RU2139593C1 - Method for continuous chemical liquid cleaning of polymer-covered surfaces of parts, primarily semiconductor plates - Google Patents

Abstract

FIELD: electronic engineering. SUBSTANCE: method involves electrochemical treatment of sulfuric acid aqueous solution, treatment of plate surfaces with activated solution obtained, repeated electrochemical treatment, and cleaning of solution. Sulfur acid concentration is 5.5 to 6.5 V; plates are treated at temperature of 80 to 100 C. In this way, photoresist is easily removed from plates. Spent solutions are reconditioned for their reuse in continuous process; composition of etching solution is maintained stable. EFFECT: improved efficiency of process. 8 dwg, 5 tbl

Description

The invention relates to semiconductor microelectronics, and in particular to the technology of liquid chemical removal (hereinafter, LC) of polymer layers (for example, photoresistive high molecular weight layers, etc.) from the surface of semiconductor wafers after technological operations of liquid etching of silicon oxide layers of various compositions in the manufacture of integrated circuits, and also in other industries where removal of organic polymer coatings from the surface of products is required:
in the manufacture of printed circuit boards;
in the manufacture of parts and components of electronic devices, etc.

 Currently, GHS processes in modern technological routes of production of VLSI are widespread and, as practice has shown, have advantages over other known methods of "dry" or plasma-chemical removal of layers of photoresists due to their universality to the appearance of photoresists and the absence of undesirable side effects ( radiation damage, secondary pollution, the possibility of deposition of atomized non-volatile compounds on the surface to be cleaned, etc.).

A known method of LCS [1] photoresists from the surface of semiconductor wafers using acidic solutions of hydrogen peroxide. The LCS method [1] includes:
preparation of a solution of the composition H ₂ SO ₄ : H ₂ O ₂ in a ratio of 1: 3, respectively, i.e. preparation of a KAPO mixture, accompanied by a significant exothermic effect;
immersion in a hot solution and holding semiconductor wafers with a photoresistive film on the surface for 10 minutes;
washing semiconductor wafers in hot and, further, cold running deionized water.

 The advantage of the method [1] is that the KAPO mixture is a universal mixture according to the spectrum of purified impurities and the type of photoresist films.

However, the efficiency of photoresist removal processes according to the method [1] is unstable in time. So, the KAPO mixture obtained by this method, containing 75% concentrated sulfuric acid, after exposure at a temperature of 130 ^o C for 1-2 hours of LC processes is diluted sulfuric acid due to the decomposition of the second component of the mixture - hydrogen peroxide, originally present in concentration of 25%.

 A known method of removing organic coating [2], including processing the substrate with a photoresistive film in a mixture of KAPO. In another embodiment of the method [2], the substrate with a photoresist film is treated with ozone or in oxygen plasma and, further, in a washing solution, a KAPO mixture, followed by rinsing in water saturated with ozone.

 The advantage of the method [2] is its versatility in the spectrum of etched photoresistive films.

 The main disadvantages of the method [2] are the short life time of the active components of the washing solutions and the high material costs of the equipment used for its implementation.

 The known method of LCS from the surface of semiconductor wafers [3] using an spent solution containing ultrapure sulfuric acid, sulfuric acid and ultrapure water from the production process, purifying it and reintroducing it into the LCC process to maintain a constant concentration of ultrapure acid with a certain purity and composition of the solution . The method and device, protected by a US patent, include processes of high-temperature distillation, condensation, sublimation, electrochemical production of supra sulfuric acid for carrying out LCS processes.

The disadvantages of the method are the need to use for its implementation of false material-intensive equipment for cleaning the spent solution, as well as its high energy consumption associated with the need to heat the working solution in pickling baths to 150 ^o C; in separators - up to 280 ^o C; distillation columns up to 300 ^o C and above, since the boiling point of sulfuric acid is 338 ^o C.

 In addition, the technological processes carried out in this method have a high fire hazard due to the high temperature heating of toxic and aggressive solutions of sulfuric acid. It is this drawback of a method based on a high-temperature distillation regeneration system that has led to fires at technological complexes for the production of integrated circuits at enterprises in Western Europe.

Closest to the proposed solution in technical essence is the LCF method, including the electrochemical treatment of an aqueous solution of sulfuric acid, the effect of an activated solution on the surface of a semiconductor wafer at a concentration of the initial solution of 3-7 M, and the spent solution is reactivated and purified by sequentially feeding it through the cathode and the anode chamber of the electrochemical cell at an anode current density of 3.0 to 4.8 kA / m ² [4].

