Abstract: Questionable Patent is also referred to as a “probabilistic patent” or a “weak patent”, which is a patent that is improperly granted, including that does not conform to the provisions of the present patent law, and a patent right may be granted but has a wide scope of claims. The causes of questionable patent include internal and external factors. The inherent factor is that the patent right is not a fully enforceable right, and the patent holder actually has the right to be able to claim his right in court. However, due to the uncertainty of the result of the litigation, the validity of the patent right cannot be determined accordingly, which is the root cause of the questionable patent. The external factors are referred to the sharp increase in the number of patent applications, insufficient quality of the inspector, limited review time, insufficient authorization and retrieval conditions, and the defects of related policies, etc.The questionable patent could bring great harm to the society, such as hindering the innovation or increasing the cost of innovation, increasing the formation of the patent jungles, creating new difficulties of the patent license, and increasing the uncertainty of the patent prospect. Charging royalties or threatening litigation through questionable patent would frustrate competition from existing or potential firms. Therefore, the questionable patent is one of the most serious problems facing the patent system.In view of all kinds of social and economic problems caused by questionable patent, scholars have done a great deal of research on this problem, and most of them advocate that the questionable patent should be governed by strengthening the examination process or increasing judicial input. However, most of the existing literatures assume that the patent quality (the validity of the patent) is exogenous, but few scholars discuss on the endogenous problem of patent quality. If we can realize the endogenous quality of patent, the causes of questionable patent could be analyzed at a more essential level, which may bring some new conclusions and enlightenment.The concept of "height of invention" is introduced in this paper. If an invention attempts to obtain a patent, it must satisfy the three factors of "novelty", "practicability" and "non-obvious", and the height of invention is a comprehensive concept containing the above three factors. Questionable patent is not without any innovation, but not with enough height of invention, which often requires enough R&D investment to achieve. Patent quality is not given naturally. R&D investment determines the height of invention and the height of invention further determines the quality of patent. Therefore, there is always a positive correlation between R&D investment and patent quality. If we regard R&D investment as an exogenous variable, we could transform height of invention into an endogenous variable and realize the endogenous quality of patent. On this basis, we could analyze the impact of R&D investment, market structure and existing technology on patent quality in the process of patent marketization.In this paper, we assume that there is a positive correlation between invention height and R&D investment, and a positive correlation between patent quality and invention height, thus establishing a relationship between patent quality and R&D investment，and then endogenous quality of patent is achieved. This paper takes game theory as the main research method，and analyzes R&D investment, patent licensing behavior of innovative firms and the strategic choice of non-innovative firms by a duopoly game model, and then compares the quality of patent in different situations. This paper mainly discusses the following two aspects: on one hand, what are the factors that determine the quality of the patent? How do these factors affect R&D investment and firm behavior? On the other hand, what is the impact of questionable patent on social welfare? At the same time, questionable patents often accompany patent litigation, and what role does the court play in this process?The results show that it is possible for innovation firms to provide questionable patent in any case, which is mainly due to the high requirement forheight of invention or the low R&D efficiency of related innovation activities, or both. The establishment of court compensation for patent infringement greatly affects the behavior of potential infringers and then determines the market structure. When the amount of compensation for patent infringement is low, the expected loss caused by the infringement of non-innovative firms is small, so the incentive of infringement will be increased, resulting in the increase of the number of firms in the market. From the perspective of social welfare，questionable patent does not always have a negative effect. When significant innovation is very difficult, complete quality patents need to invest a lot of R&D costs, and these costs are difficult to be compensated from the process of patent marketization. At this time, incomplete quality patents could realize the optimization of social welfare.The main revelation of this paper is that the impact of market structure on R&D depends on the specific characteristics of different industries or related innovations from the perspective of patent quality. There is a certain relationship between market structure and patent quality, but this correlation is not monotonous. From the point of view of social welfare, questionable patent is not always harmful, its existence is reasonable sometimes. Therefore, the questionable patent should be treated differently under different circumstances. Courts should be strict with questionable patent when significant innovation is easy, such as setting a lower amount of patent compensation or the trial results are conducive to reducing the market power of patent firm. When significant innovation is difficult, the court should take a more lenient attitude towards questionable patent, such as setting a higher amount of patent compensation or the trial result is relatively favorable to the patent firm. The existing governance mode of questionable patent is relatively simple, and it may be difficult to achieve a more satisfactory result. Therefore, more diversified measures should be taken for the governance of questionable patent.

Key words:  questionable patent, height of invention, R&D investment, endogenous quality of patent