High-dose Ion-implanted Photoresist Research Articles

(Digital Presentation) High-Dose Ion-Implanted Photoresist Stripping Technology Employing High Temperature Single-Wafer SPM System

[Introduction]Requirements for high-dose ion-implanted photoresist stripping technology in the field of semiconductor manufacturing are to achieve with no residue, low damage and less chemical consumption. To realize these requirements simultaneously have been extremely difficult for decades. It is said that highly ion-implanted photoresist is highly carbonized and hardened, so it is extremely difficult to strip this. Traditional plasma ashing process tends to introduce a plasma damage, and batch wet process tends to cause much chemical consumption. Therefore, we tackled with this issue by employing single-wafer wet process system with high temperature SPM (Sulfuric acid and hydrogen Peroxide Mixture) process, and obtained interesting results.SPM produces a strong oxidant, peroxymonosulfuric acid, to decompose organic matter [1]. The reactivity of SPM with the photoresist increases at higher temperatures. However, at temperatures above 220°C, oxidant in the SPM becomes deactivated [2] and photoresist-stripping performance is said to be deteriorated, resulting in longer processing times, thereby increasing use of chemicals.Whereas, we have confirmed that a high-dose photoresist with a dose quantity of 1E16atoms/cm2 can be stripped successfully using SPM at a temperature of about 250°C. In light of that, we have evaluated the etching characteristics in the high temperature region and from the results, we have discussed the mechanism of the deactivation process of active species and photoresist strip performance.[Experimental procedure]The sample structure used are shown in Tab.1. We evaluated the etching characteristics considering amorphous-Carbon (a-Carbon) as a completely hardened form of photoresist layer. Tab.2 shows the chemicals that we used, Fig.1 shows outline of the experimental concept and Fig.2 shows the evaluation flow. The SPM was formed by mixing of H2SO4 and H2O2 in advance at room temperature, and then supplied them on a wafer, rapidly heated with a lamp heater, thereby confirming the temperature dependence of the etching rate. The thickness of a-Carbon film was measured before and after treatment with a spectroscopic ellipsometer (Onto Innovation Inc.: Atlas). And then we calculated the etching rate. Photoresist samples were observed with cross sectional SEM images (Hitachi High-Tech Corp.: S-5500) at an accelerating voltage of 10 kV. Accordingly, we checked for the presence of resist residue after treatment.[Results and Discussion]As shown in Fig.3, temperature dependence of the etching rate of the a-Carbon shows that increases from 165 to around 210 °C and then decreases at higher temperature. A concentration of H2SO4 included in SPM is about 81 wt%, and the boiling point of 81 wt% of the H2SO4-aq is about 209 °C [3]. When the boiling point of SPM is assumed same as that of H2SO4-aq, the temperature dependence of the etching rate of a-Carbon has a peak at boiling point. According to the Arrhenius plot of the etching rate of a-Carbon [Fig.4], a mechanism of the etching of a-Carbon is changed at the boiling point. We discuss the reaction mechanism at higher than 210 °C, the original boiling point of SPM, below. SPM is consisted of mixture of H2SO4, H2O2 and H2O. By heating up at 210 °C, first H2O is evaporated away and H2O2 will be start to be evaporated, and then the H2SO4 concentration will increase. It means that the boiling point of SPM will shift to higher temperature. Existence of H2SO4 and H2O2 is essential to form oxidant: peroxymonosulfuric acid that is delivered from SPM, at higher than 210 °C the concentration of the oxidant becomes lower. This will lead a decrease of the a-Carbon etching rate. At higher than 210 °C, a difference of actual data and the extrapolated plot from Arrhenius plot shows a concentration of the oxidant when all the oxidant contribute to the reaction. Temperature dependence of estimated concentration of the oxidant is plotted at Fig. 5. The concentration of the oxidant is estimated about 10 % at 250 °C. On the contrary, a good stripping performance shows at 250 °C; the oxidant concentration becomes very low. It is thought that the stripping performance of the high-dose photoresist is determined not only by the oxidant concentration, but also another factor. We will discuss in detail the mechanism of this at the presentation.[1] N. Hayamizu et al., Toshiba Review, 64, 38 (2009).[2] M. Uchida, Japanese Patent No. 5668914 (2014).[3] C.M. Gable, et al., J. Am. Chem. Soc., 72, 1445, (1950). Figure 1

(Digital Presentation) High-Dose Ion-Implanted Photoresist Stripping Technology Employing High Temperature Single-Wafer SPM System

We found that the high-dose ion-implanted photoresist can be removed with high efficiency without residue by using SPM heated to 250 °C. We evaluated the reaction mechanism in detail, and proposed a model: a heat consumption by vaporization of H2O, a heat gain by decomposition of H2O2, and a character change of SPM itself interact, the high-dose ion-implanted photoresist along with dopants is removed.

