JP2003209212A - Magazine type plasma cleaning system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform plasma cleaning while containing the surface of BGA, CSP, MCM substrate or the lead frame for QFP or SOP in a magazine. <P>SOLUTION: Distance between a high frequency power applying electrode 1' and a ground electrode 2 is variable in the range of 5-25 cm and the frequency of high frequency power is selected optimally in the range of 10-160 kHz in order to prevent the occurrence of a sheath in each substrate 4 contained in a magazine 3. Plasma is generated uniformly on the surface of each substrate in the magazine 3 and the surface of each substrate or the lead frame is cleaned. Bonding reliability of gold wire is enhanced while enhancing bonding reliability of molding resin. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

The present invention relates to an IC chip or a semiconductor chip such as BGA, CSP, MCM or QFP, SOP.
In the process of assembling into packages such as BGA, CSP resin substrate or QFP, S for mounting IC chips etc.
The plasma cleaning is applied to the OP lead frame before wire bonding or as mold pretreatment to improve the reliability of the wire bonding or the bonding of the mold resin to the substrate. .

[0002]

2. Description of the Related Art Conventionally, contamination of a resin substrate or a lead frame with an organic substance is generally performed by using a mixed gas of oxygen gas (O ₂ ) and argon gas (Ar) with high frequency applied power of 13.56 MHz. Plasma is generated and removed. In addition, metals and metal compounds are argon gas (A
r) and a fluorine compound such as CF ₄ , a chlorine-based compound such as C ₂ Cl ₃ F ₃ or a mixed gas of hydrogen gas (H ₂ ) is used to similarly generate plasma to remove oxides and the like. ing. When removing these unnecessary substances, there are a method of removing by a chemical reaction by plasma particles, a method of accelerating charged particles such as ions by an electric field to collide with a sample, and removing by RIE effect (reactive ion etching). Has been done.

Particularly in IC packages such as BGA and CPS, the lower the Au plating is, the higher the solder connection reliability is because the cost of the substrate is reduced and the connection reliability of the solder balls is improved. 3 to plating
After applying a thickness of 5 μm, the Au plating was about 0.2 mm thick by the conventional reduction electroless plating method, but 0.0 by the substitution type electroless plating method (flash plating method).
Since it has come to be thinly plated with 5 to 0.03 μm,
There is a problem with the connection reliability of the wire bonder. Therefore, not only the organic substances on the substrate surface are removed, but also Ni compounds etc. on the Au plating are removed by plasma treatment to improve the reliability of wire bonding and the reliability of connection of solder balls on the substrate surface. Is being done.

Further, conventionally, in order to remove a metal compound such as Ni on Au plating, a parallel plate electrode is installed in a chamber capable of reducing pressure, and Ar is applied at a gas pressure of about several tens Pa.
Plasma is generated by applying high frequency power of 13.56 MHz using gas to generate a negative self-bias voltage on the high frequency application electrode side, and the substrate to be cleaned stored in the magazine is placed on this electrode. Approximately 1 or 2 sheets are taken out and installed individually, and RIE such as Ar ion is performed.
Depending on the effect, these metal compounds such as Ni (O
H) ₂ , NiO, Ni ₂ O _{3 and the} like are removed to increase the Au concentration and improve the bondability.

A package substrate or lead frame generally used in a packaging process for semiconductor chips or IC chips is stored in a magazine so that it can be easily transported (generally, about 20 to 30 substrates can be stored). However, when cleaning with plasma, as shown in FIG. 1, the substrates (4) are transferred one by one from one magazine (3) on one side. After taking out, one or two sheets are placed on the high-frequency power applying electrode (1) one by one in a single plasma treatment, and then the chamber is evacuated from the exhaust port (10). Gas is introduced from the gas introduction port (9), electric power is applied to generate plasma, the substrate is plasma-treated, and then the substrate is stored again in another magazine (3 ') to remove each substrate. After all plasma processing, they are transported to the next process for each magazine.