 This method [4] can significantly simplify the process of recovery of spent solutions; Maintain a stable component composition of an LCL solution; less material and energy intensive.

 The disadvantage of the prototype is that this LCF solution is not applicable for the process of removing layers of photoresists from the surface of semiconductor wafers.

So, it has been experimentally shown that the process of removal of polymers occurs under the action of active radicals O ₃ , O, O ^• , OH ^• , O $\genfrac{}{}{0ex}{}{\text{-}}{\text{2}}$ - etc.), which are formed at elevated temperatures in the decomposition of oxidizing agents in etching solutions - activated solutions of concentrated H ₂ SO ₄ .

It was shown that the optimum etching rate and the efficiency of the LCS process are achieved at a temperature of 80-100 ^o C and at a constant flow rate of a freshly prepared activated solution of 8-10 l / h. The implementation of these processes by the method [4] is practically impossible and economically disadvantageous.

 The technical problem solved by the invention is to increase the efficiency of the process of removing layers of photoresists from the surface of semiconductor wafers; increasing the efficiency of the processes of regeneration of spent solutions for their re-introduction into the continuous production process of LCS and maintaining a stable component composition of the etching solution.

This task is achieved by the fact that in the known method (including electrochemical activation of a solution of sulfuric acid, the effect of the obtained activated solution on the surface of the plates, repeated electrochemical treatment and cleaning of the solution), sulfuric acid with a concentration of 15-18 M is used, the electrochemical treatment and cleaning of the solution are carried out at a voltage of electrodes from 5.5 to 6.5 V, the process of exposure of the obtained activated solution to the surface of the plates is carried out at a temperature of from 80 to 100 ^o C.

The essence of the proposed method lies in the fact that when using the inventive method, it is possible to obtain a GHS solution with high oxidizing properties sufficient to conduct complete oxidation of the layers of photoresist films to CO ₂ and H ₂ O. According to the proposed method, the process of electrochemical activation of concentrated sulfuric acid is carried out at the voltage at the electrodes is more than 5.0 V, namely in the range from 5.0 to 6.5 V. According to the prototype method, the voltage at the electrodes upon activation of 3-7 M sulfuric acid solutions with sets from 3.0 to 5.0 V.

 The differences in the features of the processes of electrochemical activation by the method of the prototype and by the proposed method are in the reactions of the formation of oxidizing components of solutions of LC and LC.

Me(HSO $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$ )_адс _→ Me(SO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4}}$ )_адс+H⁺ (2)
2Me(HSO $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$ )_адс _→ Me+H₂S₂O₈ (3)

Высокая эффективность десорбционной активности моющего раствора по отношению к примесям, имеющимся на очищаемой поверхности (это в достаточной мере подтверждено банком данных, полученных методами спектроскопии поверхности: ОЖЕ-спектроскопии и ВИМС), полученного по способу прототипа [5], достигается за счет генерирования продуктов - мононадсерной кислоты, надсерной кислоты и перекиси водорода - в области потенциалов от 3,0 до 5,0 В, где они синтезируются, устойчивы и далее не окисляются.The reaction mechanism for the formation of oxidizing components of the prototype represents several main stages. The first stage is the discharge and adsorption of sulfate or hydrosulfate ions according to reactions (1a) and (1b). In the double layer field, the adsorbed hydrosulfate radical can undergo deprotonization with the formation of chemisorbed sulfate anion radicals by reaction (2). Therefore, it is fundamentally not essential whether sulfate or hydrosulfate ions are present that are simultaneously discharged - an equilibrium state will be established in solution (2). The removal of these intermediate particles occurs as a result of subsequent reactions - recombination (3) or electrochemical desorption (4)

Me (HSO $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$ ) _ads _ → Me (SO $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4}}$ ) _ads + H ⁺ (2)
2Me (HSO $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$ ) _ads _ → Me + H ₂ S ₂ O ₈ (3)

The high efficiency of the desorption activity of the washing solution with respect to impurities on the surface being cleaned (this is sufficiently confirmed by the data bank obtained by surface spectroscopy methods: Auger spectroscopy and SIMS) obtained by the prototype method [5] is achieved by generating products - monosulfuric acid, sulfuric acid and hydrogen peroxide - in the potential range from 3.0 to 5.0 V, where they are synthesized, stable and not further oxidized.

