Global.cs

using UnityEngine;
using System.Collections;

public class Global : MonoBehaviour
{
    private static Global instance;

    private static Plane screenPlane;

    void Awake()
    {
        if (!instance)
        {
            instance = this;
            DontDestroyOnLoad(gameObject);
            InitFirst();
        }
        else
        {
            Destroy(gameObject);
        }
    }

    void Start()
    {
        InitScene();
    }

    void OnLevelWasLoaded()
    {
        InitScene();
    }

    void InitFirst()
    {
        // for iOS
        Application.targetFrameRate = 60;
        // create plane used to detect world position
        screenPlane = new Plane(-Vector3.forward, Vector3.zero);
    }

    void InitScene()
    {
        // ...
    }

    public static Vector3 GetWorldPosition(Vector2 point)
    {
        Ray ray = Camera.main.ScreenPointToRay(point);
        float distance = 0.0f;
        if (screenPlane.Raycast(ray, out distance))
        {
            Vector3 vec = ray.GetPoint(distance);
            return vec;
        }
        return Vector3.zero;
    }

}