[0006]

However, in such a conventional method, since the substrates are taken out from the magazine one by one and processed by plasma, the productivity is poor and there is a problem in cost, which is more rational. In order to increase the productivity more efficiently, it is desired to perform plasma processing in the magazine while the substrates are stored in the magazine, instead of taking out the substrates one by one and performing the plasma processing.

However, in the conventional method, generally 13.5.
Electrons can follow voltage fluctuations because a high frequency such as 6 MHz is used as electric power, but since ions cannot follow because they are heavier than electrons, ions are trapped in the gas space between each electrode The gas space between them has a positive potential, and correspondingly, a negative potential, that is, a negative self-bias voltage is generated on the side of the high-frequency power application electrode having a small electrode area. Therefore, the ions generated in the plasma are electrically attracted to the substrate surface on this electrode,
Since the unnecessary metal compounds on the surface of the substrate are removed by physical sputtering due to the collision of the ions on the surface of the substrate, when each substrate is stored in multiple layers in the magazine, each substrate is exposed to the plasma. There is a problem that it is difficult to remove these unnecessary metals or metal compounds. In order to solve such a problem, the present invention provides plasma processing in a state where substrates are collectively stored in a magazine.

[0008]

The method of the present invention is not a method of processing the substrates one by one by taking them out from the magazine, but the reliability of bonding by performing the plasma processing with the substrates stored in the magazine. This is a magazine-type plasma cleaning method that aims to improve the adhesiveness between the substrate and the mold resin at the same time as improving the productivity. Became possible.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described with reference to FIG. Aluminum or SU that can be decompressed as shown in Fig. 2.
A thickness composed of a material such as ceramics on each opposing electrode surface between the parallel plate electrode type high frequency power applying electrode (1 ′) installed in the chamber (7) made of S and the installation electrode (2) An insulating plate having a size of about 1 mm to 3 mm and an area equivalent to the size of each electrode is placed in contact with each electrode surface. A large number of small diameter holes are formed in each electrode and each insulating plate, and the plasma reaction gas is introduced from the outside of the chamber through the high frequency electrode and the large number of holes in the insulating plate on this surface. The plasma reaction gas is discharged through the insulating plate and the installation electrode of. The insulating plate on these electrode surfaces is not necessary if it is not necessary depending on the object of plasma treatment. Further, when it is necessary to prevent partial streamer discharge or to make the electric field in the magazine more uniform, it is more effective to use an insulating plate in order to easily prevent arc discharge.

By making the distance between the electrodes variable in the range of about 5 cm to 25 cm, the optimum electrode interval can be set according to the width of the magazine. The magazine is placed between the insulating plates installed between the electrodes or between the electrodes when there is no insulating plate. The distance between each electrode or the insulating plate and the magazine is set within a range from several mm to several cm so that the optimum value of the distance can be set according to the purpose.
Since these intervals are closely related to the uniformity of plasma gas flow, they are generally set based on experimental data.

[0011] In general, to remove unwanted organic matter deposited on the substrate is used O ₂ gas and Ar gas or a mixed gas, H ₂ O the organic matter in the plasma
The gas or CO ₂ gas is used for removal, but this method can be used for removal in the magazine in exactly the same manner. Further, unnecessary metal compounds and impurities (including smear etc.) can be efficiently reduced, decomposed or destroyed by N2 gas (or Ar gas) H2 gas etc. by the plasma surface mutual reaction (ion assist reaction). The gas pressure is preferably in the range of 0.1 Pa to several hundreds of Pa depending on the purpose. In addition, the frequency of the high frequency power is 10 kHz to 160 k with respect to the above electrode spacing
Select the optimum frequency in the range of Hz. The electric power needs to be several hundred W to several kW depending on the capacity of the target.