But experimental studies have established that the oxidizing activity of solutions obtained by the prototype method (see examples of specific performance) is not enough for conducting liquid chemical oxidation (or GHS) processes of photoresistive films or other polymer coatings from the surface of semiconductor wafers. It has been shown that the oxidation of these products — reaction products of (1) - (4) of the prototype method — occurs at potentials higher than 5.0 V; leads to the formation of higher polyoxides that oxidize photoresist films to CO ₂ and H ₂ O.

Индексом * показан путь движения кислорода из аниона в анодно выделяющийся кислород.In this potential region (from 5.0 to 6.5 V), part of the sulfate ions, discharged by reaction (1b), do not turn into peroxodisulfate ions, but, giving their oxygen to the surface of the anode, again turn into sulfate ions as a result electrochemical desorption by discharging water molecules

Index * shows the path of oxygen movement from the anion to the anode released oxygen.

и т.д. до образования полиокисей гомологического ряда H₂O_n, где n≥2. Первым членом данного гомологического ряда является перекись водорода H₂O₂.Further, the proposed method follows the mechanism of formation of higher polyoxides

etc. to the formation of polyoxides of the homological series H ₂ O _n , where n≥2. The first member of this homologous series is hydrogen peroxide H ₂ O ₂ .

 According to the prototype of the reaction, the formation of higher polyoxides, ozone, and oxygen were considered parasitic.

 All products of the presented reactions according to the prototype (1) - (4) and the proposed method (5) - (8) are strong oxidizing agents, since they are sources of oxygen formation - the main oxidizing agent of organic compounds.

But the hallmark of the proposed method is that the oxygen activity is different from the prototype. According to the proposed method, active oxygen is also a product of the decomposition reaction of peroxodisulfate ions
S ₂ o $\genfrac{}{}{0ex}{}{\text{2-}}{\text{eight}}$ + H ⁺ _ → HS ₂ O $\genfrac{}{}{0ex}{}{\text{-}}{\text{eight}}$ _ → HSO $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$ + SO $\genfrac{}{}{0ex}{}{\text{•}}{\text{4}}$ (nine)
SO $\genfrac{}{}{0ex}{}{\text{•}}{\text{4}}$ _ → O ₃ +1/2 O ₂ (10)
SO $\genfrac{}{}{0ex}{}{\text{•}}{\text{4}}$ + H ₂ O _ → H ₂ SO ₅ (11)
According to the prototype method, the course of these reactions is almost impossible due to the influence of water, leading to the decomposition of sulfuric acid by the reaction
H ₂ S ₂ O ₈ + H ₂ O ← _ → H ₂ SO ₄ + H ₂ O ₂ (12)
The next distinguishing feature of the proposed method is that the additional source of oxygen is not water (the mechanism implemented in the prototype method), but hydrogen peroxide.

According to the proposed method, the mechanism of oxygen formation is as follows:
2H ₂ O ₂ _ → 2H ₂ O + O ₂ (13)
H ₂ SO ₅ + H ₂ O ₂ _ → H ₂ SO ₄ + H ₂ O + O ₂ (14)
H ₂ S ₂ O ₈ + H ₂ O ₂ _ → 2H ₂ SO ₄ + O ₂ (15)
Similarly, the decomposition reactions of higher polyoxides of the homologous series: H ₂ O _n , formed in activated concentrated solutions of sulfuric acid, characterized by high oxidative ability and stability of oxygen: atomic, molecular and its radicals proceed.

The process of oxidation of photoresistive films from the surface of semiconductor substrates by the proposed method proceeds according to the radical-chain mechanism "with degenerate forking". To initiate oxidation reactions of organic compounds for the first time, it is proposed to heat the solution — activated concentrated sulfuric acid — to a temperature of from 80 ^o C-100 ^o C.

 The effect of heating is to weaken bonds in the molecules of oxidizing agents - products of reactions (1) - (4) and (5) - (8) - and the formation of active atoms and radicals as a result.

 According to the reactions proceeding according to the radical-chain mechanism with degenerate branching, a part of the intermediate peroxo compounds also decomposes into radicals, thereby giving rise to new reaction chains, and the rest - to the final products and, therefore, is not involved in the further initiation of the process.