Ion-Implanted Photoresist Stripping by Using Organic Solvents

In the ion implantation process, accelerated ions attack not only target materials but also photoresist, causing deformation of the molecular structure of the photoresist. It is difficult to remove crust layer formed by ion implantation in the standard photoresist removal methods. A typical method of removing high-dose ion-implanted photoresist is to use plasma ashing followed by the treatment in the mixture of hot sulfuric acid and hydrogen peroxide [1]. However, this process etches III-V semiconductors and dielectric materials [2]. Therefore, in this study, organic solvent-based mixtures were developed to remove high-dose ion-implanted photoresist without damaging the substrate. Trench-patterned GaAs was used as a substrate and KrF photoresist was coated on trench patterned GaAs. Then, P+ ion implantation with a dose of 5×1015 ions/cm2 was conducted on the photoresist. A mixed solution of dimethyl sulfoxide (DMSO) and acetonitrile (AcN) (DMSO: AcN = 6: 4) was used to remove the ion-implanted photoresist and 1 vol% HCl or HF was used as an additive. Photoresist removal was measured using an optical microscope and in-situ MIR-FTIR. When a mixed solvent of DMSO and AcN was used to remove ion-implanted photoresist, the photoresist was not completely removed and the residue and crust remained on the trench-pattern (Fig. 1). In order to improve the removal efficiency of ion-implanted photoresist, 1 % HCl or HF was added to DMSO+AcN solution. When 1 % HCl was added to the DMSO+AcN solution, the ion-implanted photoresist with an implantation energy of 30 and 50 keV was completely removed without residue. However, when the ion implantation energy was 70 keV, the photoresist crust was not removed in DMSO+AcN+HCl solution. When 1 % HF was added to DMSO+AcN solution, the ion-implanted photoresists was completely removed regardless of the ion implantation energy. Removal performance was quantitatively analyzed by in-situ MIR-FTIR as shown in Fig. 2. First, the ion-implanted photoresist was not removed more than 40 % for 10 min in DMSO. In DMSO+AcN solution, most of the photoresist was removed in 6 min, and all photoresist was removed in 4 min when DMSO+AcN+HCl was used. In the case of DMSO+AcN+HF, all of the ion-implanted photoresists were completely removed in 2 min and 30 s. Therefore, it is confirmed that DMSO+AcN+HF solution removes ion-implanted photoresist more quickly and effectively than other solutions used in this study. In addition, it is suggested that the largest interaction energy between DMSO+AcN+HF and the ion-implanted photoresist resulted in improvement of the removal efficiency of the photoresist.

Ion-Implanted Photoresist Stripping by Using Organic Solvents

Various organic solvent-based mixture solutions were used to remove the ion-implanted photoresist on trench-patterned GaAs wafers. The mixture of acetonitrile (AcN) and dimethyl sulfoxide (DMSO) could remove bulk photoresist and crust but left a small amount of photoresist residue. However, the combination of AcN and DMSO with a small amount of HF completely removed high-dose ion-implanted photoresist even at 30 °C (ion-implantation dose of 5×1015 ions/cm2 and the energy of 70 keV). Moreover, there was no material loss of GaAs wafer observed in AcN+DMSO+HF solution during the removal of ion-implanted photoresist.