Conventionally, a high frequency such as 13.56 MHz is often used, but in this case, the plasma cleaning of the substrate surface could not be performed properly when the substrate was stored in the magazine. Since plasma was not generated in the magazine and the plasma particles generated outside the magazine were introduced into the magazine, the reaction on the substrate surface was weak, and reduction, decomposition, destruction, etc. of metal compounds, etc. were particularly insufficient. Therefore, it was almost impossible to achieve the purpose.

The reason is that in the case of a high frequency such as 13.56 MHz, the electrons generated in the plasma can follow the voltage fluctuation, but the ions whose mass is about 1800 times heavier than the electron cannot follow the voltage fluctuation. . The reason is that ions move only about 20 μm in a half cycle, so they are trapped in the gas space between the electrodes, the plasma space has a positive potential, and the corresponding high frequency power application electrode side with a small area ratio is negatively charged, A negative self-bias voltage is generated on the electrodes. Further, since the magazine is also made of metal and is a conductor, it is negatively charged, and since both sides of each substrate are wired with copper or the like, they are similarly negatively charged. For this reason, the distance between each substrate in the magazine is generally 5
It is as narrow as ~ 6 mm, and a sheath is generated on the surface of each substrate along with the magazine itself to prevent the introduction of electrons between each substrate.
Since it is difficult for electrons or ions having strong energy for generating plasma to enter the gaps between the substrates, it is impossible to generate plasma between the substrates. Even if it occurred, it was non-uniform.

Since it is possible to generate plasma under the same conditions if the distance between the substrates is 8 mm or more, or if copper wiring or the like of each substrate is eliminated and the substrate is a full-face insulator, it is possible to generate plasma with each magazine. It is necessary to prevent the sheath from being generated in the substrates and to introduce electrons into the gaps between the substrates. In addition, widening the substrate interval to 8 mm or more impairs productivity, and is not practical.

From the above, in order to prevent the sheath from being generated on the magazine and each substrate surface, it is necessary to prevent the negative self-bias voltage from being generated, and the ions are also generated between the electrodes. It is important to be able to follow the fluctuation of the voltage, and the distance between the electrodes is in the range of 5 cm to 25 cm, and when plasma is generated, the frequency corresponds to the distance between the electrodes in the range of 10 kHz to 160 kHz. It was found that it is better to select the optimum frequency.

The width of a standard magazine is generally 2 cm to 1
Since the length is about 0 cm and the length is about 20 cm, it is sufficient if the distance between the electrodes is 5 cm to 25 cm even if these magazines are arranged between the electrodes and an insulating plate of several mm is sandwiched between them. It is preferable to set the optimum inter-electrode distance according to the above. In addition, some magazines do not have slits for introducing gas on the side surface.Therefore, a magazine is arranged between the electrodes in the longitudinal direction so that gas can flow through the electrodes between the substrates to generate plasma. It is preferable to generate it.

A method for determining the optimum frequency for these electrode intervals will be described below. If the transit time of the electrons is longer than the half cycle (T / 2 sec) of the high frequency voltage, the electrons are trapped between the electrodes. The cutoff frequency (Fec) at this time is generally expressed as follows. Fec = μeE / πd (1 / sec) (1) μe: Electron mobility (m ² / V · sec) d: Electrode distance E: Electric field μeE: Driving speed and frequency at which ions are captured between electrodes ( Similarly, Fic = μiE / πd (1 / sec) (2) μi = ion mobility (m ² / V · sec) μiE = driving speed. However, the electron mobility and the ion mobility vary depending on the collision frequency of electrons and ions depending on the gas pressure. Also, because ions are heavier than electrons,
The frequency (Fi) that is trapped in the gas space between the electrodes of the ions
Since c) is smaller than the electron cutoff frequency (Fec), the optimum frequency should be determined based on the ion Fic.

