substrate so that the circuit side of the wafer faces downward. The flip-chip bonding (FC) is also called C4 bonding (C〇ntr〇lled c〇Uapse hip) Connection), which was developed by the American IBM Corporation, and belongs to the Pingji Array) bonding, instead of such as wire bonding and tape-and-die bonding technology (TAB) can only provide peripheral type (peripherai Therefore, flip-chip bonding can be applied to extremely high-density fabrication and fabrication processes, and in the future construction wiring and bonding technology, flip-chip bonding technology is expected to have a very high proportion of applications. The basic principle is to first generate 0 (Solder Bump) on 1 (: metal pad of the wafer, After the κ wafer is placed on the structured substrate and the zinc-tin bumps are aligned with the pads of the substrate, the surface tension effect when reflowing (1 mile with fresh tin tin) is used to make the solder balls merge. Form the joint of IC chip and the structure substrate.

There are many types of solder. The common types are 95% lead-5% tin alloy with high melting point, or 51% indium-32.5% bismuth-16.5% tin, 63% lead-37% tin, 50% with low melting point. Lead -50% indium alloy. The method of manufacturing solder bumps is to first coat a glass protective layer on the surface of the IC chip to provide sealing protection and prevent any solder wetting. After opening a pilot hole on the metal pad position, then sputter plating such as chromium-copper-gold ^ Multi-layer metal film (commonly known as Under Bump Metallurgy; UBM) to provide adhesion, diffusion barriers, enhance solder wetting and prevent oxidation, etc., followed by evaporation, immersion (dipping) or ultrasonic spot welding ( (Ultrasonic Soldering) technology is used to plate the bismuth-tin alloy (Pb-Sn) with a thickness of 100 to 125. In the subsequent bonding heat treatment process, the surface tension effect when the solder melts will change the solder layer. Spherical solder bumps. The flip-chip-bonded structural substrate pads also need to be plated with multiple layers of thin metal (referred to as Top Surface Metallurgy; TSM), in order to record the wettability of several pieces during bonding. Tin bumps In the reliability analysis of electronic components, most of them are at the solder joints. Because, after the solder ball is combined with the substrate, the difference between the chemical potentials of the same substance is caused by the movement of the mass atoms, which is the so-called diffusion behavior. Therefore, the intermetallic metal is formed by the relationship of the reaction. Compound (Intermetal 丨 ic compound); The formation of this intermetallic compound will vary in thickness, kind, composition, and properties due to the relationship between 埶 and ^. In the process of electronic assembly, solder joints often break and break. . Therefore, how to establish an effective way to prepare test strips to obtain the main = for atomic force microscope (AFM) photos is the most important thing in this kind of analysis work.

1234852

Page 7 1234852, Description of Invention (4) ^ It can increase the sharpness of photos. The traditional square piece is etched to remove the deformed layer on the test piece. To use a chemical solution to etch a specific test piece, since chemical etching = uses a specific chemical solution method, it must be determined according to the type of material and desired. Second, the selective chemical solution etching of the engraved square, that is, specific == ,,,. The structure has the effect of etching only when different compositions are used. Therefore, when using 舳 = ^ 疋 material or a kind of element or phase of the test piece, the result of the opening x is used to deal with multiple problems. In addition, chemical dissolution :: = uniform problems of injury and pollution are other issues that should be paid attention to. [It is also related to the environment in the "Glass gland strip office electron microscope test strip preparation technology ...", which proposes a kind of: (Grading) and polishing (Polishing) two methods to prepare an electron microscope, but The generation of conformal layers after cutting, grinding and polishing of general test pieces affects the observation of atomic force microscope (AFM). [Summary of the Invention] The main object of the present invention is to provide a flip chip for atomic force microscope (AF Μ). The method of detecting the joint surface. Another purpose is to solve the problem of preparing atomic force microscope (AFM) test strips by traditional chemical method. Another purpose is to solve the deformation layer pair produced by cutting, grinding and polishing of general test strips. Effect of Atomic Force Microscopy (AFM) Observation. The method disclosed in the present invention is to use ion beam money engraving to surface-treat a flip-chip test piece, which mainly uses the interaction between the ion beam energy and money engraving time. Fit, plus a specific test strip and ion beam