Thermally Activated Gas-Phase Chemical Reactions in a Single-Wafer Wet Clean Process

A conventional high-dose ion-implanted photoresist strip and clean process flow consists of using a dry remote plasma ashing tool to remove the crust layer and underlying photoresist formed during the implant, followed by sulfuric-peroxide mixture (SPM) and ammonium hydroxide-hydrogen peroxide (SC1) chemistries applied on a wet clean tool to remove the remaining bulk photoresist and implant residues. However, film loss control and residue removal are increasingly difficult to maintain for sub-10 nm devices with conventional approaches. We investigate an alternative sequence that replaces the remote plasma strip with a thermally activated, atmospheric gas-phase oxidation process and replaces the SPM chemistry with a de-ionized water/ozone (DIO3) wet clean. The alternative sequence is entirely processed in a modified single-wafer wet clean chamber. For the oxidation process, a dry O3/O2 gas mixture decomposes and reacts directly with the ion-implanted photoresist wafer that is heated above 300°C from the backside. 9,000 Å/min strip rates are measured on 193 nm photoresist, and 650 Å/min are measured for the implanted crust removal. Based on both X-ray photoelectron spectroscopy (XPS) analysis of the reacted surface as well as the composition of the gas phase products by residual gas analysis (RGA), the crust layer and bulk photoresist are removed by oxidation mechanisms similar to remote oxygen plasma strip processes. Above 350°C, it is found that O2 gas itself reacts with the photoresist but to a lesser extent than the O3/O2 mixture. Reactivity to silicon is lower than a typical remote oxygen plasma based on ~1 Å silicon loss measured on blanket films. This in-situ thermally activated oxidation reaction allows for subsequent wet processing without any queue time delay. The integrated process completely removes 5E15-1E16 at/cm2 high-dose implanted 193 nm photoresist with no residue on patterned substrates. Fig 1: In-situ thermally activated oxidation reaction in a single-wafer wet clean chamber process sequence consisting of (a) a dry ozone-oxygen gas environment, (b) ozone-oxygen decomposition and reaction on a heated wafer surface, (c) ozonated de-ionized water rinse in an N2 environment, (d) ammonium hydroxide-hydrogen peroxide clean in an N2 environment, and (e) a de-ionized water rinse and spin dry. Figure 1

Effect of Microbubbles on Ozonized Water for Photoresist Removal

In this paper, we describe the use of ozone microbubbles in photoresist removal from silicon wafers. Ozonized water has attracted much attention as an environmental friendly cleaning method in semiconductor manufacturing. However, it would be desirable to enhance the oxidative ability of ozonized water for practical application. The existence of microbubbles in ozonized water has been shown to significantly enhance the photoresist removal rate due to an elevated dissolved ozone concentration (approximately 2.5 times that of ordinary ozone bubbling) and a direct effect of the microbubbles (removal rate is approximately 1.3 times faster than water with the same concentration of dissolved ozone without microbubbles). Additionally, the ozone microbubble solution was able to effectively remove a high-dose ion-implanted photoresist, which is extremely resistant to removal by ozonized water and other wet chemicals because of its amorphous carbon-like layer, or “crust”. Electron spin resonance experiments were al...

Mechanical behaviors of crust layer in high-dose ion implanted-photoresist (HDI-PR) upon exposure to atmospheric pressure oxygen plasma: A measuring “pop-in” point in nanoindentation

The use of existing processes for cleaning high-dose ion implanted-photoresist (HDI-PR) on semiconductor has some associated problems. When the PR surface is damaged by ion bombardment, its near surface changes to a hard layer called the crust layer. In this study, we suggest a nanoindentation-based method for measuring the “pop-in” point of force–depth graph of HDI-PR. It was observed that the thickness and load force of the crust layer decrease continually due to the atmospheric pressure plasma as the plasma treatment time increases from 0 to 120s. Further, the mechanical property such as hardness or elastic modulus of the HDI-PR has changed suddenly after a treatment time of 60s.

The effects of supercritical CO2 formulations on the removal of high-dose ion-implanted photoresists