The frequency (Fic) for capturing ions in the gas space between the electrodes is about 20 when the distance (d) between the electrodes is 1 cm and the gas pressure (P) is 1 atm (760 Torr).
It is reported that the frequency will be 0 kHz. When the gas pressure (P) is a low pressure of about 1 Pa, 400 kHz
Then, the amplitude of the ion becomes 2 cm. Therefore, the electrode spacing is now 1
When the ion collision wave number (1 / see) νmi is set to 0 cm, νmi> ω for high pressure, and Fic≈20 kHz, νmi <ω for low pressure, and Fic≈80 kHz, which is the most effective range. It was proved.
The frequency at which ions are not trapped in the gas space between the electrodes is in the above range. Here, ω represents the angular frequency of the applied high frequency power. Therefore, when plasma is processed by disposing a magazine between the electrodes with a distance of 10 cm, by setting the frequency of the high frequency power to about 20 kHz to 80 kHz according to the type and flow rate of the gas used, that is, the gas pressure,
There is no capture of ions in the gas space between the electrodes, no self-bias voltage is generated in the magazine or each substrate surface, it is easy to introduce electrons between each substrate, and plasma can be generated between each substrate As a result, not only organic substances on the surface of the substrate but also metal compounds can be efficiently reduced, decomposed and destroyed, and the desired cleaning results could be achieved.

However, when the gas pressure is higher than necessary, the number of collisions of electrons and ions increases, and the problem of heat generation occurs. Therefore, in the case of substrate processing in a magazine, a gas pressure of several hundred Pa or less is used. Is preferable.

In order to make the discharge sustaining voltage as low as possible and increase the plasma density, it is better to increase the frequency. Particularly, at 50 kHz or higher, the discharge sustaining voltage decreases according to the frequency. As small as possible,
It is also important to consider that the gas pressure is made as low as possible so that ions are not trapped between the electrodes even if the frequency is increased.

The discharge starting voltage changes depending on the type of gas, the gas pressure (P) and the inter-electrode distance (d). There is Paschen's law that the minimum value is obtained by the product of Pd.
Therefore, it is desirable to set the pressure (P) and the inter-electrode distance (d) as close as possible to the minimum value of the Pd product.

For example, in the case of N ₂ gas, the minimum value of Pd is about 0.7 Pa · m, the pressure (P) becomes about 7 Pa when the distance (d) is 10 cm, and the minimum value of Pd value is about 7 Pa in O ₂ gas. It is 0.5 Pa · m and about 5 Pa at 10 cm, H ₂
The minimum value for gas is about 1.5 Pa · m and about 15 for 10 cm.
It becomes Pa. As described above, it is important to determine the optimum frequency of the high frequency power according to the type of gas, the gas pressure and the distance between the electrodes.

Further, the pressure is high in the region where the Pd product is large,
Since the electrode interval is large, a thin streamer discharge is likely to occur, and conversely, when the Pd product is small, the glow discharge becomes dominant. If the Pd product is large and the electric field distribution is likely to be uneven, corona discharge is likely to occur.

Generally, plasma may be generated without inserting an insulating plate between the electrodes. However, by inserting an insulating plate between the electrodes, an electric field between the electrodes is generated before the plasma generation as in the case without the insulating plate. An electric field is strongly generated linearly in the gas portion in proportion to the high-frequency voltage applied from the outside, and plasma is generated between the electrodes and between the substrates in the magazine. Plasma is generated, and ions, electrons, and active particles are generated, and electrons or ions are alternately charged and discharged on the insulating plate surface every ¼ cycle, and discharge occurs near the maximum and minimum applied voltages. Is stopped and the discharge is intermittent, so that it is possible to prevent corona discharge and streamer discharge, control heat generation, and easily prevent arc discharge.

Since the voltage is carried on the back side, the average electric field in the plasma generating section becomes small and the electric field in the magazine also becomes small. Therefore, it is possible to reduce the damage due to the charged particles to each substrate in the magazine. Therefore, by preventing excessive plasma generation, heat generation can be controlled and plasma generation on each substrate surface can be easily made uniform. The magazine may be connected to each insulating plate, but in order to make the distribution and generation of plasma particles on each substrate surface in the magazine uniform, it is better to place them at a certain distance from each insulating plate. The results are obtained, and there is no problem in plasma generation without the insulating plate, but the movements of electrons and ions, etc. become alternately and frequently due to the charge and discharge in every 1/4 cycle, and in addition, on each substrate surface. It became easier to make the activation uniform.