Page 8 1234852 V. Inflammation angle of description (5) in order to treat the surface of the test piece. It has the following advantages: ~ Excellent item. Ion beam etching belongs to non-selective etching, so it is suitable for multi-phase test pieces. Even at 7M 2. Since the energy and current of the ion beam can be adjusted, it is suitable for precision etching with low etching rate, so that the damage to the test t will also be reduced. The completed 3. Control of process parameters is more accurate and convenient, and the test piece has high reproducibility. If so, it can also be etched in a small area, so it can be etched for a specific area. [Embodiments] The detailed technical content of the present invention and its preferred embodiments are illustrated in the drawings below. Said: Please refer to the "Figure 1" first, here only the single solder ball (solder bump, or solder bump) in the flip-chip bonding technology and the bonding structure of the metal pad and solder pad connected to it example. It can be understood from the structure in the figure that, generally, a surface of a metal pad (such as a copper pad, Cu pad) 2 of a silicon chip 10 has a layer of nitride (Nitride) dielectric protection layer 30, and one with The contact window of the metal ㈣ communicates with ntaefHQle), which is used for this purpose: The opening of the contact window will be plated with a multi-layer metal thin layer (called Top Surface Metal Lurgy, TSM) to facilitate the solder bump bonding process. Wet, this multi-layer metal thin layer includes: using a physical vapor deposition (physical vapor deposition) method to sequentially generate a metal layer of titanium (Ti) 4o and copper ((: 11) 50), followed by recording (Ni ) Layer 60 'then generates a solder ball 70,

Page 9 1234852 V. Description of the invention (6) This completes the so-called solder bump. On the surface of the PCB chip 80, a solder pad (such as a copper pad, which has a bump phase on the surface of the solder pad 90) corresponding to a solder pad (such as a copper pad) is formed. ΒΓ bonding and the soldering of the flipped wafer 10. The bumps are aligned on the substrate 8: ::: Reflow (Ref low) method to bond them, $ 90. Technology Because the solder joints of materials are often damaged by intermetallic compounds (lnter tray metalcmpund; IMcf) j during reflow (Refl0w), how to accurately observe the micrograph of this part Structure, ', to understand the formation mechanism of this intermetallic compound', and then analyze its composition, state and structure, the impact on the electrical performance and reliability of the IC chip. Therefore, how to establish an effective way to prepare the test piece Obtaining clear AFM photos' is a very important subject in this kind of analysis. The technology of the present invention basically includes two steps, which are: 1 Test piece of flip-chip bonding structure 丨 〇 0 , And after rough grinding and polishing, the thickness of the chip-on-chip test piece is between 1 ~ 2mm. (2 · Using the ion beam 14 etch (ion beam etch) way, the chip-on-chip test piece that has been ground and polished in the previous step 1 〇0, control the energy of the number of ions and the etching time, and cooperate with the incident angle 0 of the incident ion beam 1 0 and the test piece 1 0 0 (as shown in the "Figure 2"), where the incident angle is 25 Degrees to 35 degrees, but the optimal angle is 30 degrees to remove the solder balls and the adjacent metal pads, due to grinding and polishing

Page 10 1234852 V. Description of the invention (7) The deformed part caused surface treatment of the intermetallic compound and the change of each metal layer to obtain the required test piece of atomic force microscope. The present invention uses an ion beam etch method, which mainly controls the energy of the number of ions and the etching time, and cooperates with the incident ion beam. 〇i (for example, using argon (Ar) as the reaction gas of the ion source) ) The pattern of the incident angle Θ with the test piece 丨 〇〇 'is to treat the surface of the test piece; to remove the deformed part caused by grinding and polishing between the solder ball and its adjoining metal pad Part 'especially for intermetallic compounds generated between solder balls (s 〇1 derba 1 1) and their adjoining metal pads (pads) can obtain the shape of the test piece that is more favorable for atomic force microscope analysis. The following example is given as an illustration. First of all, please refer to "Figure 3", which is the AFM image of the test piece after general grinding and polishing. The composition of each layer in the figure is the copper layer of the tin ball 70, the intermetallic compound 102, the nickel layer 91, and the pad 90, from the top to the bottom. As can be seen from the figure, although the differences between the layers can be roughly compared However, the deformation caused by 2 = grinding makes the image of the nickel layer 9 丨 and the pad 9〇 less> Yue Chu, it is not obvious what the nickel layer 91 and the steel layer of the pad 90 are. difference. Therefore, the test piece needs to be further processed. , Is set to 2.0. The conditions set in this example are: a mode in which the incident current of the fixed ion beam is I = 0 micro-female (uA), and the mode for polishing the flip-chip test piece i0 ^ is 30 degrees, Use 4, 5, 6, 7 respectively to hit the test piece for 3 minutes with the shooting angle Θ, and then show via atomic force :: =): can FM image. The respective AρM images obtained are "45th