Highlights3wt.% EH-3 and TMN-3 are chosen to form scCO2 microemulsions.2vol.% DMSO and IPA can enhance HDI PR efficiency by 75.5% and 64.2%.DMSO and EH-3 show a positive synergistic effect on the stripping efficiency.scCO2 density tuned by temperature and pressure affects HDI PR stripping behavior.100% Removal efficiency is achieved at 60?C and 25MPa for 20min. A new effective method for high-dose ion-implanted photoresist (HDI PR) stripping is reported based on supercritical CO2 (scCO2) microemulsions consisting of branched hydrocarbon surfactant 2-ethylhexanol polyoxyethylene-polyoxypropylene (EH-3) and co-solvent dimethylsulfoxide (DMSO). The optimized concentrations of EH-3 and DMSO in scCO2 were 3wt.% and 2vol.%, respectively, and the weight ratio of H2O and EH-3 is 1:1. The HDI PRs containing crust layers were removed in the scCO2 microemulsions at 10-25MPa and 40-80?C for 20min. The results revealed that the stripping efficiency of HDI PR increased with the pressure and temperature (T) when T is lower than 60?C. The removal efficiency of 96.8% was achieved at 60?C and 20MPa and then it gradually decreased as T higher than 60?C due to the lower scCO2 density at higher T. Magnetic agitation was favorable for the HDI PR removal. The optimum stripping parameters were determined to be 60?C and 25MPa for 20min at which 100% removal efficiency was achieved in EH-3/DMSO/scCO2 microemulsions. This environmentally benign method provides a promising alternative to effectively strip HDI PR for devices with nano-sized feature dimensions in microelectronic processing.

The Stripping Behavior of High-Dose Ion-Implanted Photoresists in Supercritical CO2 Formulations

A new type of high-dose ion-implanted photoresist (HDI PR) stripping method was reported based on supercritical CO2 (scCO2) microemulsions consisting of branched hydrocarbon surfactant 2-ethyl hexanol polyoxyethylene-polyoxypropylene (EH-3) and co-solvent dimethylsulfoxide (DMSO). The optimized concentration of EH-3 and DMSO in scCO2 were 3 wt% and 2 vol.%, respectively, and the ratio of H2O and EH-3 is 1:1. The post-etch HDI PRs containing crust polymer were removed in the scCO2 microemulsions under the pressure of 10–25 MPa at 40–80°C for 10 min. The results revealed that the stripping efficiency increased with the pressure and temperature when the temperature is lower than 60°C. The 94% removal efficiency was achieved at 60°C and 20 MPa and then it gradually decreased as the temperature is higher than 60°C due to the lower scCO2 density at higher temperature. The magnetic agitation and pressure pulsation were favorable to HDI PR removal. The optimum stripping parameters were determined to be 60°C and 25 MPa for 20 min at which 100% photoresist removal efficiency was achieved in EH-3/DMSO/scCO2 microemulsions. This environmentally benign method provides a promising alternative to effectively strip the HDI PR for nano-sized devices in microelectronics processing.

Stripping of High-Dose Ion-Implanted Photoresist Using a Combination of Dry and Wet Single-Wafer Processing

High dosed ion implanted (HDI) photoresist is well-known to be difficult to remove by conventional wet stripping e.g. sulfuric–peroxide mixture without substrate loss because of their carbonized crust layer. To overcome this issue, several methods are proposed such as dry etch + wet, assist of physical cleaning. In this paper, a new method to strip HDI photoresist using a combination of dry and wet processing in the same chamber is introduced to achieve HDI photoresist removal without substrate loss. A new process consists of two steps. The first process removes the carbonized crust layer by atmospheric-pressure inductively coupled hydrogen plasma, and the second process removes remaining the bulk photoresist by a sulfuric–ozone mixture. The new process enables removal of the HDI photoresist without any substrate loss, and with significantly shorter times.

Effect of Microbubbles on Ozonized Water for Photoresist Removal

In this paper, we describe the use of ozone microbubbles in photoresist removal from silicon wafers. Ozonized water has attracted much attention as an environmental friendly cleaning method in semiconductor manufacturing. However, it would be desirable to enhance the oxidative ability of ozonized water for practical application. The existence of microbubbles in ozonized water has been shown to significantly enhance the photoresist removal rate due to an elevated dissolved ozone concentration (approximately 2.5 times that of ordinary ozone bubbling) and a direct effect of the microbubbles (removal rate is approximately 1.3 times faster than water with the same concentration of dissolved ozone without microbubbles). Additionally, the ozone microbubble solution was able to effectively remove a high-dose ion-implanted photoresist, which is extremely resistant to removal by ozonized water and other wet chemicals because of its amorphous carbon-like layer, or crust. Electron spin resonance experiments were also performed without the influence of serious metal contamination and indicated the presence of hydroxyl radicals, which are thought to be formed by interaction of ozone with hydroxide ions adsorbed at the gas-water interface upon collapse of the microbubbles. The hydroxyl radicals play an important role in photoresist removal by the ozone microbubble treatment.