When a metal oxide or the like is reduced by H ₂ gas plasma, positive ions and negative ions to which electrons are attached are present in H ₂ gas together with electrons, excited atoms, molecules and free radicals. It is important to effectively use not only ions but also negative ions.
Plasma treatment at low frequencies is more effective than the method utilizing a negative self-bias voltage at Hz.

When the plasma comes into contact with the container wall or the solid surface, the composition of the plasma particles greatly changes depending on the surface state. Especially for H ₂ gas, excited hydrogen H ^* ,
Negative ion hydrogen H ⁻ , hydrogen atom H, and positive ion hydrogen H ⁺ are present, and their component ratios change greatly. Therefore, the material of the electrodes and the insulator on the electrode surface, especially the condition of the wall surface of the chamber is important, and it is very important to perform a passive surface treatment such as Al ₂ O ₃ for stable plasma cleaning. It is a factor.

Further, in plasma generation by high frequency glow discharge, it is very important to control the plasma density and the component ratio of plasma particles according to the purpose. Therefore, it is important to control the power of high frequency applied power, frequency, distance between electrodes, presence or absence of insulating plate between electrodes, chamber material, capacity between electrodes and chamber, temperature, gas type, gas pressure and gas flow rate, etc. Becomes Especially, the relationship between the gas pressure and the flow rate is important, and it is preferable to control the flow rate together with the gas pressure. Gas is introduced from the outside of the chamber between the ground electrodes to generate plasma through the high-frequency power application electrode having a large number of small holes and an insulating plate, and the substrate in the magazine is plasma-processed, and these plasma gas particles are ground electrode. When the gas is discharged from the side, the discharge spreads from the upstream side to the downstream side of the gas flow and spatially changes the plasma component ratio.
Active species such as electrons, ions, excited atoms (molecules), and radical particles generated between the electrodes have their respective lifetimes and their density does not decrease with time, so that they flow downstream and at the same time, new gas is generated between the electrodes. It is generated as plasma and flows downstream. Therefore, when the gas flow rate is low, the reaction species are often insufficient, and the reaction speed becomes slow. Also, when the gas flow rate is too high, the residence time of the reaction species in the reaction region decreases and the reaction species react before they react. Evacuation results in a slower reaction rate. That is, control of the optimum gas pressure and the optimum gas flow rate is important for stable plasma cleaning.

As described above, in order to uniformly clean many substrates housed in the magazine, rather than introducing plasma particles only from the outside of the magazine, the plasma is evenly distributed on each substrate surface in the magazine. Is generated, and it is most important to make the component ratio of plasma particles on the substrate surface as uniform as possible.

[0030]

EXAMPLES Examples of specific plasma processing of the present invention will be described below. As shown in FIG. 2, a high frequency applying electrode (1 ′) and a ground electrode (8) are installed in a parallel plate type in a chamber (7), and an alumina insulating plate (thickness 3 mm, width 2
4 cm, height 20 cm) and the electrode spacing is about 10
cm, and the magazine (3) at a position approximately in the middle of each insulating plate.
(Width 5.5 cm, length 20 cm, height 14.5 cm, each substrate (4) spacing 6 mm, 20 sheets can be stored, material Al) substrate (4) (thickness 0.36 mm, width 5.0 cm, length) 18.
7 cm, material BT, Cu wiring thickness 10 μm, Ni plating 4 μm, plating 0.05 μm) housed in a magazine of 20 pieces is placed in a chamber (7) via an insulator (8). Diameter 1mmφ for each electrode (1) (2) and insulating plate
A large number of small holes were formed at intervals of 4 mm so that the introduction of gas was made uniform. Gas flow rate is H ₂ gas 200 cc /
sec, N ₂ gas 50 cc / sec, gas pressure 40P
a, high frequency power (5) 340 W, frequency 40 kHz, processing time 3 minutes, chamber temperature 25 ° C initially, 38 ° C after processing
Rose to. The plasma-treated substrate under the above conditions was Au.
A bonding test was performed with a general ultrasonic bonder machine at a line of 28 μm. The tensile strength of the bonder before the treatment had an average value of 3.2 g with a standard deviation of 1.7 to 2.2, but the plasma treated under the above conditions had an average value of 13.8.
g, the standard deviation improved to 0.9. These had almost the same tensile strength as that of a conventionally used substrate having an Au plating thickness of 0.2 μm.