But since the concentration of molecules of the oxidizable substance compared with the concentration of radicals is small, the oxidation of films of photoresist and other organic compounds is reduced mainly to the formation of final, stable reaction products, i.e. CO ₂ and H ₂ O.

Глубина окисления органических соединений - полимерных слоев фоторезистов на поверхности кремниевых пластин - зависит от диффузионных факторов, а именно от скорости подвода активного кислорода в зону реакции окисления.

The depth of oxidation of organic compounds — the polymer layers of photoresists on the surface of silicon wafers — depends on diffusion factors, namely, the rate of supply of active oxygen to the oxidation reaction zone.

In turn, the rate of supply of active oxygen depends on the concentration of active components in the LC solution at the time of the oxidation process. The determination of the component composition of the solution obtained by the proposed method is carried out by titrimetric analysis methods; impurity composition of the solution is estimated:
on organic impurities - by the method of liquid phase titration;
for inorganic impurities - by the method of inversion voltammetry.

 (Methods and means of controlling the composition of the solution are given in examples of specific performance).

Конечной стадией взаимодействия данного раствора, получаемого по предлагаемому способу, с неорганическими примесями, попадающими в него при проведении техпроцессов ЖХС, являются хелатные, комплексные соединения, где молекулярный анион имеет формулу: O $\genfrac{}{}{0ex}{}{\text{2-}}{\text{2}}$ - перекисную; O $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ - озонидную; O $\genfrac{}{}{0ex}{}{\text{-}}{\text{2}}$ - надперекисную.Thus, the final stage of the interaction of an activated solution of concentrated (15-18 M) sulfuric acid with polymer layers of photoresists and other organic compounds on the surface of semiconductor substrates is:

The final stage of the interaction of this solution obtained by the proposed method with inorganic impurities that enter it during the process of LCS are chelated, complex compounds, where the molecular anion has the formula: O $\genfrac{}{}{0ex}{}{\text{2-}}{\text{2}}$ - peroxide; O $\genfrac{}{}{0ex}{}{\text{-}}{\text{3}}$ - ozonide; O $\genfrac{}{}{0ex}{}{\text{-}}{\text{2}}$ - superperoxide.

 The chelating mechanism of the formation of complexes having a three-membered closed cycle leads to the fact that the bonded (metal ion, metal atom, anion, etc. stripped from the surface) are firmly held in the complex and cannot be absorbed back by the surface treated with activated solutions of concentrated sulfuric acids. Further, impurities bound to chelate complexes are removed while they are destroyed during washing of semiconductor wafers in flowing ultrapure water.

 It is important to emphasize that when processing semiconductor wafers in activated sulfuric acid solutions, all organic and inorganic contaminants are oxidized to the highest degree of oxidation, their oxidized forms prevent the process of secondary contamination of the surface when rinsing in running water; eliminates the possibility of the formation of local critical areas on the cleaned surface.

 The inventive method is illustrated by the following drawings.

 In FIG. 1 shows a block diagram of a recovery unit for implementing the proposed method.

 In FIG. 2 shows the results of studies of the surface of silicon wafers by Auger spectroscopy when exposed to the photoresist layers of a solution obtained by the proposed method.

 In FIG. 3 shows the results of determining the total organic carbon in solutions obtained by the prototype, the method of liquid phase titration.

In FIG. 4 shows the results of a study of the effectiveness of the processes of regeneration of sulfuric acid solutions by the method of inversion voltammetry:
FIG. 4a - voltammogram of the initial solution of sulfuric acid to electrochemical activation;
FIG. 4b is a voltammogram of an activated sulfuric acid solution obtained by the proposed method.

 In FIG. 5 shows the results of the determination of impurities of total organic carbon in solutions obtained by the proposed method, during the two-month period of its operation. The determination method is liquid phase titration.

In FIG. 6 shows the results of studies of the effectiveness of the processes of regeneration of spent solutions of activated sulfuric acid by the proposed method by inversion voltammetry:
FIG. 6a - voltammogram of the spent solution of sulfuric acid;
FIG. 6b is a voltammogram of the regenerated solution.

In FIG. 7 shows the relationship between the number ratio of H ₂ SO ₄ molecules; HSO ions $\genfrac{}{}{0ex}{}{\text{-}}{\text{4}}$ ; SO ions $\genfrac{}{}{0ex}{}{\text{2-}}{\text{4}}$ in an aqueous solution of H ₂ SO ₄ .