High-dose ion-implanted photoresist stripping in environmentally benign supercritical CO2 nonfluorous surfactant microemulsions

The stripping behavior of high-dose ion-implanted (HDIM) photoresist was investigated in a scCO2 microemulsion with 3wt.% surfactant of 2-ethyl hexanol polyoxyethylene-polyoxypropylene and 2vol.% co-solvent of dimethylsulfoxide. The removal efficiency was studied under the pressure of 10-25MPa at 40-60^oC. The results of surface characterization and composition analysis for the cleaned samples by SEM, FTIR, and XPS revealed that the removal rate increased with time with a sharp increase during the first 10min. Temperature and pressure are favorable to the stripping within 20min. The photoresist removal efficiency at 50^oC improved from 40% to 85% as the pressure range of 10-25MPa due to the higher scCO2 density at higher pressure which enhances the photoresist solubility in scCO2. The magnetic agitation and pressure pulsation generated a higher fluid velocity to provide a larger physical force. This new environment-friendly photoresist stripping method allows dramatic reduction in water and chemicals consumption and overcomes the obstacle in the conventional microelectronics processing due to the shrinkage in the feature size.

Novel Single-Wafer Single-Chamber Dry and Wet Hybrid System for Stripping and In Situ Cleaning of High-Dose Ion-Implanted Photoresists

There is increasing demand for moving from batch immersion tools to single-wafer spin tools for silicon wafer cleaning, etching, and photoresist/residue removal in advanced semiconductor manufacturing. However, high-dose ion-implanted photoresist removal using a conventional single-wafer spin tool is very difficult. We have developed a novel single-wafer single-chamber dry and wet hybrid system in combination with dry ashing and moderate-temperature wet-cleaning treatments by implementing an atmospheric-pressure plasma unit into a conventional single-wafer spin cleaning tool. This compact single-wafer single-chamber system can completely remove the hardened photoresist due to high-dose ion-implantation by an atmospheric-pressure plasma ashing process followed by an in situ wet chemical process in the same single chamber within 2 min. This single-wafer single-chamber dry/wet hybrid system offers less than 1/3 smaller footprint, less than 1/4 shorter cycle time (for 50 wafer processing), and potentially better process control and less contamination risk, as well as lower equipment cost, compared to the conventional combination of two separate dry- and wet-processing systems.

Plasma를 이용한 세정액의 활성화와 시료 표면의 탄성계수 및 강도 변화에 대한 연구

현대의 반도체 산업에서 공정 중 가장 큰 비중을 차지하며, 가장 많은 자본과 인력을 소비하는 것이 바로 세정 공정이다. 세정공정은 소자의 작동에 영향을 미치고 기능을 저하시킬 수 있는 이물질 입자들을 제거하는 것이다. 특히 소자를 식각하기 위한 Photoresist(PR) 과정이 끝날 때마다 항상 세정 과정이 포함되어야 했다. 또한, Photoresist(PR) 공정 중 생성된 HDI-PR(high dose implanted photoresist)은 세정 과정에서 제거가 힘들기 때문에 현대의 고밀도 집적회로 세정 공정에서는 건식 세정과 습식 세정을 혼용하여 여러 단계의 세정 공정을 거치게 된다. 이 논문에서는 기존 플라즈마 방식으로 대표되는 건식 세정과 약액으로 대표되는 습식 세정을 동시에 사용하는 방식을 사용하여 약액활성화 방법(Plasma Liquid-Vapor Activation; PLVA)을 제안하여 실험을 실시하였고 HDI-PR을 활성화된 용액에 담근 후 Nano-Indenter를 이용하여 표면강도와 탄성계수를 측정했다. Nano-indenter는 특정한 기하학적 형태를 가지는 Tip을 표면에 압입한 후 압입하중과 압입깊이를 측정함으로서 시료의 표면강도와 탄성계수를 측정하였다. 그 결과 plasma로 활성화된 PR stripper 용액으로 strip한 후의 시료의 표면 강도가 크게 줄어든 것을 확인하였다. 이는 이후 물리적 표면 세정 공정을 후 공정으로 사용한다면 보다 효율적인 HDI-PR을 제거할 수 있을 것으로 사료된다. In the modem semiconductor industry, the progress that consumes the most capital and labor is cleansing process. Cleansing process is to remove impurities that can affect the operation of the device and deteriorate its function. Especially, Photoresist (PR) progress that etches the device always requires cleansing at the end of the progress. Also, HDI-PR (High-Dose Ion-implanted Photoresist) created from PR progress is difficult to remove. Thus, in modem IC cleansing, many steps of cleansing are used, including dry and wet cleansing. In this paper, we suggested to combine existing dry-cleansing and wet-cleansing, each represented by plasma cleansing and stripper solution, as Plasma Liquid-Vapor Activation (PLVA). This PLVA method enhances the effect of existing cleansing solution, and decreases the amount of solution and time required to strip. We stripped HDI-PR by activated solution and measured surface hardness and Young's modulus by Nano-indenter. Nano-indenter is the equipment that determines the hardness and the modulus of elasticity by indenting nano-sized tip with specific shape into the surface and measuring weight and z-axis displacement. We measured the change of surface hardness and Young's modulus before and after the cleansing. As a result, we found out that the surface hardness of the sample sharply decreased after the cleansing by plasma-activated PR stripper solution. It can be considered that if physical surface-cleansing process is inserted after this, more effective elimination of HDI-PR is possible.