[0031]

Since the present invention has the characteristics as described above, from the above results, it is possible to perform efficiently and without any problem in quality by the plasma processing in the state where a large number of substrates are stored in the magazine. By being able to bond the Au wire, the productivity is greatly improved and the cost can be reduced.

In the above description, one magazine is installed between the electrodes in the chamber for plasma processing.
Even if a large number of magazines are arranged between the electrodes, it is possible to perform the plasma processing in the same manner. Also, it can be applied to many kinds of magazines by setting the inter-electrode distance according to the size of the magazine and applying high frequency power at the optimum frequency, and it is possible to perform plasma processing extremely rationally and Au wire. It was possible to improve not only the reliability of bonding but also the adhesion between the mold resin and the substrate. Moreover, when removing only unnecessary organic substances on the substrate surface, Ar is used as a used gas.
A mixed gas of gas and O ₂ gas is effective.

[Brief description of drawings]

FIG. 1 is a sectional view of the inside of a chamber of a single-wafer RIE type plasma cleaning embodiment.

FIG. 2 is a sectional view of a magazine-type plasma cleaning system according to an embodiment of the present invention.

[Explanation of symbols]

1: High frequency power application electrode 1 ': High frequency power application electrode (shower structure) 2: Installed electrode (shower structure) 3,3 ': Magazine 4: substrate or lead frame 5: High frequency power source 6: Blocking condenser 7: Chamber 8: Insulator 9: Gas inlet 10: Gas outlet

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Rikuo Sakurai             585-, Tsurunomai, Kita-Nagai, Miyoshi-cho, Iruma-gun, Saitama Prefecture             9 Within Mori Engineering Co., Ltd. F-term (reference) 5F004 AA13 BA06 BA07 BB11 BB29                       BD01                 5F067 AA04 DA00

Claims (3)

[Claims] 1. A BGA, CS when assembling an IC chip or a semiconductor chip on a package or a circuit board.
A magazine in which a P or MCM substrate or a QFP or SOP lead frame is housed is installed between a power applying electrode and a ground electrode arranged in a parallel plate type in a vacuum chamber via an inner wall of the chamber and an insulator. 2. An electrode for introducing a plasma generating gas to each electrode,
Alternatively, a large number of small-diameter holes for discharging plasma gas are formed so that gas is introduced from the outside of the chamber through the hole of one electrode and discharged through the hole of the other electrode to the outside of the chamber. Alternatively, gas is introduced from the outside of the chamber through the holes of both electrodes, and a hole for discharging the gas is formed in a part of the chamber so that the gas is discharged to the outside of the chamber. The distance between electrodes should be variable between 5 cm and 25 cm. 3. Electrons and ions, which are charged particles among various active particles generated by plasma in every half cycle of the frequency of the applied AC power, follow the fluctuations of the applied AC power and enter the gas space between the electrodes. The frequency was selected to be the optimum value within the range of 10 kHz to 160 kHz, which was adapted to the distance between the electrodes and the gas pressure so as not to be captured, so that plasma could be uniformly generated between each substrate or lead frame housed in the magazine. 3. The magazine type plasma cleaning system according to claim 1, wherein the plasma cleaning system is a magazine type plasma cleaning system.