;
фиг. 8б - результаты титриметрического определения концентрации мононадсерной кислоты

, в растворе ЖХС;
фиг. 8в - результаты титриметрического определения концентрации надсерной кислоты

в растворе ЖХС;
фиг. 8г - результаты титриметрического определения концентрации перекиси водорода

в растворе ЖХС.In FIG. Figure 8 shows the results of studies on the stability of activated solutions during the day with a determination interval of 2 hours - (C _ok ≈ f (t)):
FIG. 8a - the results of titrimetric determination of the total content of oxidizing agents:

;
FIG. 8b - the results of titrimetric determination of the concentration of mononadsulfuric acid

in a solution of LCS;
FIG. 8c - results of titrimetric determination of the concentration of sulfuric acid

in a solution of LCS;
FIG. 8g - results of titrimetric determination of the concentration of hydrogen peroxide

in a solution of LCS.

 Examples of specific performance.

 Example 1 (implementation of the method GHS surface of the semiconductor wafers of the prototype).

A solution of sulfuric acid with a concentration of 5 M (60 L) is poured into the pressure vessel 1 (see Fig. 1), equipped with solution level sensors, from which it flows by gravity with a flow rate of 5 l / h to the cathode and then to the anode chamber of the electrochemical cell 4 (Fig. 1). After filling the electrochemical cell with electrolyte, turn on a stabilized voltage source 5 (Fig. 1) and set the voltage at the electrodes to 4.0 V and a current of 300 A. The normal operation of the power supply 5 and the electrochemical cell 4 is monitored by the presence of a given potential on the instrumentation and current in the load. Further filling with the solution of the electrochemical cell 4 leads to the discharge of the activated solution through the drain hole by gravity into the storage tank 2 of FIG. 1; when a sufficient volume of activated solution has been generated, it is pumped from the tank 2 through the filter device 8 to the working bath 6. Using a pump 7, a cassette with silicon wafers having a photoresistive film on the surface (after chemically treated silicon dioxide in etchants) is immersed in an activated sulfuric acid solution, located in the working bath 6 at room temperature. The plates are kept in solution, heated to 100 ^o C, 10 minutes Then they are washed in a bath with running deionized water for 15 minutes and dried. The spent solution again enters the pressure tank 1.

The quality control of the process of removing photoresist films was carried out visually under a microscope at a magnification of 100 ^x .

 The activated sulfuric acid solution was analyzed, which was in continuous operation to conduct LCL processes on the surface of semiconductor wafers with the aim of pre-diffusion cleaning them for the content of SDA by liquid phase titration. The determination results are shown in FIG. 3.

 Example 2 (by the proposed method).

 An analysis was made of the quality of the initial recovery system for loading, the block diagram of which is shown in FIG. 1, a concentration of 17 M sulfuric acid solution: by the method of inverse voltammetry for the content of zinc, cadmium, lead and copper; by the method of liquid-phase oxidation of organic compounds - to the content of SDA. FIG. 4a shows a voltammogram of the original 17 M sulfuric acid solution; in FIG. 5 - the content of the JMA.

A solution of 17 M sulfuric acid was obtained by passing through the cathode and anode chambers of the electrochemical cell 4 shown in the block diagram (Fig. 1) at a voltage of 6.0 V on the electrodes. The normal operation of the power source 5 and the electrochemical cell 4 is controlled by the presence of -measurement devices of a given potential and current in the load. The activated solution through a drain hole in the electrochemical cell by gravity enters the storage tank 2 (Fig. 1); when a sufficient volume (more than 20 l) of activated solution has been generated, it is pumped out of the container 2 by means of a pump 7 through a filtering device 8 into a working bath 6. A cassette with KEF 4.5 silicon wafers with a diameter of 100 mm and a photoresist film on the surface (after plasma-chemical cleaning Silicon dioxide chromatography in appropriate etchants) is immersed in an activated 17 M solution of H ₂ SO ₄ located in a working bath and heated to a temperature of 90 ^o C. The plates are kept in solution for 10 minutes. Then they are washed in a bath with running water for 15 minutes and dried. The spent solution enters the pressure tank 1.

 The cleanliness of the surface of silicon wafers being cleaned after the LC process according to the proposed method was carried out by Auger spectroscopy. The Auger spectra of the surface of the silicon wafers shown in FIG. 2.