Stripping and Cleaning of High-Dose Ion-Implanted Photoresists Using a Single-Wafer, Single-Chamber Dry/Wet Hybrid System

Stripping high-dose ion-implanted (HDI) photoresists is considered as one of the most challengeable processes in the semiconductor manufacturing due to the difficulty of both removing crust (or carbonized layer) formed during the ion implantation and preventing the silicon recess after subsequent cleaning. The HDI photoresists are conventionally removed by using a two-step process, low-pressure plasma ashing in a single-wafer tool followed by SPM-based wet stripping in a batch immersion tool. Alternative HDI-resist strip methods have been proposed, such as a combination of physical-force pretreatments followed by more traditional wet cleaning steps [1], a SPM-based all-wet process at extremely high temperature (≥ 200°C) [2], and supercritical CO2 combined with chemical additive formulations [3].

The effect of ultrasonic agitation on the stripping of photoresist using supercritical CO 2 and co-solvent formulation

A novel technology for removing high-dose ion-implanted photoresist (HDI PR) from semiconductor wafers using supercritical carbon dioxide (scCO 2) and several co-solvent formulations have been described. A combination of ultrasonic agitation with scCO 2/co-solvent stripping was found to be an effective method for photoresist removal. Ultrasonic agitation was an efficient technique for achieving higher stripping rates. The effects of temperature, pressure, reaction time and the type of organic co-solvent on the stripping rate of HDI PR were investigated. The microstructures of sample wafers after stripping were characterized by scanning electron microscopy and energy dispersive X-ray spectrometer.

Liquid Clean Formulations for Stripping High-Dose Ion-Implanted Photoresist from Microelectronic Devices

Liquid clean formulations comprised of an organosilane, , and toluene have been found to selectively strip thin films of high-dose arsenic-implanted polyhydroxystyrene photoresist and hardened carbonized crust without silicon/oxide loss. During the cleaning process, self-assembled monolayers (SAMs) of organosilanes were attached to the surface silanol groups in the regions of patterned wafers. It was determined that a higher surface density of organosilane SAMs improves the cleaning efficiency, and halide leaving groups further assist with photoresist and crust removal possibly due to chemical interactions with the arsenic ions. Wafers treated with the formulation containing resulted in the best cleaning efficiency (assessed by optical and scanning electron microscopy), yielding greater than 90% overall photoresist/crust removal in static cleaning mode and 100% removal in a dynamic cleaning mode, under optimal reaction conditions (, ). The SAMs were then stripped from the wafers. A solution of in DMSO was found to depassivate the wafer after (static mode) while maintaining the structural integrity of the underlying silicon-containing layers. The surfaces were characterized by contact angle to determine wettability, atomic force microscopy, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy.

Photoresist Removal using Atomic Hydrogen Generated by Heated Catalyzer

In this paper, a novel method for high-dose ion-implanted photoresist removal using atomic hydrogen generated by a heated tungsten catalyzer is proposed. It is found that photoresist doped as high as 1×1015 cm-2 can be removed completely. Quadruple mass spectroscopy detected hydrocarbon fragments during the atomic hydrogen treatment.