1234852 V. Description of the invention (8) ~ 7A, 8A picture "," Figures 6B, 7B, 8B "are AFM enlarged images of the nickel layer and copper layer of the solder pad of" pictures µ, 7A, 8A ", respectively. After processing the test piece through the above-mentioned engraving process, the grain shape and arrangement of the copper layer of the recording layer 91 and the bonding pad 90 can be clearly observed. For the nickel layer 91, the growth pattern of the columnar crystals can be clearly distinguished. In addition, it can be found that the recording of the copper layer near the pad 90 and the 102 layer of the intermetallic compound have different characteristics. The grain size, nickel near the pad 90, has smaller grains, and nickel near the intermetallic compound 102 layer has a larger grain size. In addition, for the copper layer of the pad 90, The shape of the grains can be clearly seen. Therefore, the observation of the formation mechanism 介 of the intermetallic compound 102 as a basis for the reliability analysis of the electronic assembly will be of great help. In summary, the AFM images obtained from the above different etching conditions can be known, the best AFM scanning images, the conditions can be set to the ion beam should be selected 7keV 'incidence current is 200 microamperes, and Test piece inclination angle. -Of course, the above example is used to illustrate the preferred embodiment of the present invention: ^^ :: 2 = conditional parameters, including: ion beam energy, etch j, and fresh material bumps used for flip-chip bonding and their The connected metal 塾 "::: The above is only a preferred embodiment of the present invention and it has been implemented. That is, all changes and modifications in accordance with the scope of the patent application for the present invention are covered by the scope of the patent for the present invention. Special

^ 234852 Simple illustration of the diagram ^ Simple illustration of the diagram] Figure 1, the joint of the Yan φ solder ball and the metal pads and solder pads connected to it.

Q0 ^ \ P figure. Schematic diagram of the incident angle 0 between the ion beam and the test piece. Figure 3 shows the test moon and image. AFM image after grinding, polishing—like grinding and polishing $ 4Picture ’shows 4 + a a

image. Figure 5 of the AFM of the test piece bombarded by the energy of thousands of keV (keV) is shown in 5 images. Afm of the test piece bombarded by energy of keV (picture 6A)

image. The afM of the test piece bombarded by the energy of volts (keV)

Figure 7A is an enlarged AFM image of the 0, catalogue layer, and copper layer of the solder pad. image. The afM of the test piece that had been bombarded with energy of 7 kilovolts (keV). Figure 7B is 8A ®

Fig. 8A is, θ ,. AFM image of the round brocade layer and the detailed copper layer. image. ., AfM Figure 8β of the test piece bombarded by energy of ^^ 8 kilovolts (keV), which is Figure 8A # [Schematic> 4 »AFM enlarged image of scale layer and copper layer of pad. 1 0 · Description of chain number] 20 30 Chip Metal pad Dielectric protection layer

Page 131234852 Brief description of drawings 40: Titanium (Ti) 5 0: Copper (Cu) 6 0, 9 1 · Ni (N i) layer 7 0: Tin ball 80: Substrate 9 0: Pad 1 0 0 : Test piece 101: Ion beam 1 0 2: Intermetallic compound 0: Incident angle Φ

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Claims (1)

1234852 1. A method for detecting a flip-chip joint surface applied to an atomic force microscope (AFM), which is used to process a test piece of a flip-chip bond (FC chip structure), including: cutting to obtain the flip-chip bond Structure, and grinding and polishing into a flip-chip test piece; and in the manner of ion beam etch, the flip-chip test piece which has been polished and polished in the previous step is incident on the ion beam and the test piece at an incident angle. Surface treatment is performed to obtain the required test piece of the atomic force microscope (AF M). 2. The method for detecting a flip-chip joint surface applied to an atomic force microscope (can ^ AFM) as described in item 1 of the scope of patent application, wherein the thickness of the flip-chip test piece is between 1 and 2 mm. 3. The method for detecting a flip chip joint surface applied to an atomic force microscope (AFM) according to item 1 of the scope of the patent application, wherein the ion beam engraved ion beam energy is 4 keV to 8 keV, and the etching time is 3 cents. 4 · The method for detecting a flip-chip joint surface applied to an atomic force microscope (AFM) as described in item 1 of the scope of the patent application, wherein the incident angle is 25 degrees to 35 degrees, and the optimal angle is 30 degrees.