тигриметрическим методом анализа. Получали вольтамперограмму полученного активированного раствора серной кислоты (фиг. 4б), содержание СОУ (фиг. 5) и суммарное содержание окислителей (фиг. 8).Conducted an analysis of the quality of the resulting solution by inversion voltammetry; the content of total organic carbon was determined by the method of liquid-phase oxidation of organic compounds, the content of oxidizing agents was determined

tigrimetric analysis method. Received voltammogram obtained activated sulfuric acid solution (Fig. 4b), the content of the SDA (Fig. 5) and the total content of oxidizing agents (Fig. 8).

 The GCF processes of photoresists from the surface of silicon wafers were carried out in the same volume (60 L) of a solution located in the recovery system in continuous operation for two months of the system operation (Fig. 1). The test results of the working parties of the plates are presented in table. fifteen.

 An analysis of the activated solution of LCS (determination frequency of 5-7 days), located in a continuously operating recovery system with a maximum production load for conducting LCS processes for the content of SDA by liquid-phase oxidation, was carried out. The determination results are shown in FIG. 5.

 An analysis was made of the quality of the LC solution (frequency of determinations 1-2 times a day) by inversion voltammetry. Voltammograms were obtained similar to those shown in FIG. 6.

 Voltammogram 6a - the results of measuring the content of zinc, cadmium, lead and copper before regeneration; 6b - after carrying out the processes of regeneration of the GHS solution, including the processes of its purification and activation.

 Comparative indicators of the effectiveness of films from the surface of silicon wafers and the standard process LCS - processing in a mixture of KAPO - are given in table. fifteen.

 The results obtained in the implementation of examples of specific performance of the proposed GHS method indicate the achievement of the goal — increasing the efficiency of the process of removing photoresist layers from the surface of semiconductor substrates, creating energy-saving GHS technology with 100% recovery of solutions. This is confirmed by the results of the electrophysical parameters of semiconductor structures - MOS transistors, are given in table. fifteen; Auger spectroscopy and inversion voltammetry methods.

For the example of FIG. 2 shows the results of studies of the surface of KEF 4.5 silicon wafers with a diameter of 100 mm, processed by the LC method in activated concentrated solutions of H ₂ SO ₄ . From the above Auger spectra (Fig. 2) it follows that when carrying out LCS processes according to the proposed method in a solution containing higher polyoxides and saturated with active radicals, a carbon peak is not detected. From the data of Auger spectra (2a, 2b, 2c), we can conclude that there is no carbon on the samples of silicon wafers processed by the proposed method, which is determined at the detection limit (0.1%) by Auger spectroscopy. Thus, a sufficiently effective removal of photoresist layers occurs using electrochemically activated concentrated sulfuric acid solutions. The process of oxidation of photoresist films proceeding according to the radical chain mechanism according to reaction (16) guarantees effective removal of polymer layers from the surface.

 As a proof of the effectiveness of oxidation processes occurring in the volume of an LCS solution, the processes of purification of this solution and its regeneration of solutions from organic polymer layers, the liquid-phase titration method was used.

 The results of determining the total content of organic impurities by this method are shown in FIG. 5; the results of the determination of JMA in the solutions of LCL - according to the prototype method; ZhHS - according to the proposed method are shown respectively in FIG. 3 and FIG. 5.

From the results of the analyzes shown in FIG. 3, it follows that with the continuous operation of the LCL solution in the recovery system according to the prototype method, a steady increase in the concentration of SOA from 0.5 mg / l (content in the initial 42% H ₂ SO ₄ ) to 3.0 mg / l after two months is observed operation of the solution.

 It should be noted that the LCL solution was not used for the removal of photoresist layers — LCC processes, where the content of SDA would undoubtedly be large.

From the results presented in FIG. 5, it follows that during continuous operation of solutions of activated concentrated H ₂ SO _{4, the} concentration of SOA remains constant (within the limits of the error of determination) during electrochemical activation of 15-18 M solutions by the proposed method, i.e. at the level of 0.5 mg / l or less.

From the data shown in FIG. 3 and FIG. 5, it follows that the oxidation of organic compounds is most effective in concentrated solutions of H ₂ SO ₄ . The processes of oxidation of organic compounds in activated solutions proceed according to the radical-chain mechanism of interaction both on the surface of silicon wafers and in the volume of a GHS solution in continuous operation; recuperation processes based on the electrochemical generation of oxidizing components of a GHS solution proceed quite efficiently; the processes of regeneration of LC solutions containing a stable component composition of oxidizing agents, radicals, etc., have fairly good reproducibility.

приведенной в прототипе.An analysis of the quality of the initial solution of sulfuric acid, the solution after primary activation and its regeneration for the content of impurities carried out by the method of inversion voltammetry, proves the efficiency of the cleaning processes of the washing solution flowing in the cathode chamber of the electrochemical cell 4 by reaction

given in the prototype.

 It should be noted that the analysis of the quality of the solution by this method was carried out before each GHS process of photoresists from the surface of the silicon wafers given in the examples of a specific design, and before each changeable loading of working batches (one working batch corresponds to 40 plates), regardless of which washing cycle the washing cycle was subjected to solution.

 By way of example, as evidence of the effectiveness of photoresist removal processes in FIG. 4 a, b and 6 a, b show voltammograms of zinc, cadmium, lead, copper, found in the initial solution (see Fig. 4a); the solution that underwent primary activation — peaks of zinc, cadmium, lead, and copper — are absent (Fig. 4b); there are no peaks of these elements and in the regenerated according to the conditions of example 2 (Fig. 6b) of the proposed LC method, tested according to the conditions of this example of a solution containing impurities of zinc, lead and copper - voltammogram in FIG. 8a.

 In addition, before each process of GCF photoresists from the surface of silicon wafers, according to specific examples, a titrimetric analysis of the content of the oxidizing components of the solution was carried out - the total content of oxidizing agents (see Fig. 8a), supra sulfuric acid (see Fig. 8b), mononadic acid (see Fig. 8c) and hydrogen peroxide (see Fig. 8g). The data shown in FIG. 8a, b, c, d, indicate that the composition of the oxidizing agents is stable over time, the reproducibility of the activation processes — the operating time of the active components in the LC solution — is quite high.

The correct choice of the range of concentrations of sulfuric acid from 15 to 18 M is confirmed by the following:
the stability of the activation parameters and the composition of the GHS solution according to the content of oxidizing agents (see Fig. 8), which does not depend on the degree of contamination of the substrate being cleaned and the etching solution;
the dependence of the molarity of the dissolved particles in sulfuric acid solutions on the concentration of H ₂ SO ₄ shown in FIG. 7.

From the results of the dependence (Fig. 7) of the molarity of the dissolved particles in H ₂ SO ₄ solutions on its concentration, the optimal ratio of the quantities of H ₂ SO ₄ molecules, HSO ₄ ^- ions, SO ₄ ^2- ions in aqueous sulfuric acid solutions is determined, i.e. the content of ionic and molecular forms of dissociated particles.

From FIG. 7 it follows that as the stoichiometric concentration of sulfuric acid increases, the relative concentration of SO ₄ ^2- ions (as a result of the process with the second stage of dissociation of solutions) passes through a maximum at 49.5%; drops and completely disappears when the stoichiometric concentration reaches 84%. In the concentration range from 15 M to 18 M, the concentration of SO ₄ ^2- ions is practically equal to 0; the concentration of HSO ₄ ions ^- decreases to zero as the concentration of H ₂ SO ₄ molecules increases, i.e. in 17-18 M solutions of sulfuric acid, the dissociation of its molecules is practically absent.

 For the first time in this method, the possibility of carrying out the process of electrochemical generation of concentrated solutions of sulfuric acid is shown. Electrolysis processes of concentrated sulfuric acid are not described in the literature. The use of the process of electrochemical generation of concentrated solutions for carrying out the LCF and LC processes is shown for the first time and is not described in the literature.

 The correct choice of the indicated concentration range according to the proposed method is confirmed not only by the optimal ratio of dissociates or, rather, by their absence in the used initial solutions, eliminating the influence of water - the main initiating agent of the decomposition of oxidizing components, and, as a consequence, their stability in the proposed solutions of LC.

 This conclusion is also confirmed by the Auger spectra of the surface of silicon wafers shown in FIG. 2.

As follows from the Auger spectra, the main residual impurity (at the level of its detection limit - 0.1%) is carbon, which is absent on the surface of the plates processed by the proposed method in 15 M, 17 M and 18 M H ₂ SO ₄ solutions .

The choice of the temperature range of the GHS solution from 80 to 100 ^o C due to the following:
at a temperature of less than 80 ^o C, the initiation of oxidation reactions proceeding according to the radical chain mechanism is insufficient to weaken the bonds in the molecules of the GHS solution and to form active radicals and oxygen atoms as a result;
at temperatures above 100 ^o C, along with the acceleration of the main reaction - the oxidation of photoresists - the decomposition of hydrogen peroxide and other side reactions and processes can occur and intensify, as there is an imbalance between the rate of oxidation and the rate of decomposition of oxidizing components, i.e. conditions are created for the transition of the LCS process to the diffusion region. The latter leads to the appearance of other harmful substances, as evidenced by the appearance in the washing solution of under-oxidized reaction products, which, as a rule, have colored photoresist formations with oxidizing components of the solution.

Based on the foregoing, it is hardly advisable to use photoresists at temperatures above 100 ^o C and below 80 ^o C. when carrying out GHS processes.

An important distinguishing feature of the proposed method is the fact that the optimally selected parameters are: sulfuric acid concentration; the temperature of the processes GHS; electrochemical activation parameters - provide 100% recovery of LC solutions. According to the prototype method, to achieve such an indicator is not possible for a number of reasons already mentioned. The proposed method does not require periodic replacement of the technological solution of LCS; the decrease in its volume in the recovery system, associated with entrainment on the plates and cassettes, is compensated by the addition of the initial solution of concentrated H ₂ SO ₄ .

 It is the parameters of electrochemical activation, the processes that occur during the primary activation of a 15-18 M sulfuric acid solution, when the activated solution acts on the surface and, mainly, when reactivated solutions are reactivated and cleaned at the same time, are the basis of the entire mechanism for the complete recovery cycle of the proposed new ZhHS technical process photoresistive layers from the surface of semiconductor wafers.

The results of the experiments and analyzes presented in table. 1-5, comparative indicators of the quality of the cleaned surface of the silicon wafers indicate:
1. On the practical feasibility of processes for the complete removal of photoresistive films from the surface of silicon wafers by the proposed method GHS.

2. On the feasibility and feasibility of replacing the standard material- and energy-intensive technology of LCS in the KAPO mixture with the proposed method of LCS in activated 15-18 M solutions of H ₂ SO ₄ in connection with obtaining comparable results in terms of the threshold voltage of MOS transistors; according to the results of functional control; according to the results of charge states of current-voltage characteristics; by moving and effective charge, etc. etc.

The results of production tests performed on the example of the LCS of hundreds of working batches of plates in the existing technological lines for the production of VLSIs convincingly show the efficiency of the new LCS process and its practical significance, which allows:
significantly simplify the technological process of LCS photoresists and the design of technological equipment that implements it;
to achieve a 100% indicator of the process of recovery of LC solutions;
reduce the cost of expensive reagents from 95 to 100%;
stabilize the GHS process and increase the shelf life of the activated solution of concentrated sulfuric acid;
solve a number of environmental problems of enterprises of domestic microelectronics.

 The introduction of the proposed method of GHS surface and photoresistive layers instead of expensive uneconomical methods of GHS will allow to obtain large energy and resource saving at domestic standard plants that produce modern integrated circuits.

Used sources of information
1. Poltavtsev V.G., Knyazev A.S. Surface technology in microelectronics. - K .: Technique, 1990, p. 5-28.

2. "Ozone Cleaning of the Si-SiO ₂ System" J. Applied Physics, Solid and Surfaces, 1987, A 43, p. 223-226.

 3. US patent N 4828660, With 25 V 1/28, publ. 1989, "Method and apparatus for continuous chemical processing in the production process of ultrapure liquids."

 4. Patent of Russia N 2024993, H 01/21/312, publ. 1994, "Method for cleaning products, mainly semiconductor wafers."

Claims (1)

The method of continuous liquid chemical removal of polymer layers from the surface of products, mainly semiconductor wafers, including the electrochemical treatment of an aqueous solution of sulfuric acid, the effect of the obtained activated solution on the surface of the plates, repeated electrochemical processing and cleaning of the solution, characterized in that sulfuric acid with a concentration of 15-18 M is used , the electrochemical treatment and cleaning of the solution are carried out at a voltage on the electrodes of 5.5 to 6.5 V, and the process of exposure to activated solution to the surface of the plates is carried out at a temperature of from 80 to 100 ^o C.

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